labeled diagram for photosynthesis

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Photosynthesis

Photosynthesis is a process by which phototrophs convert light energy into chemical energy, which is later used to fuel cellular activities. The chemical energy is stored in the form of sugars, which are created from water and carbon dioxide.

Table of Contents

• What is Photosynthesis?
• Site of photosynthesis

What Is Photosynthesis in Biology?

The word “ photosynthesis ” is derived from the Greek words  phōs  (pronounced: “fos”) and σύνθεσις (pronounced: “synthesis “) Phōs means “light” and σύνθεσις   means, “combining together.” This means “ combining together with the help of light .”

Photosynthesis also applies to other organisms besides green plants. These include several prokaryotes such as cyanobacteria, purple bacteria and green sulfur bacteria. These organisms exhibit photosynthesis just like green plants.The glucose produced during photosynthesis is then used to fuel various cellular activities. The by-product of this physio-chemical process is oxygen.

A visual representation of the photosynthesis reaction

• Photosynthesis is also used by algae to convert solar energy into chemical energy. Oxygen is liberated as a by-product and light is considered as a major factor to complete the process of photosynthesis.
• Photosynthesis occurs when plants use light energy to convert carbon dioxide and water into glucose and oxygen. Leaves contain microscopic cellular organelles known as chloroplasts.
• Each chloroplast contains a green-coloured pigment called chlorophyll. Light energy is absorbed by chlorophyll molecules whereas carbon dioxide and oxygen enter through the tiny pores of stomata located in the epidermis of leaves.
• Another by-product of photosynthesis is sugars such as glucose and fructose.
• These sugars are then sent to the roots, stems, leaves, fruits, flowers and seeds. In other words, these sugars are used by the plants as an energy source, which helps them to grow. These sugar molecules then combine with each other to form more complex carbohydrates like cellulose and starch. The cellulose is considered as the structural material that is used in plant cell walls.

Where Does This Process Occur?

Chloroplasts are the sites of photosynthesis in plants and blue-green algae.  All green parts of a plant, including the green stems, green leaves,  and sepals – floral parts comprise of chloroplasts – green colour plastids. These cell organelles are present only in plant cells and are located within the mesophyll cells of leaves.

Also Read:  Photosynthesis Early Experiments

Photosynthesis Equation

Photosynthesis reaction involves two reactants, carbon dioxide and water. These two reactants yield two products, namely, oxygen and glucose. Hence, the photosynthesis reaction is considered to be an endothermic reaction. Following is the photosynthesis formula:

Unlike plants, certain bacteria that perform photosynthesis do not produce oxygen as the by-product of photosynthesis. Such bacteria are called anoxygenic photosynthetic bacteria. The bacteria that do produce oxygen as a by-product of photosynthesis are called oxygenic photosynthetic bacteria.

Structure Of Chlorophyll

The structure of Chlorophyll consists of 4 nitrogen atoms that surround a magnesium atom. A hydrocarbon tail is also present. Pictured above is chlorophyll- f,  which is more effective in near-infrared light than chlorophyll- a

Chlorophyll is a green pigment found in the chloroplasts of the  plant cell   and in the mesosomes of cyanobacteria. This green colour pigment plays a vital role in the process of photosynthesis by permitting plants to absorb energy from sunlight. Chlorophyll is a mixture of chlorophyll- a  and chlorophyll- b .Besides green plants, other organisms that perform photosynthesis contain various other forms of chlorophyll such as chlorophyll- c1 ,  chlorophyll- c2 ,  chlorophyll- d and chlorophyll- f .

Also Read:   Biological Pigments

Process Of Photosynthesis

At the cellular level,  the photosynthesis process takes place in cell organelles called chloroplasts. These organelles contain a green-coloured pigment called chlorophyll, which is responsible for the characteristic green colouration of the leaves.

As already stated, photosynthesis occurs in the leaves and the specialized cell organelles responsible for this process is called the chloroplast. Structurally, a leaf comprises a petiole, epidermis and a lamina. The lamina is used for absorption of sunlight and carbon dioxide during photosynthesis.

Structure of Chloroplast. Note the presence of the thylakoid

“Photosynthesis Steps:”

• During the process of photosynthesis, carbon dioxide enters through the stomata, water is absorbed by the root hairs from the soil and is carried to the leaves through the xylem vessels. Chlorophyll absorbs the light energy from the sun to split water molecules into hydrogen and oxygen.
• The hydrogen from water molecules and carbon dioxide absorbed from the air are used in the production of glucose. Furthermore, oxygen is liberated out into the atmosphere through the leaves as a waste product.
• Glucose is a source of food for plants that provide energy for  growth and development , while the rest is stored in the roots, leaves and fruits, for their later use.
• Pigments are other fundamental cellular components of photosynthesis. They are the molecules that impart colour and they absorb light at some specific wavelength and reflect back the unabsorbed light. All green plants mainly contain chlorophyll a, chlorophyll b and carotenoids which are present in the thylakoids of chloroplasts. It is primarily used to capture light energy. Chlorophyll-a is the main pigment.

The process of photosynthesis occurs in two stages:

• Light-dependent reaction or light reaction
• Light independent reaction or dark reaction

Stages of Photosynthesis in Plants depicting the two phases – Light reaction and Dark reaction

Light Reaction of Photosynthesis (or) Light-dependent Reaction

• Photosynthesis begins with the light reaction which is carried out only during the day in the presence of sunlight. In plants, the light-dependent reaction takes place in the thylakoid membranes of chloroplasts.
• The Grana, membrane-bound sacs like structures present inside the thylakoid functions by gathering light and is called photosystems.
• These photosystems have large complexes of pigment and proteins molecules present within the plant cells, which play the primary role during the process of light reactions of photosynthesis.
• There are two types of photosystems: photosystem I and photosystem II.
• Under the light-dependent reactions, the light energy is converted to ATP and NADPH, which are used in the second phase of photosynthesis.
• During the light reactions, ATP and NADPH are generated by two electron-transport chains, water is used and oxygen is produced.

The chemical equation in the light reaction of photosynthesis can be reduced to:

2H 2 O + 2NADP+ + 3ADP + 3Pi → O 2 + 2NADPH + 3ATP

Dark Reaction of Photosynthesis (or) Light-independent Reaction

• Dark reaction is also called carbon-fixing reaction.
• It is a light-independent process in which sugar molecules are formed from the water and carbon dioxide molecules.
• The dark reaction occurs in the stroma of the chloroplast where they utilize the NADPH and ATP products of the light reaction.
• Plants capture the carbon dioxide from the atmosphere through stomata and proceed to the Calvin photosynthesis cycle.
• In the Calvin cycle , the ATP and NADPH formed during light reaction drive the reaction and convert 6 molecules of carbon dioxide into one sugar molecule or glucose.

The chemical equation for the dark reaction can be reduced to:

3CO 2 + 6 NADPH + 5H 2 O + 9ATP → G3P + 2H+ + 6 NADP+ + 9 ADP + 8 Pi

* G3P – glyceraldehyde-3-phosphate

Calvin photosynthesis Cycle (Dark Reaction)

Also Read:  Cyclic And Non-Cyclic Photophosphorylation

Importance of Photosynthesis

• Photosynthesis is essential for the existence of all life on earth. It serves a crucial role in the food chain – the plants create their food using this process, thereby, forming the primary producers.
• Photosynthesis is also responsible for the production of oxygen – which is needed by most organisms for their survival.

Frequently Asked Questions

1. what is photosynthesis explain the process of photosynthesis., 2. what is the significance of photosynthesis, 3. list out the factors influencing photosynthesis., 4. what are the different stages of photosynthesis, 5. what is the calvin cycle, 6. write down the photosynthesis equation..

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PPM stands for Parts-Per-Million. It corresponds to saying that 300 PPM of carbon dioxide indicates that if one million gas molecules are counted, 300 out of them would be carbon dioxide. The remaining nine hundred ninety-nine thousand seven hundred are other gas molecules.

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Microbe Notes

Photosynthesis: Equation, Steps, Process, Diagram

Photosynthesis is defined as the process, utilized by green plants and photosynthetic bacteria, where electromagnetic radiation is converted into chemical energy and uses light energy to convert carbon dioxide and water into carbohydrates and oxygen.

• The carbohydrates formed from photosynthesis provide not only the necessary energy form the energy transfer within ecosystems, but also the carbon molecules to make a wide array of biomolecules.
• Photosynthesis is a light-driven oxidation-reduction reaction where the energy from the light is used to oxidize water, releasing oxygen gas and hydrogen ions, followed by the transfer of electrons to carbon dioxide, reducing it to organic molecules.
• Photosynthetic organisms are called autotrophs because they can synthesize chemical fuels such as glucose from carbon dioxide and water by utilizing sunlight as an energy source.
• Other organisms that obtain energy from other organisms also ultimately depend on autotrophs for energy.
• One of the essential requirements for photosynthesis is the green pigment ‘chlorophyll’ which is present in the chloroplasts of green plants and some bacteria.
• The pigment is essential for ‘capturing’ sunlight which then drives the overall process of photosynthesis.

Table of Contents

Interesting Science Videos

Photosynthesis equations/reactions/formula

• The process of photosynthesis differs in green plants and sulfur bacteria.
• In plants, water is utilized along with carbon dioxide to release glucose and oxygen molecules.
• In the case of sulfur bacteria, hydrogen sulfide is utilized along with carbon dioxide to release carbohydrates, sulfur, and water molecules.

Oxygenic Photosynthesis

The overall reaction of photosynthesis in plants is as follows:

Carbon dioxide + Water  + solar energy → Glucose + Oxygen

6CO 2 + 6H 2 O  +  solar energy   →   C 6 H 12 O 6 + 6O 2

Carbon dioxide + Water  + solar energy → Glucose + Oxygen + Water

6CO 2 + 12H 2 O+ solar energy    →    C 6 H 12 O 6 + 6O 2 + 6H 2 O

Anoxygenic Photosynthesis

The overall reaction of photosynthesis in sulfur bacteria is as follows:

CO 2 + 2H 2 S + light energy   →    (CH 2 O)  + H 2 O  + 2S

Video Animation: Photosynthesis (Crash Course)

Photosynthetic pigments

• Photosynthetic pigments are the molecules involved in absorbing electromagnetic radiation, transferring the energy of the absorbed photons to the reaction center, resulting in photochemical reactions in the organisms capable of photosynthesis.
• The molecules of photosynthetic pigments are quite ubiquitous and are always composed of chlorophylls and carotenoids.
• In addition to chlorophyll, photosynthetic systems also contain another pigment, pheophytin (bacteriopheophytin in bacteria), which plays a crucial role in the transfer of electrons in photosynthetic systems.
• Moreover, other pigments can be found in particular photosynthetic systems, such as xanthophylls in plants.

Image Source: Simply Science .

Chlorophyll

• Chlorophyll is the pigment molecule, which is the principal photoreceptor in the chloroplasts of most green plants.
• Chlorophylls consist of a porphyrin ring, which is bounded to an ion Mg 2+ , attached to a phytol chain.
• Chlorophylls are very effective photoreceptors because they contain networks of alternating single and double bonds.
• In chlorophyll, the electrons are not localized to a particular atomic nucleus and consequently can more readily absorb light energy.
• In addition, chlorophylls also have solid absorption bands in the visible region of the spectrum.
• Chlorophylls are found either in the cytoplasmic membranes of photosynthetic bacteria, or thylakoid membranes inside plant chloroplasts.

Bacteriorhodopsin

• Bacteriorhodopsin is another class of photosynthetic pigment that exists only in halobacteria.
• It is composed of a protein attached to a retinal prosthetic group.
• This pigment is responsible for the absorption of light photons, leading to a conformational change in the protein, which results in the expulsion of the protons from the cell.

Phycobilins

• Cyanobacteria and red algae employ phycobilins such as phycoerythrobilin and phycocyanobilin as their light-harvesting pigments.
• These open-chain tetrapyrroles have the extended polyene system found in chlorophylls, but not their cyclic structure or central Mg 2+ .
• Phycobilins are covalently linked to specific binding proteins, forming phycobiliproteins, which associate in highly ordered complexes called phycobilisomes that constitute the primary light-harvesting structures in these microorganisms.

Carotenoids

• In addition to chlorophylls, thylakoid membranes contain secondary light-absorbing pigments, or accessory pigments, called carotenoids.
• Carotenoids may be yellow, red, or purple. The most important are β -carotene, which is a red-orange isoprenoid, and the yellow carotenoid lutein.
• The carotenoid pigments absorb light at wavelengths not absorbed by the chlorophylls and thus are supplementary light receptors.

Factors affecting photosynthesis

Blackman formulated the Law of limiting factors while studying the factors affecting the rate of photosynthesis. This Law states that the rate of a physiological process will be limited by the factor which is in the shortest supply. In the same way, the rate of photosynthesis is also affected by a number of factors, which are namely;

• As the intensity of light increases, the rate of light-dependent reactions of photosynthesis and in turn, the rate of photosynthesis increases.
• With increased light intensity, the number of photons falling on a leaf also increases. As a result, more chlorophyll molecules are ionized, and more ATPs and NADH are generated.
• After a point, however, the rate of photosynthesis remains constant as the light intensity increases. At this point, photosynthesis is limited by some other factors.
• Besides, the wavelength of light also affects the rate of photosynthesis.
• Different photosynthetic systems absorb light energy more effectively at different wavelengths.

Carbon dioxide

• An increase in the concentration of carbon dioxide increases the rate at which carbon is incorporated into carbohydrates in the light-independent reactions of photosynthesis.
• Thus, increasing the concentration of carbon dioxide in the atmosphere rapidly increases the rate of photosynthesis up to a point after which it is limited by some other factors.

Temperature

• The light-independent reactions of photosynthesis are affected by changes in temperature as they are catalyzed by enzymes, whereas the light-dependent reactions are not.
• The rate of the reactions increases as the enzymes reach their optimum temperature, after which the rate begins to decrease as the enzymes tend to denature.

Process/ Steps of Photosynthesis

The overall process of photosynthesis can be objectively divided into four steps/ process:

1. Absorption of light

• The first step in photosynthesis is the absorption of light by chlorophylls that are attached to the proteins in the thylakoids of chloroplasts.
• The light energy absorbed is then used to remove electrons from an electron donor like water, forming oxygen.
• The electrons are further transferred to a primary electron acceptor, quinine (Q) which is similar to CoQ in the electron transfer chain.

2. Electron Transfer

• The electrons are now further transferred from the primary electron acceptor through a chain of electron transfer molecules present in the thylakoid membrane to the final electron acceptor, which is usually NADP + .
• As the electrons are transferred through the membrane, protons are pumped out of the membrane, resulting in the proton gradient across the membrane.

3. Generation of ATP

• The movement of protons from the thylakoid lumen to the stroma through the F 0 F 1 complex results in the generation of ATP from ADP and Pi.
• This step is identical to the step of the generation of ATP in the electron transport chain .

4. Carbon Fixation

• The NADP and ATP generated in steps 2 and 3 provide energy, and the electrons drive the process of reducing carbon into six-carbon sugar molecules.
• The first three steps of photosynthesis are directly dependent on light energy and are thus, called light reactions, whereas the reactions in this step are independent of light and thus are termed dark reactions.

Types/ Stages/ Parts of photosynthesis

Figure: Photosynthesis takes place in two stages: light-dependent reactions and the Calvin cycle. Light-dependent reactions, which take place in the thylakoid membrane, use light energy to make ATP and NADPH. The Calvin cycle, which takes place in the stroma, uses energy derived from these compounds to make GA3P from CO 2 . Image Source: OpenStax (Rice University) .

Photosynthesis is divided into two stages based on the utilization of light energy:

1. Light-dependent reactions

• The light-dependent reactions of photosynthesis only take place when the plants/ bacteria are illuminated.
• In the light-dependent reactions, chlorophyll and other pigments of photosynthetic cells absorb light energy and conserve it as ATP and NADPH while simultaneously, evolving O 2 gas.
• In the light-dependent reactions of photosynthesis, the chlorophyll absorbs high energy, short-wavelength light, which excites the electrons present inside the thylakoid membrane.
• The excitation of electrons now initiates the transformation of light energy into chemical energy.
• The light reactions take in two photosystems that are present in the thylakoid of chloroplasts.

Figure: Light-dependent reactions of photosynthesis in the thylakoid membrane of plant cells. Image Source: Wikipedia (Somepics) .

Photosystem II

• Photosystem II is a group of proteins and pigments that work together to absorb light energy and transfer electrons through a chain of molecules until it finally reaches an electron acceptor.
• Photosystem II has a pair of chlorophyll molecules, also known as P680 as the molecules best absorb light of the wavelength 680 nm.
• The P680 donates a pair of electrons after absorbing light energy, resulting in an oxidized form of P680.
• Finally, an enzyme catalyzes the splitting of a water molecule into two electrons, two hydrogen ion, and oxygen molecules.
• This pair of electrons then are transferred to P680, causing it to return to its initial stage.

Photosystem I

• Photosystem I is a similar complex like photosystem II except for that photosystem I have a pair of chlorophyll molecules known as P700 as they best absorb the wavelength of 700 nm.
• As photosystem I absorb light energy, it also becomes excited and transfers electrons.
• The now oxidized form of P700 then accepts an electron from photosystem II, restring back to its initial stage.
• The electrons from photosystem I are then passed in a series of redox reactions through the protein ferredoxin.
• The electrons finally reach NADP + , reducing them to NADPH.

2 H 2 O + 2 NADP +  + 3 ADP + 3 P i  + light → 2 NADPH + 2 H +  + 3 ATP + O 2

Video Animation: The Light Reactions of Photosynthesis (Ricochet Science)

2. Light independent reactions (Calvin cycle)

Light independent reactions of photosynthesis are anabolic reactions that lead to the formation of a sex-carbon compound, glucose in plants. The reactions in this stage are also termed dark reactions as they are not directly dependent on the light energy but do require the products formed from the light reactions.

Figure: Overview of the Calvin cycle pathway. Image Source: Wikipedia (Mike Jones) .

This stage consists of 3 further steps that lead to carbon fixation/ assimilation.

Step 1: Fixation of CO 2 into 3-phosphoglycerate

• In this step, one CO 2 molecule is covalently attached to the five-carbon compound ribulose 1,5-biphosphate catalyzed by the enzyme ribulose 1,5-biphosphate carboxylase, also called rubisco.
• The attachment results in the formation of an unstable six-carbon compound that is then cleaved to form two molecules of 3-phosphoglycerate.

Step 2: Conversion of 3-phosphoglycerate to glyceraldehydes 3-phosphate

• The 3-phosphoglycerate formed in step 1 is converted to glyceraldehyde 3-phosphate by two separate reactions.
• At first, enzyme 3-phosphoglycerate kinase present in the stroma catalyzes the transfer of a phosphoryl group from ATP to 3-phosphoglycerate, yielding 1,3-bisphosphoglycerate.
• Next, NADPH donates electrons in a reaction catalyzed by the chloroplast-specific isozyme of glyceraldehyde 3-phosphate dehydrogenase, producing glyceraldehyde 3-phosphate and phosphate (Pi).
• Most of the glyceraldehyde 3-phosphate thus produced is used to regenerate ribulose 1,5-bisphosphate.
• The rest of the glyceraldehyde is either converted to starch in the chloroplast and stored for later use or is exported to the cytosol and converted to sucrose for transport to growing regions of the plant.

Step 3: Regeneration of ribulose 1,5-biphosphate from triose phosphates

• The three-carbon compounds formed in the previous steps are then converted into the five-carbon compound, ribulose 1,5-biphosphate through a series of transformations with intermediates of three-, four,-, five-, six-, and seven-carbon sugar.
• As the first molecules in the process, if regenerated, this stage of photosynthesis results in a cycle (Calvin cycle).

3 CO 2 + 9 ATP + 6 NADPH + 6 H +     →     glyceraldehyde-3-phosphate (G3P) + 9 ADP + 8 P i  + 6 NADP +  + 3 H 2 O

A G3P molecule contains three fixed carbon atoms, so it takes two G3Ps to build a six-carbon glucose molecule. It would take six turns of the cycle to produce one molecule of glucose.

Video Animation: The Calvin Cycle (Ricochet Science)

Products of Photosynthesis

The outcomes of light-dependent reactions of photosynthesis are:

The products of light-independent reactions (Calvin cycle) of photosynthesis are:

• glyceraldehyde-3-phosphate (G3P) / Glucose (carbohydrates)

The overall products of photosynthesis are:

• Glucose (carbohydrates)
• Sulfur (in photosynthetic sulfur bacteria)

Photosynthesis Examples

Photosynthesis in green plants or oxygenic bacteria.

• In plants and oxygenic bacteria like cyanobacteria, photosynthesis takes place in the presence of green pigment, chlorophyll.
• It takes place in the thylakoids of the chloroplasts, resulting in products like oxygen gas, glucose, and water molecules.
• Most of the glucose units in plants are linked to form starch or fructose or even sucrose.

Photosynthesis in sulfur bacteria

• In purple sulfur bacteria, photosynthesis takes place in the presence of hydrogen sulfur rather than water.
• Some of these bacteria like green sulfur bacteria have chlorophyll whereas other purple sulfur bacteria have carotenoids as photosynthetic pigments.
• The result of photosynthesis in these bacteria are carbohydrates (not necessarily glucose), sulfur gas, and water molecules.

Importance of photosynthesis

• Photosynthesis is the primary source of energy in autotrophs where they make their food by utilizing carbon dioxide, sunlight, and photosynthetic pigments.
• Photosynthesis is equally essential for heterotrophs, as they derive their energy from the autotrophs.
• Photosynthesis in plants is necessary to maintain the oxygen levels in the atmosphere.
• Besides, the products of photosynthesis contribute to the carbon cycle occurring in the oceans, land, plants, and animals.
• Similarly, it also helps maintain a symbiotic relationship between plants, animals, and humans.
• Sunlight or solar energy is the primary source of all other forms of energy on earth, which is utilized through the process of photosynthesis.

Artificial photosynthesis

Artificial photosynthesis is a chemical process that mimics the biological process of utilization of sunlight, water and carbon dioxide to produce oxygen and carbohydrates.

Image Source: Phys .

• In artificial photosynthesis, photocatalysts are utilized that are capable of replicating the oxidation-reduction reactions taking place during natural photosynthesis.
• The main function of artificial photosynthesis is to produce solar fuel from sunlight that can be stored and used under conditions, where sunlight is not available.
• As solar fuels are prepared, artificial photosynthesis can be used to produce just oxygen from water and sunlight, resulting in clean energy production.
• The most important part of artificial photosynthesis is the photocatalytic splitting of a water molecule, resulting in oxygen and large quantities of hydrogen gas.
• Further, light-driven carbon reduction can also be performed to replicate the process of natural carbon fixation, resulting in carbohydrates molecules.
• Thus, artificial photosynthesis has applications in the production of solar fuels, photoelectrochemistry, engineering of enzymes, and photoautotrophic microorganisms for the production of microbial biofuel and biohydrogen from sunlight.

Video Animation: Learning from leaves: Going green with artificial photosynthesis

Photosynthesis vs. Cellular respiration

Image Source: Khan Academy .

Video Animation: Photosynthesis vs. Cellular Respiration Comparison (BOGObiology)

FAQs (Revision Questions)

Where does photosynthesis occur? Photosynthesis occurs in the thylakoid membrane of the chloroplasts.

What are the products of photosynthesis? The products of photosynthesis are carbohydrates (glucose), oxygen, and water molecules.

What are the reactants of photosynthesis? The reactants of photosynthesis are carbon dioxide, water, photosynthetic pigments, and sunlight.

How are photosynthesis and cellular respiration related? Photosynthesis and cellular respiration are essentially the reverses of one another where photosynthesis is an anabolic process resulting in the formation of organic molecules. In contrast, cellular respiration is a catabolic process resulting in the breaking down of organic molecules to release energy.

• Berg JM, Tymoczko JL, Stryer L. Biochemistry. 5th edition. New York: W H Freeman; 2002. Section 17.2, Entry to the Citric Acid Cycle and Metabolism Through It Are Controlled.Available from: https://www.ncbi.nlm.nih.gov/books/NBK22347/
• Nelson DL and Cox MM. Lehninger Principles of Biochemistry. Fourth Edition.
• Montero F. (2011) Photosynthetic Pigments. In: Gargaud M. et al. (eds) Encyclopedia of Astrobiology. Springer, Berlin, Heidelberg
• Lodish H, Berk A, Zipursky SL, et al. Molecular Cell Biology. 4th edition. New York: W. H. Freeman; 2000. Section 16.3, Photosynthetic Stages and Light-Absorbing Pigments.Available from: https://www.ncbi.nlm.nih.gov/books/NBK21598/

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How can we say that 6 calvin cycles are needed to produce 1 glucose molecule why not 2?

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Photosynthetic Cells

Cells get nutrients from their environment, but where do those nutrients come from? Virtually all organic material on Earth has been produced by cells that convert energy from the Sun into energy-containing macromolecules. This process, called photosynthesis, is essential to the global carbon cycle and organisms that conduct photosynthesis represent the lowest level in most food chains (Figure 1).

What Is Photosynthesis? Why Is it Important?

Most living things depend on photosynthetic cells to manufacture the complex organic molecules they require as a source of energy. Photosynthetic cells are quite diverse and include cells found in green plants, phytoplankton, and cyanobacteria. During the process of photosynthesis, cells use carbon dioxide and energy from the Sun to make sugar molecules and oxygen. These sugar molecules are the basis for more complex molecules made by the photosynthetic cell, such as glucose. Then, via respiration processes, cells use oxygen and glucose to synthesize energy-rich carrier molecules, such as ATP, and carbon dioxide is produced as a waste product. Therefore, the synthesis of glucose and its breakdown by cells are opposing processes.

However, photosynthesis doesn't just drive the carbon cycle — it also creates the oxygen necessary for respiring organisms. Interestingly, although green plants contribute much of the oxygen in the air we breathe, phytoplankton and cyanobacteria in the world's oceans are thought to produce between one-third and one-half of atmospheric oxygen on Earth.

What Cells and Organelles Are Involved in Photosynthesis?

Chlorophyll A is the major pigment used in photosynthesis, but there are several types of chlorophyll and numerous other pigments that respond to light, including red, brown, and blue pigments. These other pigments may help channel light energy to chlorophyll A or protect the cell from photo-damage. For example, the photosynthetic protists called dinoflagellates, which are responsible for the "red tides" that often prompt warnings against eating shellfish, contain a variety of light-sensitive pigments, including both chlorophyll and the red pigments responsible for their dramatic coloration.

What Are the Steps of Photosynthesis?

Photosynthesis consists of both light-dependent reactions and light-independent reactions . In plants, the so-called "light" reactions occur within the chloroplast thylakoids, where the aforementioned chlorophyll pigments reside. When light energy reaches the pigment molecules, it energizes the electrons within them, and these electrons are shunted to an electron transport chain in the thylakoid membrane. Every step in the electron transport chain then brings each electron to a lower energy state and harnesses its energy by producing ATP and NADPH. Meanwhile, each chlorophyll molecule replaces its lost electron with an electron from water; this process essentially splits water molecules to produce oxygen (Figure 5).

Once the light reactions have occurred, the light-independent or "dark" reactions take place in the chloroplast stroma. During this process, also known as carbon fixation, energy from the ATP and NADPH molecules generated by the light reactions drives a chemical pathway that uses the carbon in carbon dioxide (from the atmosphere) to build a three-carbon sugar called glyceraldehyde-3-phosphate (G3P). Cells then use G3P to build a wide variety of other sugars (such as glucose) and organic molecules. Many of these interconversions occur outside the chloroplast, following the transport of G3P from the stroma. The products of these reactions are then transported to other parts of the cell, including the mitochondria, where they are broken down to make more energy carrier molecules to satisfy the metabolic demands of the cell. In plants, some sugar molecules are stored as sucrose or starch.

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Chloroplast Function, Definition, and Diagram

Chloroplasts are cellular organelles that are responsible for the process of photosynthesis . They are the reason Earth is a flourishing, green planet that supports diverse life forms.

Chloroplast Definition

A chloroplast is a type of organelle known as a plastid, predominantly found in plant cells and algae. It is the site of photosynthesis, a process where light energy is converted into chemical energy , fueling the organism’s activities.

Discovery and Word Origin

The discovery of chloroplasts dates back to the 19th century, with early microscopic observations made by botanists such as Julius von Sachs. The term “chloroplast” comes from the Greek words “chloros,” meaning green, and “plastes,” meaning formed or molded, highlighting their characteristic green color and intricate structure.

Organisms with Chloroplasts

Chloroplasts are primarily found in plants and algae. They are absent in animals and fungi .

Some types of lichens contain chloroplasts. Chloroplasts are present when the photosynthetic partner is green algae, but not when cyanobacteria are the partner.

A few types of protozoa and animals contain chloroplasts, but they acquired them through a process known as secondary endosymbiosis. This process involves a eukaryotic host cell engulfing a photosynthetic eukaryotic cell. The primary examples of such organisms are:

• Euglenoids: Euglena, a well-known genus in this group, contains chloroplasts derived from green algae. These single-celled organisms live in freshwater and have both plant-like (photosynthetic) and animal-like (motile and heterotrophic) characteristics.
• Dinoflagellates: Dinoflagellates are a group of mainly marine plankton. Some species have chloroplasts originating from green algae and diatoms.
• Apicomplexans: This is a bit of an unusual case. Most apicomplexans, like the malaria parasite Plasmodium, are not photosynthetic. However, they retain a vestigial organelle called the apicoplast, which comes from chloroplasts. The apicoplast does not perform photosynthesis but it is essential for other vital cellular functions.
• Certain Marine Animals: There are rare instances where marine animals incorporate chloroplasts into their cells in a process called kleptoplasty. For example, sea slugs like Elysia chlorotica consume algae and then retain the chloroplasts within their own cells. While the chloroplasts perform photosynthesis for some time, the integration is not permanent, so the sea slugs must regularly consume algae to maintain their photosynthetic capability.

Location and Number in a Cell

Chloroplasts are a cell’s cytoplasm. The number varies greatly, ranging from one large chloroplast in some algae to hundreds in a single leaf cell of a higher plant, depending on the species and environmental conditions.

Structure of a Chloroplast

Chloroplasts have a lens shape in plants, although they have different shapes in algae, like a cup, a net, or a spiral. A typical chloroplast size is 3-10 μm in diameter and 1–3 μm thick. Each chloroplast contains at least three membrane systems: the outer membrane, inner membrane, and thylakoid system. A chloroplast’s structure is complex, comprising several distinct components:

• Outer Membrane: The outer membrane is a semi-permeable barrier that encases the organelle.
• Inner Membrane: The inner membrane is located just inside the outer membrane. It regulates material entry and exit.
• Stroma: The stroma is a fluid-filled space inside the outer and inner membranes containing enzymes, ribosomes, and DNA . The thylakoid system floats within the stroma. The Calvin cycle occurs in the stroma.
• Thylakoid Membrane (Thylakoids): The thylakoid membrane consists of a system of interconnected membranes where the light-dependent reactions of photosynthesis occur. There are two types of thylakoids. Granal thylakoids are the pancake-like stacks, while stromal thylakoids are helical sheets that wrap around the grana.
• Grana: Grana (singular: granum) are stacks of disc-like structures formed by thylakoid membranes. Each granum has between two to a hundred thylakoids, although stacks of 10-20 thylakoids are common.
• Thylakoid Space: The thylakoid space or lumen is the interior of the granum. It contains proteins that drive the electron transport chain in photosynthesis.
• Lamellae: Lamellae are membrane bridges connecting the grana.
• DNA: Chloroplast DNA is packaged into nucleoids in the stroma. Each organelle may contain many nucleoids.
• Ribosomes: The ribosomes in the stroma of a chloroplast are smaller than those in the cell’s cytoplasm. They synthesize some of the chloroplast’s proteins.
• Starch Granules: Most chloroplasts contain starch granules. Starch granules account for up to 15% of a chloroplast’s volume. They accumulate in the stroma and grow in size during the daytime. Some hornworts and algae contain pyrenoids, which are structures that serve as the site of starch accumulation.
• Plastoglobuli: Plastoglobuli (singular: plastoglobulus) are spheres of proteins and lipids. While they occur in all chloroplasts, they are more common in older organelles or ones under oxidative stress.

Functions of Chloroplasts

The primary function of chloroplasts is photosynthesis, comprising two stages: the light-dependent reactions occurring in the thylakoids, and the light-independent Calvin Cycle happening in the stroma. They also play roles in fatty acid synthesis, amino acid synthesis, and the immune response. Chloroplasts also act as sensors for gravity and defense functions.

Pigments in Chloroplasts

Chlorophyll is the pigment responsible for the green color of plants and algae and the key molecule involved in photosynthesis. However, chloroplasts contain several pigments besides chlorophyll, which play crucial roles in photosynthesis and in protecting the cell from damage caused by sunlight. These pigments include:

• Carotenoids: These are yellow, orange, and red pigments that serve multiple functions. They absorb light energy for use in photosynthesis. They also provide photoprotection by dissipating excess light energy that could otherwise damage chlorophyll or interact with oxygen to produce harmful reactive oxygen species. Carotenoids include compounds like beta-carotene and xanthophylls.
• Phycobilins: Found in the chloroplasts of red algae and cyanobacteria, phycobilins are water-soluble pigments that are present in phycobiliproteins. These pigments, which include phycocyanin and phycoerythrin, absorb different wavelengths of light than chlorophyll. They extend the range of light that can be used for photosynthesis.
• Accessory Pigments: These are additional pigments that help in capturing light energy. They transfer the energy to chlorophyll for the photosynthetic process. While not directly involved in the conversion of light energy into chemical energy, they are essential for efficient photosynthesis, especially under low-light conditions or in water, where light penetration is different from that on land.

Comparison with Other Plastids

Chloroplasts are a type of plastid that are distinct from others like chromoplasts (responsible for pigment synthesis and storage) and leucoplasts (involved in storage and biosynthesis of various molecules). Unlike these other plastids, chloroplasts contain the pigment chlorophyll, essential for photosynthesis.

Comparison with Mitochondria

Both mitochondria and chloroplasts are organelles that have their own DNA and likely originated from endosymbiotic events, but they serve different functions. While chloroplasts are the centers of photosynthesis, mitochondria are the powerhouses of the cell, responsible for cellular respiration. Both organelles occur in plant cells.

Theories of Chloroplast Evolution

The prevailing theory of chloroplast evolution is the endosymbiotic theory. It suggests that chloroplasts originated from photosynthetic bacteria living symbiotically inside eukaryotic cells. This theory is supported by the presence of their own DNA, double membrane, and similarities to cyanobacteria.

Interesting Chloroplast Facts

Here are some interesting and useful facts about chloroplasts:

• Dynamic Movement within Cells: Chloroplasts are not static within cells. They move in response to light intensity in a phenomenon known as chloroplast photorelocation movement. In low light, they spread out to maximize light absorption, while in intense light, they align along cell walls to minimize damage from excessive light.
• Role in Gravity Perception: In some plant cells, especially in the root cap, chloroplasts perform gravity sensing. They settle at the bottom of cells, which helps the plant determine its growth direction.
• Environmental Stress Response: Chloroplasts play a critical role in the plant’s response to environmental stresses. They signal to the nucleus to change the expression of certain genes in response to factors like drought, temperature changes, and light stress.
• Chloroplast DNA: The DNA within chloroplasts is circular, similar to bacterial DNA. It encodes some of the essential proteins and enzymes needed for photosynthesis. This DNA is inherited maternally in most plants, meaning it’s passed down from the mother plant to its offspring.
• Secondary Metabolite Synthesis: Chloroplasts play a role in the synthesis of secondary metabolites, which are important for plant defense, such as flavonoids and terpenoids.
• Ancient Origins: Fossil evidence suggests that early plants contained chloroplasts over a billion years ago.
• Size and Shape Variability: The size and shape of chloroplasts varies between different species and even within different tissues of the same plant. Also, not all plant cells contain chloroplasts.
• Chloroplast Genome Reduction: Over evolutionary time, many chloroplast genes have transferred to the nucleus of the host cell, significantly reducing the size of the chloroplast genome.
• Non-Photosynthetic Chloroplasts: Some plant cells contain chloroplasts that do not perform photosynthesis, especially in roots and non-green tissues. These chloroplasts participate in other biochemical pathways, such as amino acid and fatty acid synthesis.
• Alberts, B. (2002). Molecular Biology of the Cell (4th ed.). New York: Garland. ISBN 0-8153-4072-9.
• Burgess, J. (1989). An Introduction to Plant Cell Development . Cambridge: Cambridge University Press. ISBN 0-521-31611-1.
• Campbell, N.A.; Reece, J.B.; et al. (2009). Biology (8th ed.). Benjamin Cummings (Pearson). ISBN 978-0-8053-6844-4.
• Hoober, J.K. (1984). Chloroplasts . New York: Plenum. ISBN 9781461327677.
• Nakayama, T.; Archibald, J.M. (2012). “Evolving a photosynthetic organelle”. BMC Biology . 10 (1): 35. doi: 10.1186/1741-7007-10-35

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Photosynthesis

• Macromolecules
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Description

This multipart animation series explores the process of photosynthesis and the structures that carry it out.

Photosynthesis converts light energy from the sun into chemical energy stored in organic molecules, which are used to build the cells of many producers and ultimately fuel ecosystems. After providing an overview of photosynthesis, these animations zoom inside the cells of a leaf and into a chloroplast to see where and how the reactions of photosynthesis happen. The animations detail both the light reactions and the Calvin cycle, focusing on the flow of energy and the cycling of matter.

This animation series contains seven parts, which can be watched individually or in sequence. The first three parts are appropriate for middle school through college-level students. The remaining parts are appropriate for high school through college-level students; Parts 5 and 6 are recommended for more advanced students. Depending on students’ background, it may be helpful to pause the animations at various points to discuss different steps or structures.

The accompanying “Student Worksheet” incorporates concepts and information from the animations. The animations are also available in a YouTube playlist or as a full-length YouTube video .

The “Resource Google Folder” link directs to a Google Drive folder of resource documents in the Google Docs format. Not all downloadable documents for the resource may be available in this format. The Google Drive folder is set as “View Only”; to save a copy of a document in this folder to your Google Drive, open that document, then select File → “Make a copy.” These documents can be copied, modified, and distributed online following the Terms of Use listed in the “Details” section below, including crediting BioInteractive.  

Student Learning Targets

• Summarize the overall purpose of photosynthesis, as well as its inputs and outputs.
• Describe the structures used to perform photosynthesis in plants. 
• Describe the main components of the light reactions and Calvin cycle, and how they contribute to photosynthesis.  

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ATP, Calvin cycle, chlorophyll, chloroplast, electron transport chain, G3P, light reactions, NADPH, photosystem, rubisco

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The resource is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International license . No rights are granted to use HHMI’s or BioInteractive’s names or logos independent from this Resource or in any derivative works.

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The Process of Photosynthesis in Plants (With Diagram)

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The Process of Photosynthesis in Plants!

Introduction:

Life on earth ultimately depends on energy derived from sun. Photosynthesis is the only process of biological importance that can harvest this energy.

Literally photosynthesis means ‘synthesis using light’. Photosynthetic organisms use solar energy to synthesize carbon compound that cannot be formed without the input of the energy.

Photosynthesis (Photon = Light, Synthesis = Putting together) is an anabolic, endergonic process by which green plant synthesize carbohydrates (initially glucose) requiring carbon dioxide, water, pigments and sunlight. In other words, we can say that photosynthesis is transformation of solar energy/radiant energy/light energy (ultimate source of energy for all living organisms) into chemical energy.

Simple general equation of photo synthesis is as follows:

According to Van Neil and Robert Hill, oxygen liberated during photosynthesis comes from water and not from carbon dioxide.

Thus, the overall correct biochemical reaction for photosynthesis can be written as:

Some photosynthetic bacteria use hydrogen donor other than water. Therefore, photosynthesis is also defined as the anabolic process of manufacture of organic compounds inside the chlorophyll containing cells from carbon dioxide and hydrogen donor with the help of radiant energy.

Significance of Photosynthesis:

1. Photosynthesis is the most important natural process which sustains life on earth.

2. The process of photosynthesis is unique to green and other autotrophic plants. It synthesizes organic food from inorganic raw materials.

3. All animals and heterotrophic plants depend upon the green plants for their organic food, and therefore, the green plants are called producers, while all other organisms are known as consumers.

4. Photosynthesis converts radiant or solar energy into chemical energy. The same gets stored in the organic food as bonds between different atoms. Photosynthetic products provide energy to all organisms to carry out their life activities (all life is bottled sunshine).

5. Coal, petroleum and natural gas are fossil fuels which have been produced by the application of heat and compression on the past plant and animal parts (all formed by photosynthesis) in the deeper layers of the earth. These are extremely important source of energy.

6. All useful plant products are derived from the process of photosynthesis, e.g., timber, rubber, resins, drugs, oils, fibers, etc.

7. It is the only known method by which oxygen is added to the atmosphere to compensate for oxygen being used in the respiration of organisms and burning of organic fuels. Oxygen is important in (a) efficient utilization and complete breakdown of respiratory substrate and (b) formation of ozone in stratosphere that filters out and stops harmful UV radiations in reaching earth.

8. Photosynthesis decreases the concentration of carbon dioxide which is being added to the atmosphere by the respiration of organisms and burning of organic fuels. Higher concentration of carbon dioxide is poisonous to living beings.

9. Productivity of agricultural crops depends upon the rate of photosynthesis. Therefore, scientists are busy in genetically manipulating the crops.

Magnitude of Photosynthesis:

Only 0.2% of light energy falling on earth is utilized by photosynthetic organisms. The total carbon dioxide available to plants for photosynthesis is about 11.2 x 10 14 tonnes. Out of this only 2.2 x 10 13 tonnes are present in the atmosphere @ 0.03%. Oceans contain 11 x 10 14 (110,000 billion) tonnes of carbon dioxide.

About 70 to 80 billion tonnes of carbon dioxide are fixed annually by terrestrial and aquatic autotrophs and it produces near about 1700 million tonnes of dry organic matter. Out of these 10% (170 million tonnes) of dry matter is produced by land plants and rest by ocean (about 90%). This is an estimate by Robinowitch (1951),According to more recent figures given by Ryther and Woodwell (1970) only 1/3 of total global photosynthesis can be attributed to marine plants.

Historical Background:

Functional Relationship between Light and Dark Reactions :

During photosynthesis water is oxidized and carbon dioxide is reduced, but where in the over­all process light energy intervenes to drive the reaction. However, it is possible to show that photo­synthesis consists of a combination of light-requiring reactions (the “light reactions”) and non-light requiring reactions (the “dark reactions”).

It is now clear that tall the reactions for the incorporation of CO 2 into organic materials (i.e., carbohydrate) can occur in the dark (the “dark reactions”). The reactions dependent on light (the “light reactions”) are those in which radiant energy is converted into chemical energy.

According to Arnon, the functional relationship between the “light” and “dark” reactions can be established by examining the requirements of the dark reactions. The “dark reactions” comprise a complex cycle of enzyme-mediated reactions (the Calvin Cycle) which catalyzes the reduction of car­bon dioxide to sugar. This cycle requires reducing power in the form of reduced nicotinamide adenine dinucleotide phosphate (NADPH) and chemical energy in the form of adenosine triphosphate (ATP).

The reduced NADP (NADPH) and ATP are produced by the “light reactions”. It is thus possible to divide a description of photosynthesis into those reactions associated with the Calvin cycle and the fixation of carbon dioxide, and those reactions (i.e., capture of light by pigments, electron transport, photophosphorylation) which are directly driven by light.

Site of Photosynthesis :

Chloroplast (Fig. 6.2) in green plants constitute the photosynthetic apparatus and act as site of photosynthesis. Chloroplasts of higher plants are discoid or ellipsoidal in shape measuring 4 —6 μ in length and 1—2 μ in thickness. It is a double membranous cytoplasmic organelle of eukaryotic green plant cells. The thickness of the two membranes including periplastidial space is approximately 300Å.

Ground substance of chloroplast is filled with a hydrophilic matrix known as stroma. It contains cp-DNA (0.5%), RNA (2—3%), Plastoribosome (70S), enzymes for carbon dioxide assimilation, proteins (50—60%), starch grains and osmophilic droplets, vitamin E and K, Mg, Fe, Mn, P, etc. in traces. In stroma are embedded a number of flattened membranous sacs known as thylakoids. Photosynthetic pigments occur in thylakoid membranes.

Aggregation of thylakoids to form stacks of coin like struc­tures known as granna. A grannum consists near about 20 — 30 thylakoids. Each thylakoid encloses a space known asloculus. The end of disc shape thylakoid is called as margin and the area where the thylakoids membranes are appressed together is called partition.

Some of the granna lamella are connected with thylakoids of other granna by stroma lamella or fret membranes. Thylakoid mem­brane and stroma lamella both are composed of lipid and proteins. In photosynthetic prokaryotes (blue-green algae and Bacteria) chloroplast is absent. Chromatophore is present in photosynthetic bacteria and photosynthetic lamellae in blue-green algae.

Mechanism of Photosynthesis :

Photosynthesis is an oxidation reduction process in which water is oxidized and carbon dioxide is reduced to carbohydrate.

Blackmann (1905) pointed out that the process of photosynthesis consists of two phases:

(1) Light reaction or Light phase or Light-dependent phase or Photochemical phase

(2) Dark reaction or Dark phase or Light independent phase or Biochemical phase.

During light reaction, oxygen is evolved and assimilatory power (ATP and NADPH 2 ) are formed. During dark reaction assimilatory power is utilized to synthesize glucose.

(i) Oxygenic photosynthesis (with evolution of O 2 ) takes place in green eukaryotes and cyanobacteria (blue-green algae).

(ii) An oxygenic photosynthesis (without the evolution of O 2 ) takes place in photosynthetic bacteria.

Photosynthetic Pigments:

Photosynthetic pigments are substances that absorb sunlight and initiate the process of photo­synthesis.

Photosynthetic pigments are grouped into 3 categories:

(i) Chlorophyl l:

These are green coloured most abundant photosynthetic pigments that play a major role during photosynthesis. Major types of chlorophylls are known to exist in plants and photosynthetic bacteria viz., Chlorophyll a, b, c, d and e, Bacteriochlorophyll a, b and g, and Chlorobium chlorophyll (Bacterio viridin).

The structure of chlorophyll was first studied by Wilstatter, Stoll and Fischer in 1912. Chemically a chlorophyll molecule consists of a porphyrin head (15 x 15Å) and phytol tail (20Å). Porphyrin consists of tetrapyrrole rings and central core of Mg. Phytol tail is side chain of hydrocarbon. It is attach to one of the pyrrole ring. This chain helps the chlorophyll molecules to attach with thylakoid membrane.

Out of various types of chlorophyll, chlorophyll a and chlorophyll b are the most important for photosynthetic process. Chlorophyll a is found in all photosynthetic plants except photosynthetic bacteria. For this reason it is designated as Universal Photosynthetic Pigment or Primary Photosynthetic Pigment.

(ii) Carotenoids :

These are yellow, red or orange colour pigments embedded in thylakoid membrane in association with chlorophylls but their amount is less. These are insoluble in water and precursor of Vitamin A. These are of two of types viz., Carotene and Xanthophyll (Carotenol/Xanthol).

Carotenes are pure hydrocarbons, red or orange in colour and their chemical formula is – C 40 H 56 Some of the common carotenes are -α, β, γ and δ carotenes, Phytotene, Neurosporene, Lycopene (Red pigment found in ripe tomato). β—carotene on hydrolysis gives Vitamin A.

Xanthophylls are yellow coloured oxygen containing carotenoids and are most abundant in nature. The ratio of xanthophyll to carotene in nature is 2:1 in young leaves. The most common xanthophyll in green plant is Lutein (C 40 H 56 O 2 ) and it is responsible for yellow colour in autumn foliage. Both carotene and xanthophylls are soluble in organic solvents like chloroform, ethyl ether, carbondisulphide etc.

(iii) Phycobilins (Biliproteins) :

These are water soluble pigments and are abundantly present in algae, and also found in higher plants. There are two important types of phycobilins-Phycoerythrin (Red) and Phycocyanin (Blue). Like chlorophyll, these pigments are open tetrapyrrole but do not contain Mg and Phytol chain.

Nature of Light (Fig. 6.3 ):

The source of light for photosynthesis is sunlight. Sun Light is a form of energy (solar energy) that travels as a stream of tiny particles. Discrete particles present in light are called photons. They carry energy and the energy contained in a photon is termed as quantum. The energy content of a quantum is related to its wave length.

Shorter the wave length, the greater is the energy present in its quantum. Depending upon the wave length electro magnetic spectrum comprises cosmic rays, gamma rays, X-rays,-UV rays, visible spectrum, infra red rays, electric rays and radio waves.

The visible spectrum ranges from 390 nm to 760 nm (3900 – 7600A), however, the plant life is affected by wave length ranging from 300 – 780 nm. Visible spectrum can be resolved into light of different colours i.e., violet (390-430 nm), blue or indigo (430-470 nm), blue green (470-500 nm), green (500 – 580 nm), yellow (580 – 600 nm), orange (600 – 650 nm), orange red (650 – 660 nm) and red (660 – 760 nm). Red light above 700 nm is called far red. Radiation shorter than violet are UV rays (100 – 390 nm). Radiation longer than those of red are called infra red (760 – 10,000 nm).

A ray of light falling upon a leaf behaves in 3 different ways. Part of it is reflected, a part transmitted and a part absorbed. The leaves absorb near about 83% of light, transmit 5% and reflect 12%. From the total absorption, 4% light is absorbed by the chlorophyll. Engelmann (1882) performed an experiment with the freshwater, multicellular filamentous green alga spirogyra.

In a drop of water having numerous aerobic bacteria, the alga was exposed to a narrow beam of light passing through a prism. The bacte­ria after few minutes aggregated more in that re­gions which were exposed to blue and red wave length. It confirms that maximum oxygen evolu­tion takes place in these regions due to high photosynthetic activities.

Absorption Spectrum :

All photosynthetic organisms contain one or more organic pigments capable of absorbing visible radiation which will initiate the photochemical reactions of photosynthesis. When the amount of light absorbed by a pigment is plotted as a function of wave length, we obtain absorption spectrum (Fig. 6.4).

It varies from pigment to pigment. By passing light of specific wave length through a solution of a substance and measuring the fraction absorbed, we obtain the absorption spectrum of that substance. Each type of molecules have a characteristic absorption spectrum, and measuring the absorption spectrum can be useful in identifying some unknown substance isolated from a plant or animal cell.

Action Spectrum :

It represents the extent of response to different wave lengths of light in photosynthesis. It can also be defined as a measure of the process of photosynthesis when a light of different wave lengths is supplied but the intensity is the same. For photochemical reactions involving single pigment, the action spectrum has same general shape as the absorption spectrum of that pigment, otherwise both are quite distinct (Fig. 6.5).

Quantum Requirement and Quantum Yield:

The solar light comes to earth in the form of small packets of energy known as photons. The energy associated with each photon is called Quantum. Thus, requirement of solar light by a plant is measured in terms of number of photons or quanta.

The number of photons or quanta required by a plant or leaf to release one molecule of oxygen during photosynthesis is called quantum requirement. It has been observed that in most of the cases the quantum requirement is 8.

It means that 8 photons or quantum’s are required to release one molecule of oxygen. The number of oxygen molecules released per photon of light during photosynthesis is called Quantum yield. If the quantum requirement is 8 then quantum yield will be 0.125 (1/8).

Photosynthetic Unit or Quantasome:

It is defined as the smallest group of collaborating pigment molecules necessary to affect a photochemical act i.e., absorption and migration of a light quantum to trapping centre where it promotes the release of an electron.

Emmerson and Arnold (1932) on the basis of certain experiments assumed that about 250 chlorophyll molecules are required to fix one molecule of carbon dioxide in photosynthesis. This number of chlorophyll molecules was called the chlorophyll unit but the name was subsequently changed to photosynthetic unit and later it was designated as Quantasome by Park and Biggins (1964).

The size of a quantasome is about 18 x 16 x l0nm and found in the membrane of thylakoids. Each quantasome consists of 200 – 240 chlorophyll (160 Chlorophyll a and 70 – 80 Chlorophyll b), 48 carotenoids, 46 quinone, 116 phospholipids, 144 diagalactosyl diglyceride, 346 monogalactosyl diglyceride, 48 sulpholipids, some sterols and special chlorophyll molecules (P 680 and P 700 ).

‘P’ is pigment, 680 and 700 denotes the wave length of light these molecule absorb. Peso and P 700 constitute the reaction centre or photo centre. Other accessory pigments and chlorophyll molecules are light gatherers or antenna molecules. It capture solar energy and transfer it to the reaction centre by resonance transfer or inductive resonance.

Photoluminescence :

It is the phenomenon of re-radiation of absorbed energy. It is of two types:

(1) Fluorescence and

(2) Phosphorescence.

The normal state of the molecule is called as ground state or singlet state. When an electron of a molecule absorbs a quantum of light it is raised to a higher level of energy a state called Excited Second Singlet State. From first singlet state excited electron may return to the ground state either losing its extra energy in the form of heat or by losing energy in the form of radiant energy. The later process is called fluorescence. The substance which can emit back the absorbed radiations is called fluorescent substance. All photosynthetic pigments have the property of fluorescence.

The excited molecule also losses its electronic excitation energy by internal conversion and comes to another excited state called triplet state. From this triplet state excited molecule may return to ground state in three ways-by losing its extra energy in the form of heat, by losing extra energy in the form of radiant energy is called phosphorescence. The electron carrying extra energy may be expelled from the molecule and is consumed in some other chemical reactions and a fresh normal electron returns to the molecule. This mechanism happens in chlorophyll a (Universal Photosynthetic Pigment).

Emerson Red Drop Effect and Enhancement Effect :

R. Emerson and Lewis (1943) while determining the quantum yield of photosynthesis in Chlorella by using monochromatic light of different wave lengths noticed a sharp decrease in quantum yield at wave length greater than 680 mμ.This decrease in quantum yield took place in the far red part of the spectrum i.e., the curve shows quantum yield drops dramatically in the region above 680 nm (Red region). This decline in photosynthesis is called Red drop effect (Emerson’s first experiment).

Emerson and his co-workers (1957) found that the inefficient far red light in Chlorella beyond 680nm could be made fully efficient if supplemented with light of short wave length. The quantum yield from the two combined beams was found to be greater than the effect of both beams when used separately. This enhancement of photosynthesis is called Emerson Enhancement Effect (Emerson’s second experiment) (Fig. 6.6).

Rate of oxygen evolution in combined beam – Rate of oxygen evolution in red beam/Rate of oxygen evolution in far red beam

E = Emerson effect.

Light Trapping Centres (PSI & PSII) :

The discovery of red drop effect and the Emerson’s enhancement effect concluded in a new concept about the role played bychlorophyll-a and accessary pigments in photosynthesis that photo­synthesis involves two distinct photochemical processes. These processes are associated with two groups of photosynthetic pigments called as Pigment system I (Photoact I or Photosystem I) and Pigment system II (Photoact II or Photosystem II).

Each pigment system consists of a central core complex and light harvesting complex (LHC). LHC comprises antenna pigments associated with proteins (viz.., antenna complex). Their main function is to harvest light energy and transfer it to their respective reaction centre. The core complex consists of reaction centre associated with proteins and also electon donors and acceptors.

Wave length of light shorter than 680 nm affect both the pigment systems while wave length longer than 680 nm affect only pigment system I. PSI is found in thylakoid membrane and stroma lamella. It contains pigments chlorophyll a 660, chlorophyll a 670, chlorophyll a 680, chlorophyll a 690, chlorophyll a 700. Chlorophyll a 700 or P 700 is the reaction centre of PS I. PS II is found in thylakoid membrane and it contains pigments as chlorophyll b 650, chlorophyll a 660, chlorophyll a 670, chlorophyll a 678, chlorophyll a 680 – 690 and phycobillins.

P 680-690 is the reaction centre of PS II. Chlorophyll a content is more in PS I than PS II. Carotenoids are present both in PS II and PS I. PS I is associated with both cyclic and non-cyclic photophosphorylation, but PS II is associated with only non-cyclic photophosphorylation.

Both the pigment systems are believed to be inter-connected by a third integral protein complex called cytochrome b – f complex. The other intermediate components of electron transport chain viz., PQ (plasto quinone) and PC (plastocyanin) act as mobile electron carriers between two pigment systems. PS I is active in both red and far red light and PS II is inactive in far red light (Fig. 6.7).

Evidence in Support of Two Phases of Photosynthesis:

1. Physical Separation of Chloroplast into Granna and Stroma Fraction:

It is now possible to separate granna and stroma fraction of chloroplast. If light is given to granna fraction in the presence of suitable hydrogen acceptor and in complete absence of carbon dioxide then assimilatory power, ATP and NADPH 2 , are produced. If these assimilatory powers are given to stroma fraction in the presence of carbon dioxide and absence of light then carbohydrate is synthesized.

2. Temperature Coefficient (Q 10 ):

Q 10 is the ratio of the rate of reaction at a given temperature and a temperature 10°C lower. Q 10 value of photosynthesis is found to be two or three (for dark reaction) when photosynthesis is fast, but Q 10 is one (for light reaction) when photosynthesis is slow.

3. Evidence from Intermittent Light:

Warburg observed that when intermittent light (flashes of light) of about 1/16 seconds were given to green algae (Chlorella vulgaris and Scenedesmus obliquus), the photosynthetic yield per second was higher as compared to the continuous supply of same intensity of light. This confirms that one phase of photosynthesis is independent of light.

4. Evidence from Carbon dioxide in Dark:

It comes from tracer technique by the use of heavy carbon in carbon dioxide (C 14 O 2 ). The leaves which were first exposed to light have been found to reduce carbon dioxide in the dark It indicates that carbon dioxide is reduced to carbohydrate in dark and it is purely a biochemical phase.

I. Light Reaction (Photochemical Phase):

Light Reaction:

Light reaction or photochemical reaction takes place in thylakoid membrane or granum and it is completely dependent upon the light. The raw materials for this reactions are pigments, water and sunlight.

It can be discussed in the following three steps:

1. Excitation of chlorophyll

2. Photolysis of water

3. Photophosphorylation

1. Excitation of Chlorophyll:

It is the first step of light reaction. When P 680 or P 700 (special type of chlorophyll a) of two pigment systems receives quantum of light then it becomes excited and releases electrons.

2. Photolysis of Water and Oxygen Evolution (Hill Reaction):

Before 1930 it was thought that the oxygen released during photosynthesis comes from carbon dioxide. But for the first time Van Neil discovered that the source of oxygen evolution is not carbon dioxide but H 2 O. In his experiment Neil used green sulphur bacteria which do not release oxygen during photosynthesis. They release sulphur. These bacteria require H 2 S in place of H 2 O.

The idea of Van Neil was supported by R. Hill. Hill observed that the chloroplasts extracted from leaves of Stellaria media and Lamium album when suspended in a test tube containing suitable electron acceptors (Potassium feroxalate or Potassium fericyanide), Oxygen evolution took place due to photochemical splitting of water.

The splitting of water during photosynthesis is called Photolysis of water. Mn, Ca, and CI ions play prominent role in the photolysis of water. This reaction is also known as Hill reaction. To release one molecule of oxygen, two molecules of water are required.

The evolution of oxygen from water was also confirmed by Ruben, Randall, Hassid and Kamen (1941) using heavy isotope (O 18 ) in green alga Chlorella. When the photosynthesis is allowed to proceed with H 2 O 18 and normal CO 2 , the evolved oxygen contains heavy isotope. If photosynthesis is allowed to proceed in presence of CO 2 18 and normal water then heavy oxygen is not evolved.

Thus the fate of different molecules can be summarized as follows:

3. Photophosphorylation:

Synthesis of ATP from ADP and inorganic phosphate (pi) in presence of light in chloroplast is known as photophosphorylation. It was discovered by Arnon et al (1954).

Photophosphorylation is of two types.

(a) Cyclic photophosphorylation

(b) Non-cyclic photophosphorylation.

(a) Cyclic Photophosphorylation (Fig. 6.8) :

It is a process of photophosphorylation in which an electron expelled by the excited photo Centre (PSI) is returned to it after passing through a series of electron carriers. It occurs under conditions of low light intensity, wavelength longer than 680 nm and when CO 2 fixation is inhibited. Absence of CO 2 fixation results in non requirement of electrons as NADPH 2 is not being oxidized to NADP + . Cyclic photophosphorylation is performed by photosystem I only. Its photo Centre P 700 extrudes an electron with a gain of 23 kcal/mole of energy after absorbing a photon of light (hv).

After losing the electron the photo Centre becomes oxidized. The expelled electron passes through a series of carriers including X (a special chlorophyll molecule), FeS, ferredoxin, plastoquinone, cytochrome b- f complex and plastocyanin before returning to photo Centre. While passing between ferredoxin and plastoquinone and/or over the cytochrome complex, the electron loses sufficient energy to form ATP from ADP and inorganic phosphate.

Halobacteria or halophile bacteria also perform photophosphorylation but ATP thus produced is not used in synthesis of food. These bacteria possess purple pigment bacteriorhodopsin attached to plasma membrane. As light falls on the pigment, it creates a proton pump which is used in ATP synthesis.

(b) Noncyclic Photophosphorylation (Z-Scheme) (Fig. 6.9) :

It is the normal process of photophosphorylation in which the electron expelled by the excited photo Centre (reaction centre) does not return to it. Non-cyclic photophosphorylation is carried out in collaboration of both photo system I and II. (Fig. 6.9). Electron released during photolysis of water is picked up by reaction centre of PS-II, called P 680 . The same is extruded out when the reaction centre absorbs light energy (hv). The extruded electron has an energy equivalent to 23 kcal/mole.

It passes through a series of electron carriers— Phaeophytin, PQ, cytochrome b- f complex and plastocyanin. While passing over cytochrome complex, the electron loses sufficient energy for the synthesis of ATP. The electron is handed over to reaction centre P 700 of PS-I by plastocyanin. P 700 extrudes the electron after absorbing light energy.

The extruded electron passes through FRS ferredoxin, and NADP -reductase which combines it with NADP + for becoming reduced through H+ releasing during photolysis to form NADPH 2 . ATP synthesis is not direct. The energy released by electron is actually used for pumping H + ions across the thylakoid membrane. It creates a proton gradient. This gradient triggers the coupling factor to synthesize ATP from ADP and inorganic phosphate (Pi).

Chemiosmotic Hypothesis:

How actually ATP is synthesized in the chloroplast?

The chemiosmotic hypothesis has been put forward by Peter Mitchell (1961) to explain the mechanism. Like in respiration, in photosynthesis too, ATP synthesis is linked to development of a proton gradient across a membrane. This time these are membranes of the thylakoid. There is one difference though, here the proton accumulation is towards the inside of the membrane, i.e., in the lumen. In respiration, protons accumulate in the inter-membrane space of the mitochondria when electrons move through the ETS.

Let us understand what causes the proton gradient across the membrane. We need to consider again the processes that take place during the activation of electrons and their transport to determine the steps that cause a proton gradient to develop (Figure 6.9).

(b) As electrons move through the photosystems, protons are transported across the membrane. This happens because the primary accepter of electron which is located towards the outer side of the membrane transfers its electron not to an electron carrier but to an H carrier. Hence, this molecule removes a proton from the stroma while transporting an electron. When this molecule passes on its electron to the electron carrier on the inner side of the membrane, the proton is released into the inner side or the lumen side of the membrane.

(c) The NADP reductase enzyme is located on the stroma side of the membrane. Along with electrons that come from the acceptor of electrons of PS I, protons are necessary for the reduction of NADP + to NADPH+ H + .These protons are also removed from the stroma.

Hence, within the chloroplast, protons in the stroma decrease in number, while in the lumen there is accumulation of protons. This creates a proton gradient across the thylakoid membrane as well as a measurable decrease in pH in the lumen.

Why are we so interested in the proton gradient?

This gradient is important because it is the breakdown of this gradient that leads to release of energy. The gradient is broken down due to the movement of protons across the membrane to the stroma through the trans membrane channel of the F 0 of the ATPase. The ATPase enzyme consists of two parts: one called the F 0 is embedded in the membrane and forms a trans-membrane channel that carries out facilitated diffusion of protons across the membrane. The other portion is called F 1 and protrudes on the outer surface of the thylakoid membrane on the side that faces the stroma.

The break down of the gradient provides enough energy to cause a conformational change in the F 1 particle of the ATPase, which makes the enzyme synthesis several molecules of energy-packed ATP. Chemiosmosis requires a membrane, a proton pump, a proton gradient and ATPase. Energy is used to pump protons across a membrane, to create a gradient or a high concentration of protons within the thylakoid lumen.

ATPase has a channel that allows diffusion of protons back across the membrane; this releases enough energy to activate ATPase enzyme that catalyzes the formation of ATP. Along with the NADPH produced by the movement of electrons, the ATP will be used immediately in the biosynthetic reaction taking place in the stroma, responsible for fixing CO 2 , and synthesis of sugars.

Where are the ATP and NADPH Used ?

We have seen that the products of light reaction are ATP, NADPH and O 2 . Of these O 2 diffuses out of the chloroplast while ATP and NADPH are used to drive the processes leading to the synthesis of food, more accurately, sugars. This is the biosynthetic phase of photosynthesis.

This process does not directly depend on the presence of light but is dependent on the products of the light reaction, i.e., ATP and NADPH, besides CO 2 and H 2 O. You may wonder how this could be verified; it is simple: immediately after light becomes unavailable the biosynthetic process continues for some time, and then stops. If then, light is made available, the synthesis starts again.

Can we, hence, say that calling the biosynthetic phase as the dark reaction is a misnomer?

II. Dark Reaction (Biosynthetic Phase)-The Second Phase of Photosynthesis:

The pathway by which all photosynthetic eukaryotic organisms ultimately incorporate CO 2 into carbohydrate is known as carbon fixation or photosynthetic carbon reduction (PCR.) cycle or dark reactions. The dark reactions are sensitive to temperature changes, but are independent of light hence it is called dark reaction, however it depends upon the products of light reaction of photosynthesis, i.e., NADPH 2 and ATP.

The carbon dioxide fixation takes place in the stroma of chloroplasts because it has enzymes essential for fixation of CO 2 and synthesis of sugar. Dark reaction is the pathway by which CO 2 is reduced to sugar. Since CO 2 is an energy poor compound; its conversion to an energy-rich carbohydrate involves a sizable jump up the energy ladder. This is accomplished through a series of complex steps involving small bits of energy.

The CO 2 assimilation takes place both in light and darkness when the substrates NADPH 2 and ATP are available. Because of the need for NADPH 2 as a reductant and ATP as energy equivalent, CO 2 fixation is closely linked to the light reactions. During evolution three different ecological variants have evolved with different CO 2 incorporation mechanism: C 3 , C 4 and CAM plants.

Calvin or C 3 Cycle or PCR (Photosynthetic Carbon Reduction Cycle):

It is the basic mechanism by which CO 2 is fixed (reduced) to form carbohydrates. It was proposed by Melvin Calvin. Calvin along with A.A. Benson, J. Bassham used radioactive isotope of carbon (C 14 ) in Chlorella pyrenoidosa and Scenedesmus oblique’s to determine the sequences of dark reaction. For this work Calvin was awarded Nobel prize in 1961. To synthesize one glucose molecule Calvin cycle requires 6CO 2 , 18 ATP and 12 NADPH 2 .

Calvin cycle completes in 4 major phases:

1. Carboxylation phase

2. Reductive phase

3. Glycolytic reversal phase (sugar formation phase)

4. Regeneration phase

1. Carboxylation phase:

CO 2 enters the leaf through stomata. In mesophyll cells, CO 2 combines with a phosphorylated 5-carbon sugar, called Ribulose bisphosphate (or RuBP). This reaction is catalyzed by an enzyme, called RUBISCO. The reaction results in the formation of a temporary 6 carbon compound (2-carboxy 3-keto 1,5-biphosphorbitol) Which breaks down into two molecules of 3-phosphoglyceric acid (PGA) and it is the first stable product of dark reaction (C 3 Cycle).

2. Reductive Phase:

The PGA molecules are now phosphorylated by ATP molecule and reduced by NADPH 2 (product of light reaction known as assimilatory power) to form 3-phospho-glyceraldehyde (PGAL).

3. Glycolytic Reversal (Formation of sugar) Phase:

Out of two mols of 3-phosphoglyceraldehyde one mol is converted to its isomer 3-dihydroxyacetone phosphate.

4. Regeneration Phase:

Regeneration of Ribulose-5-phosphate (Also known as Reductive Pentose Phosphate Pathway) takes place through number of biochemical steps.

Summary of Photosynthesis:

(A) Light Reaction takes place in thylakoid membrane or granum

(B) Dark Reaction (C 3 cycle) takes place in stroma of chloroplast.

C 4 Cycle (HSK Pathway or Hatch Slack and Kortschak Cycle) :

C 4 cycle may also be referred as the di-carboxylic acid cycle or the β-carboxylation pathway or Hatch and Slack cycle or cooperative photosynthesis (Karpilov, 1970). For a long time, C 3 cycle was considered to be the only photosynthetic pathway for reduction of CO 2 into carbohydrates. Kortschak, Hartt and Burr (1965) reported that rapidly photosynthesizing sugarcane leaves produced a 4-C compound like aspartic acid and malic acid as a result of CO 2 – fixation.

It was later supported by M. D. Hatch and C. R. Slack (1966) and they reported that a 4-C compound oxaloacetic acid (OAA) is the first stable product in CO 2 reduction process. This pathway was first reported in members of family Poaceae like sugarcane, maize, sorghum, etc. (tropical grasses), but later on the other subtropical plant like Atriplex spongiosa (Salt bush), Dititaria samguinolis, Cyperus rotundus, Amaranthus etc. So, the cycle has been reported not only in the members of Graminae but also among certain members of Cyperaceae and certain dicots.

Structural Peculiarities of C 4 Plants (Kranz Anatomy ):

C 4 plants have a characteristic leaf anatomy called Kranz anatomy (Wreath anatomy – German meaning ring or Helo anatomy). The vascular bundles in C 4 plant leaves are surrounded by a layer of bundle sheath cells that contain large number of chloroplast. Dimorphic (two morphologically distinct type) chloroplasts occur in C 4 plants (Fig. 6.13).

In Mesopyll cell:

(i) Chloroplast is small in size

(ii) Well developed grannum and less developed stroma.

(iii) Both PS-II and PS-I are present.

(iv) Non cyclic photophosphorylation takes place.

(v) ATP and NADPH 2 produces.

(vi) Stroma carries PEPCO but absence of RuBisCO.

(vii) CO 2 acceptor is PEPA (3C) but absence of RUBP

(viii) First stable product OAA (4C) produces.

In Bundle sheath Cell:

(i) Size of chloroplast is large

(ii) Stroma is more developed but granna is poorly developed.

(iii) Only PS-I present but absence of PS-II

(iv) Non Cyclic photophosphorylation does not takes place.

(v) Stroma carries RuBisCO but absence of PEPCO.

(vi) CO 2 acceptor RUBP (5c) is present but absence of PEPA (3C)

(vii) C3-cycle takes place and glucose synthesies.

(viii) To carry out C3-cycle both ATP and NADPH2 comes from mesophyll cell chloroplast.

Carbon dioxide from atmosphere is accepted by Phosphoenol pyruvic acid (PEPA) present in stroma of mesophyll cell chloroplast and it converts to oxaloacetic acid (OAA) in the presence of enzyme PEPCO (Phosphoenolpyruvate carboxylase). This 4-C acid (OAA) enters into the chloroplast of bundle sheath cell and there it undergoes oxidative decarboxylation yielding pyruvic acid (3C) and CO 2 .

The carbon dioxide released in bundle sheath cell reacts with RuBP (Ribulose 1, 5 bisphosphate) in presence of RUBISCO and carry out Calvin cycle to synthesize glucose. Pyruvic acid enters mesophyll cells and regenerates PEPA. In C 4 cycle two carboxylation reactions take place.

Reactions taking place in mesophyll cells are stated below: (1 st carboxylation)

C 4 plants are better photosynthesizes. There is no photorespiration in these plants. To synthesize one glucose molecule it requires 30 ATP and 12 NADPH 2 .

Significance of C 4 Cycle:

1. C 4 plants have greater rate of carbon dioxide assimilation than C 3 plants because PEPCO has great affinity for CO 2 and it shows no photorespiration resulting in higher production of dry matter.

2. C 4 plants are better adapted to environmental stress than C 3 plants.

3. Carbon dioxide fixation by C 4 plants requires more ATP than C 3 plants for conversion of pyruvic acid to PEPA.

4. Carbon dioxide acceptor in C 4 plant is PEPA and key enzyme is PEPCO.

5. They can very well grow in saline soils because of presence of C 4 organic acid.

Crassulacean Acid Metabolism (CAM Pathway):

It is a mechanism of photosynthesis which occurs in succulents and some other plants of dry habitats where the stomata remain closed during the daytime and open only at night. The process of photosynthesis is similar to that of C 4 plants but instead of spatial separation of initial PEPcase fixation and final Rubisco fixation of CO 2 , the two steps occur in the same cells (in the stroma of mesophyll chloroplasts) but at different times, night and day, e.g., Sedum, Kalanchoe, Opuntia, Pineapple (Fig. 6.13). (CAM was for the first time studied and reported by Ting (1971).

Characteristics of CAM Plants:

1. Stomatal movement is scoto-active.

2. Presence of monomorphic chloroplast.

3. Stroma of chloroplast carries both PEPCO and RUBISCO.

4. Absence of Kranz anatomy.

5. It is more similar to C 4 plants than C 3 plants.

6. In these plants pH decreases during night and increases during day time.

Mechanism of CAM Pathway :

PHASE I. During night:

Stomata of Crassulacean plants remain open at night. Carbon dioxide is absorbed from outside. With the help of Phosphoenol pyruvate carboxylase (PEPCO) enzyme the CO 2 is immediately fixed, and here the acceptor molecule is Phosphoenol pyruvate (PEP).

Malic acid is the end product of dark fixation of CO 2 . It is stored inside cell vacuole.

During day time the stomata in Crassulacean plants remain closed to check transpiration, but photosynthesis does take place in the presence of sun light. Malic acid moves out of the cell vacuoles. It is de-carboxylated with the help of malic enzyme. Pyruvate is produced. It is metabolized.

The CO 2 thus released is again fixed through Calvin Cycle with the help of RUBP and RUBISCO. This is a unique feature of these succulent plants where they photosynthesis without wasting much of water. They perform acidification or dark fixation of CO 2 during night and de-acidification during day time to release carbon dioxide for actual photosynthesis.

Ecological Significance of CAM P lants:

These plants are ecologically significant because they can reduce rate of transpiration during day time, and are well adapted to dry and hot habitats.

1. The stomata remain closed during the day and open at night when water loss is little due to prevailing low temperature.

2. CAM plants have parenchyma cells, which are large and vacuolated. These vacuoles are used for storing malic and other acids in large amounts.

3. CAM plants increase their water-use efficiency, and secondly through its enzyme PEP carboxylase, they are adapted to extreme hot climates.

4. CAM plants can also obtain a CO 2 compensation point of zero at night and in this way accomplish a steeper gradient for CO 2 uptake compared to C 3 plants.

5. They lack a real photosynthesis during daytime and the growth rate is far lower than in all other plants (with the exception of pineapple).

Photorespiration or C 2 Cycle or Glycolate Cycle or Photosynthetic Carbon Oxidation Cycle:

Photorespiration is the light dependent process of oxygenation of RUBP (Ribulose bi-phosphate) and release of carbon dioxide by photosynthetic organs of the plant. Otherwise, as we know, photosynthetic organs release oxygen and not CO 2 under normal situation.

Occurrence of photorespiration in a plant can be demonstrated by:

(i) Decrease in the rate of net photosynthesis when oxygen concentration is increased from 2-3 to 21%.

(ii) Sudden increased evolution of CO 2 when an illuminated green plant is transferred to dark.

Photorespiration is initiated under high O 2 and low CO 2 and intense light around the photosynthesizing plant. Photorespiration was discovered by Dicker and Tio (1959), while the term “Photorespiration” was coined by Krotkov (1963). Photorespiration should not be confused with photo- oxidation. While the former is a normal process in some green plants, the latter is an abnormal and injurious process occurring in extremely intense light resulting in destruction of cellular components, cells and tissues.

On the basis of photorespiration, plants can be divided into two groups:

(i) Plants with photorespiration (temperate plants) and plants without photorespiration (tropical plants).

Site of Photorespiration :

Photorespiration involves three cell organelles, viz., chloroplast, peroxisome and mitochondria for its completion. Peroxisome, the actual site of photorespiration, contains enzymes like glycolate oxydase, glutamate glyoxalate aminotransferase, peroxidase and catalase enzymes.

Mechanism of Photorespiration:

We know that the enzyme RUBISCO (Ribulose biphosphate carboxylase oxygenase) catalyzes the carboxylation reaction, where CO 2 combines with RuBP for calvin cycle (dark reaction of photosynthesis) to initiate. But this enzyme RUBISCO, under intense light conditions, has the ability to catalyse the combination of O 2 with RuPB, a process called oxygenation.

In other words the enzyme RUBISCO can catalyse both carboxylation as well as oxygenation reactions in green plants under different conditions of light and O 2 /CO 2 ratio. Respiration that is initiated in chloroplasts under light conditions is called photorespiration. This occurs essentially because of the fact that the active site of the enzyme RUBISCO is the same for both carboxylation and oxygenation (Fig. 6.16).

The oxygenation of RuBP in the presence of O 2 is the first reaction of photorespiration, which leads to the formation of one molecule of phosphoglycolate, a 2 carbon compound and one molecule of phosphoglyceric acid (PGA). While the PGA is used up in the Calvin cycle, the phosphoglycolate is dephosphorylated to form glycolate in the chloroplast (Fig. 6.16).

From the chloroplast, the glycolate is diffused to peroxisome, where it is oxidised to glyoxylate. In the peroxisome, the glyoxylate is used to form the amino acid, glycine. Glycine enters mitochondria where two molecules of glycine (4 carbons) give rise to one molecule of serine (3 carbon) and one CO 2 (one carbon).

The serine is taken up by the peroxisome, and through a series of reactions, is converted to glycerate. The glycerate leaves the peroxisome and enters the chloroplast, where it is phosphorylated to form PGA. The PGA molecule enters the calvin cycle to make carbohydrates, but one CO 2 molecule released in mitochondria during photorespiration has to be re-fixed.

In other words, 75% of the carbon lost by oxygenation of RuBP is recovered, and 25% is lost as release of one molecule of CO 2 . Because of the features described above, photorespiration is also called photosynthetic carbon oxidation cycle.

Minimization of Photorespiration (C4 and CAM Plants):

Since photorespiration requires additional energy from the light reactions of photosynthesis, some plants have mechanisms to reduce uptake of molecular oxygen by Rubisco. They increase the concentration of CO 2 in the leaves so that Rubisco is less likely to produce glycolate through reaction with O 2 .

C 4 plants capture carbon dioxide in cells of their mesophyll (using an enzyme called PEP carboxylase), and they release it to the bundle sheath cells (site of carbon dioxide fixation by Rubisco) where oxygen concentration is low.

The enzyme PEP carboxylase is also found in other plants such as cacti and succulents who use a mechanism called Crassulacean acid metabolism or CAM in which PEP carboxylase put aside carbon at night and releases it to the photosynthesizing cells during the day.

This provides a mechanism for reducing high rates of water loss (transpiration) by stomata during the day. This ability to avoid photorespiration makes these plants more hardy than other plants in dry conditions where stomata are closed and oxygen concentration rises.

Factors Affecting Photosynthesis:

Photosynthesis is affected by both environmental and genetic (internal) factors. The environmental factors are light, CO 2 , temperature, soil, water, nutrients etc. Internal or genetic factors are all related with leaf and include protoplasmic factors, chlorophyll contents, structure of leaf, accumulation of end product etc.

Some of the important factors are discussed below:

1. Concept of Cardinal Values :

The metabolic processes are influenced by a number of factors of the environment. The rate of a metabolic process is controlled by the magnitude of each factor. Sachs (1860) recognized three critical values, the cardinal values or points of the magnitude of each factor. These are minimum, optimum and maximum. The minimum cardinal value is that magnitudes of a factor below which the metabolic process cannot proceed.

Optimum value is the one at which the metabolic process proceeds at its highest rate. Maximum is that magnitude of a factor beyond which the process stops. At magnitudes below and above the optimum, the rate of a metabolic process declines till minimum and maximum values are attained.

2. Principle of Limiting Factors :

Liebig (1843) proposed law of minimum which states that the rate of a process is limited by the pace (rapidity) of the slowest factor. However, it was later on modified by Blackman (1905) who formulated the “principle of limiting factors”. It states that when a metabolic process is conditioned as to its rapidity by a number of separate factors, the rate of the process is limited by the pace (rapidity) of the slowest factor. This principle is also known as “Blackman’s Law of Limiting Factors.”

A metabolic process is conditioned by a number of factors. The slowest factor or the limiting factor is the one whose increase in magnitude is directly responsible for an increase in the rate of the metabolic process (here photosynthesis).

To explain it further, say at a given time, only the factor that is most limiting among all will determine the rate of photosynthesis. For example, if CO 2 is available in plenty but light is limiting due to cloudy weather, the rate of photosynthesis under such a situation will be controlled by the light. Furthermore, if both CO 2 and light are limiting, then the factor which is the most limiting of the two, will control the rate of photosynthesis.

Blackman (1905) studied the effect of CO 2 concentration, light intensity and temperature on rate of photosynthesis. All other factors were maintained in optimum concentration. Initially the photosynthetic material was kept at 20°C in an environment having 0.01% CO 2 . When no light was provided to photosynthetic material, it did not perform photosynthesis. Instead, it evolved CO 2 and absorbed O 2 from its environment. He provided light of low intensity (say 150 foot candles) and found photosynthesis to occur.

When light intensity was increased (say 800 foot candles), the rate of photosynthesis increased initially but soon it leveled off. The rate of photosynthesis could be further enhanced only on the increase in availability of CO 2 . Thus, initially light intensity was limiting the rate of photosynthesis.

When sufficient light became available, CO 2 became limiting factor (Fig. 6.17). When both are provided in sufficient quantity, the rate of photosynthesis rose initially but again reached a peak. It could not be increased further. At this time, it was found that increase in temperature could raise the rate of photosynthesis up to 35°C. Further increase was not possible. At this stage, some other factor became limiting. Therefore, at one time only one factor limits the rate of physiological process.

Objections have been raised to the validity of Blackman’s law of limiting factors. For instance:

(i) It has been observed that the rate of a process cannot be increased indefinitely by increasing the availability of all the known factors;

(ii) The principle of Blackman is not operative for toxic chemicals or inhibitors and

(iii) Some workers have shown that the pace of reaction can be controlled simultaneously by two or more factors.

3. External Factors:

The environmental factors which can affect the rate of photosynthesis are carbon dioxide, light, temperature, water, oxygen, minerals, pollutants and inhibitors.

1. Effect of Carbon dioxide:

Being one of the raw materials, carbon dioxide concentration has great effect on the rate of photosynthesis. The atmosphere normally contains 0.03 to 0.04 per cent by volume of carbon dioxide. It has been experimentally proved that an increase in carbon dioxide content of the air up to about one per cent will produce a corresponding increase in photosynthesis provided the intensity of light is also increased.

2. Effect of Light:

The ultimate source of light for photosynthesis in green plants is solar radiation, which moves in the form of electromagnetic waves. Out of the total solar energy reaching to the earth, about 2% is used in photosynthesis and about 10% is used in other metabolic activities. Light varies in intensity, quality (wavelength) and duration.

The effect of light on photosynthesis can be studied under following three headings:

(i) Intensity of Light:

The total light perceived by a plant depends on its general form (viz., height of plant and size of leaves, etc.) and arrangement of leaves. Of the total light falling on a leaf, about 80% is absorbed, 10% is reflected and 10% is transmitted. Intensity of light can be measured by lux meter.

Effect of light intensity varies from plant to plant, e.g., more in heliophytes (sun loving plants) and less in sciophytes (shade loving plants). For a complete plant, rate of photosynthesis increases with increase in light intensity, except under very high light intensity where phenomenon of Solarization’ occurs, (i.e., photo-oxidation of different cellular components including chlorophyll). It also affects the opening and closing of stomata thereby affecting the gaseous exchange. The value of light saturation at which further increase is not accompanied by an increase in CO 2 uptake is called light saturation point.

(ii) Quality of Light:

Photosynthetic pigments absorb visible part of the radiation i.e., 380 mμ, to 760 mμ. For example, chlorophyll absorbs blue and red light. Usually plants show high rate of photosynthesis in the blue and red light. Maximum photosynthesis has been observed in red light than in blue light followed by yellow light (monochromatic light). The green light has minimum effect. The rate of photosynthesis is maximum in white light or sunlight (polychromatic light). On the other hand, red algae shows maximum photosynthesis in green light and brown algae in blue light.

(iii) Duration of Light:

Longer duration of light period favours photosynthesis. Generally, if the plants get 10 to 12 hrs. of light per day it favours good photosynthesis. Plants can actively exhibit photosynthesis under continuous light without being damaged. Rate of photosynthesis is independent of duration of light.

3. Effect of Temperature:

The rate of photosynthesis markedly increases with an increase in temperature provided other factors such as CO 2 and light are not limiting. The temperature affects the velocity of enzyme controlled reactions in the dark stage. When the intensity of light is low, the reaction is limited by the small quantities of reduced coenzymes available so that any increase in temperature has little effect on the overall rate of photosynthesis.

At high light intensities, it is the enzyme-controlled dark stage which controls the rate of photosynthesis and there the Q 10 = 2. If the temperature is greater than about 30°C, the rate of photosynthesis abruptly falls due to thermal inactivation of enzymes.

4. Effect of Water:

Although the amount of water required during photosynthesis is hardly one percent of the total amount of water absorbed by the plant, yet any change in the amount of water absorbed by a plant has significant effect on its rate of photosynthesis. Under normal conditions water rarely seems to be a controlling factor as the chloroplasts normally contain plenty of water.

Many experimental observations indicate that in the field the plant is able to withstand a wide range of soil moisture without any significant effect on photosynthesis and it is only when wilting sets in that the photosynthesis is retarded. Some of the effect of drought may be secondary since stomata tend to close when the plant is deprived of water. A more specific effect of drought on photosynthesis results from dehydration of protoplasm.

5. Effect of Oxygen:

Excess of O 2 may become inhibitory for the process. Enhanced supply of O 2 increases the rate of respiration simultaneously decreasing the rate of photosynthesis by the common intermediate substances. The concentration for oxygen in the atmosphere is about 21% by volume and it seldom fluctuates. O 2 is not a limiting factor of photosynthesis.

An increase in oxygen concentration decreases photosynthesis and the phenomenon is called Warburg effect. [Reported by German scientist Warburg (1920) in Chlorella algae]. This is due to competitive inhibition of RuBP-carboxylase at increased O 2 levels, i.e., O 2 competes for active sites of RuBP-carboxylase enzyme with CO 2 . The explanation of this problem lies in the phenomenon of photorespiration. If the amount of oxygen in the atmosphere decreases then photosynthesis will increase in C 3 cycle and no change in C 4 cycle.

6. Effect of Minerals:

Presence of Mn ++ and CI – is essential for smooth operation of light reactions (Photolysis of water/evolution of oxygen) Mg ++ , Cu ++ and Fe ++ ions are important for synthesis of chlorophyll.

7. Effect of Pollutants and Inhibitors:

The oxides of nitrogen and hydrocarbons present in smoke react to form peroxyacetyl nitrate (PAN) and ozone. PAN is known to inhibit Hill’s reaction. Diquat and Paraquat (commonly called as Viologens) block the transfer of electrons between Q and PQ in PS II.

Other inhibitors of photosynthesis are monouron or CMU (Chlorophenyl dimethyl urea), diuron or DCMU (Dichlorophenyl dimethyl urea), bromocil and atrazine etc., which have the same mechanism of action as that of violates. At low light intensities potassium cyanide appears to have no inhibiting effect on photosynthesis.

4. Internal Factors:

The important internal factors that regulate the rate of photosynthesis are:

1. Protoplasmic factors:

There is some unknown factor in protoplasm which affects the rate of photosynthesis. This factor affect the dark reactions. The decline in the rate of photosynthesis at temperature.above 30°C or at strong light intensities in many plants suggests the enzyme nature of this unknown factor.

2. Chlorophyll content:

Chlorophyll is an essential internal factor for photosynthesis. The amount of CO 2 fixed by a gram of chlorophyll in an hour is called photosynthetic number or assimilation number. It is usually constant for a plant species but rarely it varies. The assimilation number of variegated variety of a species was found to be higher than the green leaves variety.

3. Accumulation of end products:

Accumulation of food in the chloroplasts reduces the rate of photosynthesis.

4. Structure of leaves:

The amount of CO 2 that reaches the chloroplasts depends on structural features of the leaves like the size, position and behaviour of the stomata and the amount of intercellular spaces. Some other characters like thickness of cuticle, epidermis, presence of epidermal hairs, amount of mesophyll tissue, etc., influence the intensity and quality of light reaching the chloroplast.

5. CO 2 Compensation Point :

It is that value or point in light intensity and atmospheric CO 2 concentration when the rate of photosynthesis is just equivalent to the rate of respiration in the photosynthetic organs so that there is no net gaseous exchange. The value of light compensation point is 2.5 -100 ft. candles for shade plants and 100-400 ft. candles for sun plants. The value of CO 2 compensation point is very low in C 4 plants (0-5 ppm), where as in C 3 plants it is quite high (25-100 ppm). A plant can not survive for long at compensation point because there is net lose of organic matter due to respiration of non-green organs and dark respiration.

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Biology Wise

Photosynthesis Explained with a Diagram

It is extremely important to know the meaning and process of photosynthesis, irrespective of the fact that whether it the part of one's curriculum or not. The diagram given in this BiologyWise article is a small pictorial elaboration of the process of photosynthesis that will prove helpful for kids and teenagers to understand this vital process of the plant kingdom.

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It is extremely important to know the meaning and process of photosynthesis, irrespective of the fact that whether it the part of one’s curriculum or not. The diagram given in this BiologyWise article is a small pictorial elaboration of the process of photosynthesis that will prove helpful for kids and teenagers to understand this vital process of the plant kingdom.

The world, our planet, and the life on it are merely a magic trick by God. Mother nature contains all living things and some non-living factors too. All the living things need some food to stay alive. We consume many different foods in a single day, cakes, burgers … well the list is almost endless. Similarly, other animals also consume some or the other type of foods. Deer eat grass, fish eat plants or small fish and microbes, that are present in the water. Animals such as lions and tigers eat other animals. But have you ever wondered, what do plants eat?

What is Photosynthesis?

The process that plants carry out in the presence of radiant energy in order to create their food is known as photosynthesis. This process is one of the reasons because of which man and other forms of life are alive on the Earth today. This process basically occurs in the green parts of leaves. This process requires the following ingredients.

Soil does not become directly involved in the process of photosynthesis, but the plant absorbs some important ingredients, that are present in the soil. Chlorophyll is actually a chemical that is found in most of the plants and imparts green color to them. The process of photosynthesis actually becomes possible due to the chlorophyll that is present in plant leaves. During this process of food generation, the following reaction takes place:

6 CO 2 + 6 H 2 O → (in the presence of sunlight) C 6 H 12 O 6 + 6 O 2

Photosynthesis Diagram

According to the diagram of photosynthesis, the process begins with three most important non-living elements: water, soil, and carbon dioxide. Plants begin making their ‘food’, which basically includes large quantities of sugars and carbohydrate, when sunlight falls on their leaves. The ‘food’ is then stored aside by the plant and some of it is consumed during the day. This process goes on till the end of the day (until sunlight is available). The ‘food’ that is prepared by the plants is always in excess and humans and other animals consume it through different sources such as fruits and vegetables. Animals and human beings in return breathe out carbon dioxide during the process of cellular respiration. This carbon dioxide is in turn used by the plants to make more food. The rains and accumulated water table provides water to the plants and the sun provides light (radiant energy) every day. This process is thus, nothing but a cycle that goes on and on. According to the facts of this phenomenon, this cycle has been going on for almost 3,500 million years, which is quite a long time.

The process of photosynthesis is the reason why all animals and human beings are alive today. Hence, it is absolutely necessary to help plants, to complete this food-producing process. We can simply follow this by not plucking their leaves and watering them every day.

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Photosynthesis 1: An Introduction (Interactive Tutorial)

Looking for a student learning guide? It’s on the main menu for your course. Use the “Courses” menu above.

Page Outline

• Introduction
• Photosynthesis as an Endergonic Redox (Oxidation-Reduction) Reaction
• Quiz: Photosynthesis the Big Picture

1. Introducing Photosynthesis

Start by labeling the diagram below. Use a process of elimination if you get stuck.

[qwiz qrecord_id=”sciencemusicvideosMeister1961-PSN, Inputs and Outputs Diagram”]

[h]Interactive Diagram: Photosynthesis inputs and outputs

[q labels = “top”]

[l] carbohydrate

[fx] No. Please try again.

[f*] Excellent!

[l] carbon dioxide

[f*] Correct!

[f*] Great!

[fx] No, that’s not correct. Please try again.

For a minute, let’s think about life.

Living things are entities that, chemically speaking, are profoundly out of equilibrium with their environment. We (meaning people, cacti, sea anemones, redwood trees, bacteria, and every other organism) are systems that are highly organized. On a molecular level, we’re composed of highly reduced compounds, each buzzing with energetic electrons. Order and potential energy like that don’t happen spontaneously. They need an energy source to sustain them.

That energy source is the sun (“1”), and photosynthesis is the biological process that brings that energy and highly organized matter into living systems. Life’s chemical energy is stored in a variety of forms: immediately as ATP, a form of energy that, for the most part, is locked within individual cells, and can’t be transferred from one cell to another. A form that can be transferred from cell to cell is sugar, and that’s one of photosynthesis’s most direct products. The sugars made by photosynthesis are simple carbohydrates, and plants can convert them into more complex carbohydrates such as starches and cellulose. In the diagram above, carbohydrate is represented by Number 5. Essentially, it’s the plant itself.

Carbohydrates (along with proteins and fats) constitute one output of the photosynthetic process. Another one is oxygen (shown at “4”). All of the oxygen in our atmosphere is there because of photosynthesis, which has been bubbling oxygen into the air for the past 3.5 billion years. Life can exist without atmospheric oxygen. But we can’t. If you get excited by anything connected with multicellular life (which ranges from the shape of an orchid to conflict resolution among primates), then you can thank photosynthesis for creating the oxygen that made the complexity of multicellular aerobic life possible.

So, that’s what comes out. Let’s think about what goes in. Photosynthesis involves carbon fixation,  which involves taking carbon dioxide gas (“2” in the diagram above) and rendering it into a solid form as carbohydrate (and, through other reactions, proteins, lipids, and nucleic acids). You can think of fixation as fixing into place .

Carbon dioxide is a minor component of our atmosphere. At the start of the industrial revolution (let’s say 1800), carbon dioxide’s atmospheric concentration was about 280 parts per million. Now it’s above 400, a product of human combustion of fossil fuel ( click here to learn more about climate change). During photosynthesis, plants and other photosynthetic organisms (including algae and photosynthetic bacteria), absorb carbon dioxide into their cells and combine it with electrons and protons ripped away from water molecules (water is the second input, shown above at “3”).

Taking unorganized, randomly moving molecules in the air and organizing them into cells and multicellular organisms is a massive push against entropy. To try to envision this, just take a look at your hand: an organized structure. The bone, skin, and muscle in your hand is mostly protein. Protein, like every other biological molecule, is a carbon-based molecule. Every one of the carbon atoms making up the molecules that make up the cells that make up the tissues that make up your hand, was once, not very long ago floating freely in the air.

It’s a good thing that the sun puts out so much energy. This is not something that most people can imagine, but think about all the energy that human beings use in a year: all the electricity powering all of the lights and machines; all of the fossil fuel-derived combustion that heats our homes and moves our vehicles, all of the energy required to make everything we use. That amount, all over our planet, sustaining our civilization, is equal to an hour and a half of sunlight striking the earth (based on the Sandia Labs Solar FRQ . The calculation was based on human energy consumption in the year 2001. We undoubtedly consume more energy now).

In what follows, we’ll drop down to the cellular and molecular details of how photosynthesis work. Let’s go.

2. Photosynthesis as an endergonic redox (oxidation-reduction) reaction

Photosynthesis is a complex process, with many intermediate steps that we’ll learn about in this and the coming modules. But let’s make sure we understand the big picture first. Here’s the chemical equation for photosynthesis:

6CO 2 + 6H 2 O + light energy –> C 6 H 12 O 6 + 6O 2 .

In words, that means that six molecules of carbon dioxide are combined with six molecules of water, producing one molecule of glucose and six molecules of oxygen. Two things about this equation tell us that the reaction is endergonic (not spontaneous, requiring an input of energy to move it forward).

• The fact that energy isn’t coming out, but gets put in. That’s evident in light energy’s place on the left side of the arrow.
• The negative entropy change. Twelve molecules go into photosynthesis. Seven molecules come out. That’s a decrease in entropy. If you had twelve piles of things in your room and you organized them into seven piles, you’d have increased your room’s level of organization. Increasing organization requires energy.

For a review of the idea of free energy, read this .

Note that photosynthesis is the inverse of what happens during cellular respiration.

• Photosynthesis: 6CO 2 + 6H 2 O + light energy –> C 6 H 12 O 6 + 6O 2
• Respiration: C 6 H 12 O 6 + 6O 2 –>  energy(ATP) + 6CO 2 + 6H 2 O

During photosynthesis, carbon dioxide is reduced . That means that carbon dioxide gains energetic electrons. Those electrons come from water, which is oxidized (loses electrons). It’s important to emphasize that water doesn’t power photosynthesis. The power comes from light, and how light powers the oxidation of water is something that we’ll visit later in this series of tutorials. Note that this is, of course, also the inverse of what happens during cellular respiration, where glucose is oxidized as water is reduced.

The reduction of carbon dioxide that happens during photosynthesis is the basis of all of life’s chemical energy. Life is based on highly reduced, energetic molecules. Think of vegetable oil or a piece of bread. In fact, for an image of a reduced matter, think of a peanut butter sandwich. It’s dripping with energy. And that’s what photosynthesis does: it takes highly oxidized carbon dioxide, commonly referred to as exhaust (what comes out of a car’s tailpipe), and transforms it into highly reduced living matter (carbohydrates, which then get transformed into proteins, lipids, and nucleic acids).

The key organelles are the chloroplast (“2”), which is the site of photosynthesis; and the mitochondria (“5”) the site of most cellular respiration. Because this is a cycle, we could begin anywhere, but let’s start with the inputs of photosynthesis. These are carbon dioxide (“6”) and water (“7”). Powered by light (“1”), a chloroplast converts these inputs into the simple sugar glucose (“3”) and oxygen (“4”). These outputs of photosynthesis become the inputs for cellular respiration, the goal of which is to produce the short-term energy molecule ATP, which would appear at “8” in this diagram.

Before entering into more detail about photosynthesis, let’s make sure you’re on top of the material covered above.

3. Quiz: Photosynthesis, the big picture

[qwiz random = “false” qrecord_id=”sciencemusicvideosMeister1961-PSN, Big Picture”]

[h]Photosynthesis: the big picture.

[q labels = “top” dataset_id=”SMV_PSN_The_Big_Picture|d2867ebbbc7b4″ question_number=”8″]

The energy for all life is provided by the ______ . Within plant cells, organelles called ________________  take up ____________________ (a gas) and   __________ . Through the process of ___________________ , ______________ create the sugar _________  and release _________ as a by-product. In both plant and animal cells,   ________________  take in _________ (gas) and __________ . During cellular _____________ , mitochondria will produce   ________ , and release the gas __________________  and __________ . These, in turn, become the inputs for _________________ .

[l] chloroplasts

[l] glucose

[l] mitochondria

[l] photosynthesis

[l] respiration

[q topic= “photosynthesis_overview” question_number=”1″ dataset_id=”SMV_PSN_The_Big_Picture|d2c08ea4423b4″] Which number represents the substance that gets reduced during photosynthesis?

[textentry single_char=”true”]

[c]ID I=[Qq]

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[c]ICo=[Qq]

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[q multiple_choice=”true” dataset_id=”SMV_PSN_The_Big_Picture|d2b88dbb627b4″ question_number=”2″] In biological processes like respiration and photosynthesis and respiration, substances that are reduced have _______ chemical energy than substances that are oxidized.

[c]IG1v cmU=[Qq]

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[c]IGxlc3M=[Qq]

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[q dataset_id=”SMV_PSN_The_Big_Picture|d2b121d57bbb4″ question_number=”3″] Which number represents the substance that gets oxidized during photosynthesis?

[c]ID M=[Qq]

[f]IFllcy4g4oCcM+KAnSByZXByZXNlbnRzIHdhdGVyLiBEdXJpbmcgcGhvdG9zeW50aGVzaXMsIHdhdGVyIGdldHMgb3hpZGl6ZWQsIGxvc2luZyBlbGVjdHJvbnMgYW5kIHByb3RvbnMsIGJlY29taW5nIG1vbGVjdWxhciBveHlnZW4gKE8= Mg== KQ==[Qq]

[f]IE5vLiBIZXJlJiM4MjE3O3MgYSBoaW50LiBUaGUgcmVkdWNlZCBwcm9kdWN0IG9mIHBob3Rvc3ludGhlc2lzIGlzIGNhcmJvaHlkcmF0ZS4gTG9vayBjYXJlZnVsbHkgYXQgdGhhdCB3b3JkLCBhbmQgc2VlIGlmIHlvdSBjYW4gZmlndXJlIG91dCBpbnB1dCBmb3IgcGhvdG9zeW50aGVzaXMgY291bGQgYmUgcmVjZWl2aW5nIGVsZWN0cm9ucyBhbmQgcHJvdG9ucyB0byBiZWNvbWUgYSBjYXJib2h5ZHJhdGUu

[q dataset_id=”SMV_PSN_The_Big_Picture|d2a9b5ef94fb4″ question_number=”4″] Which number represents the reduced product of photosynthesis?

[c]NQ ==[Qq]

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[q dataset_id=”SMV_PSN_The_Big_Picture|d2a2b9cbe8fb4″ question_number=”5″]Because photosynthesis requires energy to proceed, it’s considered to be an [hangman] process.

[c]ZW5kZXJnb25pYw==[Qq]

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[q dataset_id=”SMV_PSN_The_Big_Picture|d29b98677ebb4″ question_number=”6″]Because respiration releases energy for cellular work, it’s considered to be an [hangman] process.

[c]ZXhlcmdvbmlj[Qq]

[f]RXhjZWxsZW50LsKgQmVjYXVzZSByZXNwaXJhdGlvbiByZWxlYXNlcyBlbmVyZ3kgZm9yIGNlbGx1bGFyIHdvcmssIGl0JiM4MjE3O3MgY29uc2lkZXJlZCB0byBiZSBhbiA= ZXhlcmdvbmlj IHByb2Nlc3Mu

[q labels = “top” dataset_id=”SMV_PSN_The_Big_Picture|d2911e31fcbb4″ question_number=”7″]

[l] chloroplast

[q topic= “cellular_respiration_overview” dataset_id=”SMV_PSN_The_Big_Picture|d27de8cfe3bb4″ question_number=”9″]The correct chemical reaction for cellular respiration is

[c]IEM= Ng== SA== MTI= Tw== [Qq]6 + 6CO 2 + energy(ATP) –> 6CO 2 + 6O 2

[c]IE M= Ng== SA== [Qq] 12 O 6 + 6O 2 –> energy(ATP) + 6CO 2 + 6H 2 O

[c]IDZDTw== Mg== ICsgNkg= [Qq] 2 O –> Energy(light) + C 6 H 12 O 6 + 6O 2

[c]IDZI Mg== TyArIDZP [Qq] 2  –> 6CO 2 + 6H 2 O

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[Qq] [q topic= “cellular_respiration_overview” dataset_id=”SMV_PSN_The_Big_Picture|d2759d65877b4″ question_number=”10″]The function of cellular respiration is to produce

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[Qq] [q topic= “photosynthesis_overview” dataset_id=”SMV_PSN_The_Big_Picture|d26d9c7ca7bb4″ question_number=”11″]The equation 6CO 2 + 6H 2 O + Energy(light) C 6 H 12 O 6 + 6O 2 is for

[c]IGNlbGx1bGFyIHJlc3BpcmF0aW9u[Qq]

[c]IGxhY3RpYyBhY2lkIGZlcm1lbnRhdGlvbg==[Qq]

[c]IHBob3Rvc3 ludGhlc2lz[Qq]

[c]IGFsY29ob2wgZmVybWVudGF0aW9u[Qq]

[f]IE5vLiBJbiBjZWxsdWxhciByZXNwaXJhdGlvbiwgZ2x1Y29zZSBpcyBjb21iaW5lZCB3aXRoIG94eWdlbiB0byBwcm9kdWNlIEFUUCwgd2l0aCBjYXJib24gZGlveGlkZSBhbmQgd2F0ZXIgcHJvZHVjZWQgYXMgYSBieXByb2R1Y3QuIFRoZSBlcXVhdGlvbiBhYm92ZSBzaG93cyBob3cgY2FyYm9uIGRpb3hpZGUgaXMgY29tYmluZWQgd2l0aCB3YXRlciB0byBjcmVhdGUgZ2x1Y29zZSwgd2l0aCBveHlnZW4gYmVpbmcgcmVsZWFzZWQgYXMgYSBieXByb2R1Y3QuIFdoYXQgcHJvY2VzcyBjcmVhdGVzIGdsdWNvc2U/IEFzIGEgaGludCwgbm90ZSB0aGF0IGxpZ2h0IGRyaXZlcyB0aGUgcmVhY3Rpb24u[Qq]

[f]IE5vLiBMYWN0aWMgYWNpZCBmZXJtZW50YXRpb24gaXMgYSBraW5kIG9mIGFuYWVyb2JpYyByZXNwaXJhdGlvbi4gVGhlIGVxdWF0aW9uIGFib3ZlIHNob3dzIGhvdyBjYXJib24gZGlveGlkZSBpcyBjb21iaW5lZCB3aXRoIHdhdGVyIHRvIGNyZWF0ZSBnbHVjb3NlLCB3aXRoIG94eWdlbiBiZWluZyByZWxlYXNlZCBhcyBhIGJ5cHJvZHVjdC4gV2hhdCBwcm9jZXNzIGNyZWF0ZXMgZ2x1Y29zZT8gQXMgYSBoaW50LCBub3RlIHRoYXQgbGlnaHQgZHJpdmVzIHRoZSByZWFjdGlvbi4=[Qq]

[f]IFllcy4gVGhlIGVxdWF0aW9uIGFib3ZlIHNob3dzIHRoZSByZWFjdGlvbiBmb3IgcGhvdG9zeW50aGVzaXM6IGNhcmJvbiBkaW94aWRlIGJlaW5nIGNvbWJpbmVkIHdpdGggd2F0ZXIgdG8gY3JlYXRlIGdsdWNvc2UsIHdpdGggb3h5Z2VuIGJlaW5nIHJlbGVhc2VkIGFzIGEgYnlwcm9kdWN0Lg==[Qq]

[f]IE5vLiBBbGNvaG9sIGZlcm1lbnRhdGlvbiBpcyBhIGtpbmQgb2YgYW5hZXJvYmljIHJlc3BpcmF0aW9uLiBUaGUgZXF1YXRpb24gYWJvdmUgc2hvd3MgaG93IGNhcmJvbiBkaW94aWRlIGlzIGNvbWJpbmVkIHdpdGggd2F0ZXIgdG8gY3JlYXRlIGdsdWNvc2UsIHdpdGggb3h5Z2VuIGJlaW5nIHJlbGVhc2VkIGFzIGEgYnlwcm9kdWN0LiBXaGF0IHByb2Nlc3MgY3JlYXRlcyBnbHVjb3NlPyBBcyBhIGhpbnQsIG5vdGUgdGhhdCBsaWdodCBkcml2ZXMgdGhlIHJlYWN0aW9uLg==

[Qq] [q topic= “photosynthesis_overview” dataset_id=”SMV_PSN_The_Big_Picture|d2652bd18d3b4″ question_number=”12″]The energy that powers photosynthesis comes from

[c]IGxp Z2h0[Qq]

[f]IE5vLiBHbHVjb3NlIGlzIHRoZSBwcm9kdWN0IG9mIHBob3Rvc3ludGhlc2lzLiBUaGUgZW5lcmd5IHNvdXJjZSBmb3IgcGhvdG9zeW50aGVzaXMgaXMgYnVpbHQgaW50byB0aGUgd29yZCDigJhwaG90b3N5bnRoZXNpcy7igJkg4oCYUGhvdG/igJkgcmVmZXJzIHRvIHdoYXQ/[Qq]

[f]IE5vLiBPeHlnZW4gaXMgYSBieXByb2R1Y3Qgb2YgcGhvdG9zeW50aGVzaXMuIFRoZSBlbmVyZ3kgc291cmNlIGZvciBwaG90b3N5bnRoZXNpcyBpcyBidWlsdCBpbnRvIHRoZSB3b3JkIOKAmHBob3Rvc3ludGhlc2lzLuKAmSDigJhQaG90b+KAmSByZWZlcnMgdG8gd2hhdD8=[Qq]

[f]IE5vLiBDYXJib24gZGlveGlkZSBpcyBhbiBpbnB1dCBmb3IgcGhvdG9zeW50aGVzaXMsIG5vdCBvbmUgb2YgdGhlIHByb2R1Y3RzLiBUaGUgZW5lcmd5IHNvdXJjZSBmb3IgcGhvdG9zeW50aGVzaXMgaXMgYnVpbHQgaW50byB0aGUgd29yZCDigJhwaG90b3N5bnRoZXNpcy7igJkg4oCYUGhvdG/igJkgcmVmZXJzIHRvIHdoYXQ/[Qq]

[f]IFllcy4gTGlnaHQgZW5lcmd5IGlzIHdoYXQgZHJpdmVzIHRoZSByZWFjdGlvbnMgb2YgcGhvdG9zeW50aGVzaXMu

[Qq] [q topic= “photosynthesis_overview” dataset_id=”SMV_PSN_The_Big_Picture|d25d50296bbb4″ question_number=”13″]The correct chemical equation for photosynthesis is

[c]IEM= Ng== SA== MTI= Tw== [Qq]6 + 6CO 2 + energy(ATP) -> 6CO 2 + 6O 2

[c]IEM= Ng== SA== MTI= Tw== [Qq]6 + 6O 2 -> energy(ATP) + 6CO 2 + 6H 2 O

[c]IDZD Tw== Mg== ICsgNkg= Mg== TyArIEVuZXJneShsaWdodCkgLSZndDsgQw== [Qq]6 H 12 O 6 + 6O 2

[c]IDZI Mg== TyArIDZP MiA= IC0mZ3Q7IDZDTw== [Qq]2 + 6H 2 O

[f]IE5vLiBUaGUgcmVhY3Rpb24gYWJvdmUgc2hvd3MgZ2x1Y29zZSBiZWluZyBjb21iaW5lZCB3aXRoIGNhcmJvbiBkaW94aWRlLiBJbiBwaG90b3N5bnRoZXNpcywgY2FyYm9uIGRpb3hpZGUgaXMgY29tYmluZWQgd2l0aCB3YXRlciB0byBjcmVhdGUgZ2x1Y29zZSwgd2l0aCBveHlnZW4gcmVsZWFzZWQgYXMgYSBieXByb2R1Y3Qu[Qq]

[f]IE5vLiBJbiBwaG90b3N5bnRoZXNpcywgY2FyYm9uIGRpb3hpZGUgaXMgY29tYmluZWQgd2l0aCB3YXRlciB0byBjcmVhdGUgZ2x1Y29zZSwgd2l0aCBveHlnZW4gcmVsZWFzZWQgYXMgYSBieXByb2R1Y3Qu[Qq]

[f]IFllcy4gSW4gcGhvdG9zeW50aGVzaXMsIGNhcmJvbiBkaW94aWRlIGlzIGNvbWJpbmVkIHdpdGggd2F0ZXIgdG8gY3JlYXRlIGdsdWNvc2UsIHdpdGggb3h5Z2VuIHJlbGVhc2VkIGFzIGEgYnlwcm9kdWN0Lg==[Qq]

[f]IE5vLiBGaW5kIGEgcmVhY3Rpb24gdGhhdCBiZWdpbnMgd2l0aCBjYXJib24gZGlveGlkZSBhbmQgd2F0ZXIsIGFuZCBjb21iaW5lcyB0aG9zZSBpbnB1dHMgdG8gY3JlYXRlIGdsdWNvc2UsIHdpdGggb3h5Z2VuIHJlbGVhc2VkIGFzIGEgYnlwcm9kdWN0Lg==

[Qq] [q topic= “photosynthesis_overview” dataset_id=”SMV_PSN_The_Big_Picture|d255bf02c6bb4″ question_number=”14″]The organelle that carries out photosynthesis in a plant cell is

[c]IHRoZSB2YWN1b2xl[Qq]

[c]IHRoZSBjaGxv cm9wbGFzdA==[Qq]

[c]IHRoZSBudWNsZXVz[Qq]

[c]IHRoZSBtaXRvY2hvbmRyaWE=[Qq]

[f]IE5vLiBUaGUgdmFjdW9sZeKAmXMgcm9sZSBpcyB0byBzdG9yZSB3YXRlciBhbmQgb3RoZXIgc3Vic3RhbmNlcy4gRm9yIGEgaGludCBhYm91dCB0aGUgb3JnYW5lbGxlIHRoYXQgZG9lcyBwaG90b3N5bnRoZXNpcywgdGhpbmsgb2YgdGhlIGdyZWVuIHBpZ21lbnQg4oCYY2hsb3JvcGh5bGwu4oCZIFdoaWNoIG9yZ2FuZWxsZSBoYXMgYSBzaW1pbGFyIHNvdW5kPw==[Qq]

[f]IFllcy4gVGhlIGNobG9yb3BsYXN0LCB3aGljaCBnZXRzIGl0cyBuYW1lIGZyb20gdGhlIHBpZ21lbnQgY2hsb3JvcGh5bGwsIGlzIHRoZSBvcmdhbmVsbGUgdGhhdCBjYXJyaWVzIG91dCBwaG90b3N5bnRoZXNpcy4=[Qq]

[f]IE5vLiBUaGUgbnVjbGV1cyBpcyB0aGUgY2VsbOKAmXMgY29udHJvbCBjZW50ZXIuIEZvciBhIGhpbnQgYWJvdXQgdGhlIG9yZ2FuZWxsZSB0aGF0IGRvZXMgcGhvdG9zeW50aGVzaXMsIHRoaW5rIG9mIHRoZSBncmVlbiBwaWdtZW50IOKAmGNobG9yb3BoeWxsLuKAmSBXaGljaCBvcmdhbmVsbGUgaGFzIGEgc2ltaWxhciBzb3VuZD8=[Qq]

[f]IE5vLiBUaGUgbWl0b2Nob25kcmlhIHBsYXkgYSBrZXkgcm9sZSBpbiBjZWxsdWxhciByZXNwaXJhdGlvbi4gRm9yIGEgaGludCBhYm91dCB0aGUgb3JnYW5lbGxlIHRoYXQgZG9lcyBwaG90b3N5bnRoZXNpcywgdGhpbmsgb2YgdGhlIGdyZWVuIHBpZ21lbnQg4oCYY2hsb3JvcGh5bGwu4oCZIFdoaWNoIG9yZ2FuZWxsZSBoYXMgYSBzaW1pbGFyIHNvdW5kPw==

[Qq] [q topic= “photosynthesis_overview” dataset_id=”SMV_PSN_The_Big_Picture|d24dbe19e6fb4″ question_number=”15″]From a plant’s perspective, the purpose of photosynthesis is production of

[c]IGxpZ2h0[Qq]

[c]IGNhcmJvaHlkcmF0 ZSAoZ2x1Y29zZSk=[Qq]

[f]IE5vLiBDYXJib24gZGlveGlkZSBpcyBhbiBpbnB1dCBmb3IgcGhvdG9zeW50aGVzaXMuIEZyb20gYSBwbGFudOKAmXMgcGVyc3BlY3RpdmUsIHBob3Rvc3ludGhlc2lzIGlzIGFib3V0IG1ha2luZyBmb29kLiBXaGljaCBzdWJzdGFuY2UgaXMgYSB0eXBlIG9mIGZvb2Q/[Qq]

[f]IE5vLiBMaWdodCBpcyB0aGUgZW5lcmd5IHRoYXQgZHJpdmVzIHBob3Rvc3ludGhlc2lzLiBGcm9tIGEgcGxhbnTigJlzIHBlcnNwZWN0aXZlLCBwaG90b3N5bnRoZXNpcyBpcyBhYm91dCBtYWtpbmcgZm9vZC4gV2hpY2ggc3Vic3RhbmNlIGlzIGEgdHlwZSBvZiBmb29kPw==[Qq]

[f]IE5vLiBXYXRlciBpcyBhbiBpbnB1dCBmb3IgcGhvdG9zeW50aGVzaXMuIEZyb20gYSBwbGFudOKAmXMgcGVyc3BlY3RpdmUsIHBob3Rvc3ludGhlc2lzIGlzIGFib3V0IG1ha2luZyBmb29kLiBXaGljaCBzdWJzdGFuY2UgaXMgYSB0eXBlIG9mIGZvb2Q/[Qq]

[f]IFllcy4gRnJvbSBhIHBsYW504oCZcyBwZXJzcGVjdGl2ZSwgcGhvdG9zeW50aGVzaXMgaXMgYWJvdXQgbWFraW5nIGZvb2Q7IHRoYXQgZm9vZCBpcyBhIGNhcmJvaHlkcmF0ZSwgdHlwaWNhbGx5IGdsdWNvc2Uu[Qq]

[f]IE5vLiBPeHlnZW4gaXMgYSBieXByb2R1Y3Qgb2YgcGhvdG9zeW50aGVzaXMuIFdoaWxlIHdl4oCZcmUgdmVyeSBncmF0ZWZ1bCBmb3IgcGxhbnRzIGZvciBjcmVhdGluZyBveHlnZW4sIGl04oCZcyBub3QgdGhlIGdvYWwgb2YgdGhlIHByb2Nlc3MuIEZyb20gYSBwbGFudOKAmXMgcGVyc3BlY3RpdmUsIHBob3Rvc3ludGhlc2lzIGlzIGFib3V0IG1ha2luZyBmb29kLiBXaGljaCBzdWJzdGFuY2UgaXMgYSB0eXBlIG9mIGZvb2Q/

[Qq] [q topic= “photosynthesis_overview” dataset_id=”SMV_PSN_The_Big_Picture|d245bd31073b4″ question_number=”16″]The carbon that photosynthesis puts into carbohydrates comes from

[c]IGNhcmJvbi BkaW94aWRl[Qq]

[c]IEFUUA==[Qq]

[f]IFllcy4gQ2FyYm9uIGRpb3hpZGUgaXMgdGhlIHNvdXJjZSBvZiB0aGUgY2FyYm9uIHRoYXQgcGhvdG9zeW50aGVzaXMgcHV0cyBpbnRvIGNhcmJvaHlkcmF0ZXMu[Qq]

[f]IE5vLiBMaWdodCBpcyB0aGUgZW5lcmd5IHRoYXQgZHJpdmVzIHBob3Rvc3ludGhlc2lzLiBGb3IgdGhlIHNvdXJjZSBvZiB0aGUgY2FyYm9uIHRoYXQgZ29lcyBpbnRvIGNhcmJvaHlkcmF0ZXMsIGxvb2sgZm9yIHRoZSBjaG9pY2UgdGhhdCBpcyBib3RoIGFuIGlucHV0IGZvciBwaG90b3N5bnRoZXNpcywgYW5kIHdoaWNoIGhhcyB0aGUgZWxlbWVudCBjYXJib24gaW4gaXQu[Qq]

[f]IE5vLiBBVFAgaXMgYW4gZW5lcmd5IHNvdXJjZSBmb3IgY2VsbHMsIGJ1dCBpdOKAmXMgbm90IGEgY2FyYm9uIHNvdXJjZS4gRm9yIHRoZSBzb3VyY2Ugb2YgdGhlIGNhcmJvbiB0aGF0IGdvZXMgaW50byBjYXJib2h5ZHJhdGVzLCBsb29rIGZvciB0aGUgY2hvaWNlIHRoYXQgaXMgYm90aCBhbiBpbnB1dCBmb3IgcGhvdG9zeW50aGVzaXMsIGFuZCB3aGljaCBoYXMgdGhlIGVsZW1lbnQgY2FyYm9uIGluIGl0Lg==[Qq]

[f]IE5vLiBHbHVjb3NlIGlzIG9uZSBvZiB0aGUgcHJvZHVjdHMgb2YgcGhvdG9zeW50aGVzaXMsIG5vdCB0aGUgZW5lcmd5IHNvdXJjZSBvciBjYXJib24gc291cmNlLiBGb3IgdGhlIHNvdXJjZSBvZiB0aGUgY2FyYm9uIHRoYXQgZ29lcyBpbnRvIGNhcmJvaHlkcmF0ZXMsIGxvb2sgZm9yIHRoZSBjaG9pY2UgdGhhdCBpcyBib3RoIGFuIGlucHV0IGZvciBwaG90b3N5bnRoZXNpcywgYW5kIHdoaWNoIGhhcyB0aGUgZWxlbWVudCBjYXJib24gaW4gaXQu[Qq]

[f]IE5vLiBPeHlnZW4gaXMgYSBieXByb2R1Y3Qgb2YgcGhvdG9zeW50aGVzaXMuIEZvciB0aGUgc291cmNlIG9mIHRoZSBjYXJib24gdGhhdCBnb2VzIGludG8gY2FyYm9oeWRyYXRlcywgbG9vayBmb3IgdGhlIGNob2ljZSB0aGF0IGlzIGJvdGggYW4gaW5wdXQgZm9yIHBob3Rvc3ludGhlc2lzLCBhbmQgd2hpY2ggaGFzIHRoZSBlbGVtZW50IGNhcmJvbiBpbiBpdC4=

[Qq] [q topic= “photosynthesis_overview” dataset_id=”SMV_PSN_The_Big_Picture|d23de188e5bb4″ question_number=”17″]Photosynthesis has changed our atmosphere by adding _______ to the air.

[c]IG94eW dlbg==[Qq]

[f]IE5vLiBQaG90b3N5bnRoZXNpcyByZW1vdmVzIGNhcmJvbiBkaW94aWRlIGZyb20gdGhlIGF0bW9zcGhlcmUuIFdoYXQgZ2FzIGRvZXMgaXQgYWRkIHRvIHRoZSBhdG1vc3BoZXJlPw==[Qq]

[f]IFllcy4gUGhvdG9zeW50aGVzaXMgaXMgcmVzcG9uc2libGUgZm9yIHRoZSB1bmlxdWUsIG94eWdlbi1yaWNoIGF0bW9zcGhlcmUgb2Ygb3VyIHBsYW5ldC4=

[Qq] [q topic= “photosynthesis_overview” dataset_id=”SMV_PSN_The_Big_Picture|d23605e0c43b4″ question_number=”18″]A photosynthesizing plant is placed in a sealed chamber from which no air can enter or leave. The lights are kept continuously on, and the plant is provided with enough water. If the level of gases in the chamber is measured, which of the following should be true?

[c]IE94eWdlbiBsZXZlbHMgZ28gZG93biwgYW5kIGNhcmJvbiBkaW94aWRlIGxldmVscyBnbyB1cC4=[Qq]

[c]IE94eWdlbiBsZXZlbHMgZ28gdXAsIGFuZCBjYXJib24gZGlveGlkZSBsZXZlbHMgZ28gdXAu[Qq]

[c]IE94eWdlbiBsZXZlbHMgZ28gdXAsIGFuZCBjYX Jib24gZGlveGlkZSBsZXZlbHMgZ28gZG93bi4=[Qq]

[c]IE94eWdlbiBsZXZlbHMgZ28gZG93biwgYW5kIGNhcmJvbiBkaW94aWRlIGxldmVscyBnbyBkb3duLg==[Qq]

[f]IE5vLiBQaG90b3N5bnRoZXNpcyByZW1vdmVzIGNhcmJvbiBkaW94aWRlIGZyb20gdGhlIGFpciBhbmQgYWRkcyBveHlnZW4uIEhvdyB3b3VsZCB0aGlzIGFmZmVjdCBnYXMgbGV2ZWxzIGluIHRoZSBjaGFtYmVyPw==[Qq]

[f]IE5vLiBZb3XigJlyZSByaWdodCBhYm91dCBveHlnZW4gbGV2ZWxzLCBidXQgcGhvdG9zeW50aGVzaXMgcmVtb3ZlcyBjYXJib24gZGlveGlkZSBmcm9tIHRoZSBhaXIuIEhvdyB3b3VsZCB0aGlzIGFmZmVjdCBnYXMgbGV2ZWxzIGluIHRoZSBjaGFtYmVyPw==[Qq]

[f]IFllcy4gUGhvdG9zeW50aGVzaXMgcmVtb3ZlcyBjYXJib24gZGlveGlkZSBmcm9tIHRoZSBhaXIgYW5kIGFkZHMgb3h5Z2VuLiBUaGF0IHdpbGwgY2F1c2UgdGhlIGNhcmJvbiBkaW94aWRlIGxldmVsIHRvIGZhbGwsIGFuZCB0aGUgb3h5Z2VuIGxldmVsIHRvIHJpc2Uu[Qq]

[f]IE5vLiBZb3XigJlyZSByaWdodCBhYm91dCBjYXJib24gZGlveGlkZSwgYnV0IHBob3Rvc3ludGhlc2lzIGFkZHMgb3h5Z2VuIHRvIHRoZSBhdG1vc3BoZXJlLiBIb3cgd291bGQgdGhpcyBhZmZlY3QgZ2FzIGxldmVscyBpbiB0aGUgY2hhbWJlcj8=[Qq]

[q topic= “photosynthesis_overview” dataset_id=”SMV_PSN_The_Big_Picture|d22e4f7960fb4″ question_number=”19″]In the diagram below, which number shows a mitochondrion?

[c]IDEgwqAgwqDCoA==[Qq][c]IDIgwqAgwqDCoA==[Qq][c]ID U=

[f]IE5vLiBOdW1iZXIgMSByZWZlcnMgdG8gdGhlIHN1bi4=[Qq]

[f]IE5vLiBOdW1iZXIgMiByZWZlcnMgdG8gYSBjaGxvcm9wbGFzdC4=[Qq]

[f]IFllcy4gTnVtYmVyIDUgc2hvd3MgYSBtaXRvY2hvbmRyaW9uLg==[Qq]

[q topic= “photosynthesis_overview” dataset_id=”SMV_PSN_The_Big_Picture|d22673d13f7b4″ question_number=”20″]In the diagram below, number 4 would have to be

[c]wqB3YXRlcg==[Qq]

[f]IFllcy4gTnVtYmVyIDQgaXMgYSBnYXMgdGhhdOKAmXMgY29taW5nIG91dCBvZiBhIGNobG9yb3BsYXN0LiBUaGUgZ2FzIHJlbGVhc2VkIGZyb20gYSBjaGxvcm9wbGFzdCB3b3VsZCBoYXZlIHRvIGJlIG94eWdlbi4=[Qq]

[f]IE5vLiBDYXJib24gZGlveGlkZSBnb2VzIGludG8gY2hsb3JvcGxhc3RzLiBOdW1iZXIgNCBzaG93cyBhIGdhcyBsZWF2aW5nIGEgY2hsb3JvcGxhc3QuIFdoaWNoIGdhcyBjb21lcyBvdXQgb2YgYSBjaGxvcm9wbGFzdD8=[Qq]

[f]IE5vLiBOdW1iZXIgNCBpcyBsYWJlbGVkIGFzIGEgZ2FzLCBhbmQgZ2x1Y29zZSBpcyBhIHNvbGlkLiBIb3dldmVyLCB5b3XigJlyZSBvbiB0aGUgcmlnaHQgdHJhY2ssIGluc29mYXIgYXMgeW914oCZcmUgbG9va2luZyBmb3Igb25lIG9mIHRoZSBvdXRwdXRzIG9mIHBob3Rvc3ludGhlc2lzLg==[Qq]

[f]Tm8uIE51bWJlciA0IGlzIGEgZ2FzIHRoYXQmIzgyMTc7cyBjb21pbmcgb3V0IG9mIGEgY2hsb3JvcGxhc3QuIFdhdGVyIGlzIGEgbGlxdWlkIHRoYXQgZ29lcyBpbnRvIGEgY2hsb3JvcGxhc3QuIFdoYXQgZ2FzIGRvZXMgYSBjaGxvcm9wbGFzdCByZWxlYXNlPw==[Qq]

[q topic= “photosynthesis_overview” dataset_id=”SMV_PSN_The_Big_Picture|d21e2866e33b4″ question_number=”21″]In the diagram below, number 6 would have to be

[f]IE5vLiBPeHlnZW4gY29tZXMgb3V0IG9mIGEgY2hsb3JvcGxhc3QgKDIpIGFuZCBnb2VzIGludG8gdGhlIG1pdG9jaG9uZHJpb24gKDUpLiBOdW1iZXIgc2l4IGlzIGEgZ2FzIHRoYXQgaXMgYW4gaW5wdXQgb2YgcGhvdG9zeW50aGVzaXMgYW5kIG91dHB1dCBvZiByZXNwaXJhdGlvbi4gV2hhdCBzdWJzdGFuY2UgY291bGQgcGxheSB0aG9zZSB0d28gcm9sZXM/[Qq]

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Leaf Structures Involved in Photosynthesis

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When it comes to photosynthesis, the most important parts of the plant are the leaves. Their cells and structures are specialized to take in light and allow for gas exchange with the air around them. They also contain vascular structures that transport water from the roots into the cells that carry out photosynthesis.

1. The plant’s vascular tissues—xylem and phloem—transport water to the leaves and carry glucose away from the leaves.

Anyone who cares for plants could probably tell you that pouring water directly onto the leaves isn’t the best idea. Plants absorb water from the soil, using their roots.

As you probably already know, water is necessary for photosynthesis, which primarily occurs in the plant’s leaves. You might wonder how the water gets from the roots into the leaves, and the answer is through the plant’s vascular system! Just like the veins and arteries that circulate blood throughout our bodies, the plant’s vascular tissues move water, nutrients, and the products of photosynthesis throughout the plant.

When a plant’s roots absorb water and nutrients from the soil, these materials move up the stem and into the leaves through the xylem. Capillary action—which relies on liquid’s properties of cohesion, surface tension, and adhesion—is what allows water to “defy gravity” as it travels through the xylem and into the leaves.

Once photosynthesis has occurred, the produced sugars move through the phloem to other parts of the plant to be used in cellular respiration or stored for later.

2. Stomata, regulated by guard cells, allow gases to pass in and out of the leaf.

We may not be able to see them with the naked eye, but the leaves of plants contain tons of tiny holes, or pores, called stomata (sing. stoma). They play a central role in photosynthesis, allowing carbon dioxide to enter the leaf and oxygen to exit the leaf. The stomata also facilitate transpiration, the process by which water vapor is released through a plant’s leaves.

The stomata can be opened and closed, depending on the turgor pressure—the pressure of a cell’s contents against the cell wall—in the two guard cells that border each stoma. High turgor pressure causes these cells to bend outward, opening the stomatal pore. Low turgor pressure, due to loss of water, keeps the stomatal pores closed.

3. Cells in the mesophyll of the leaf have numerous chloroplasts.

In leaves, cells in the mesophyll (the tissue between the upper and lower epidermis) are uniquely suited to carry out photosynthesis on a large scale. This is due to their high concentration of chloroplasts, which are the sites of photosynthesis. More chloroplasts means more photosynthetic capability.

Certain types of plants (dicots and some net-veined monocots) have two different types of mesophyll tissue. Palisade mesophyll cells are densely packed together, whereas spongy mesophyll cells are arranged more loosely to allow gases to pass through them. Palisade mesophyll cells also have more chloroplasts than spongy mesophyll cells.

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External Sources

A fun and easy activity from Scientific American that allows you to observe capillary action.

An OSU page explaining turgor pressure inside plant cells.

An article on transpiration and the water cycle from the USGS.

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The Biology Corner

Biology Teaching Resources

How Does Photosynthesis Work?

In this exercise, students label the major events that happen in photosynthesis . The graphic shows a plant with elements of showing the reactants and products. Students label each element and then write the equation for photosynthesis.

Photosynthesis is the process by which green plants, algae, and some bacteria convert light energy into chemical energy stored in glucose. It occurs in two main stages: the light-dependent reactions and the light-independent reactions (also known as the Calvin cycle).

On the second graphic, students label a chloroplast showing the light-dependent reaction with its reactants (water) and products (oxygen, ATP, NADPH.)    

During the light-dependent reactions, light energy is absorbed by chlorophyll molecules in the thylakoid membranes of chloroplasts. This energy is used to split water molecules into oxygen, protons, and electrons. The electrons move through the electron transport chain, generating ATP and reducing NADP+ to NADPH.

In the light-independent reactions (Calvin cycle), ATP and NADPH produced in the light-dependent reactions are used to convert carbon dioxide into glucose. This process takes place in the stroma of the chloroplast and involves a series of enzyme-mediated steps, ultimately producing glucose, which can be used as a source of energy by the plant or stored for later use.

Other Resources on Photosynthesis

For a follow-up activity, students can connect photosynthesis to cellular respiration by examining a graphic showing how the processes are connected. The Photosynthesis and Respiration Model connects the products and the reactants for the two processes. Students highlight features of the model and answer questions.

Students can also complete the Photosynthesis Coloring where the color features of a chloroplast and the elements in an equation.

HS-LS1-6 Construct and revise an explanation based on evidence for how carbon, hydrogen, and oxygen from sugar molecules may combine with other elements to form amino acids and/or other large carbon-based molecules.

Shannan Muskopf

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Characteristics of chloroplasts

The photosynthetic machinery, chloroplast genome and membrane transport.

What is a chloroplast?

Where are chloroplasts found, do chloroplasts have dna.

• Why is photosynthesis important?
• What is the basic formula for photosynthesis?

chloroplast

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• Frontiers - Chloroplast Degradation: Multiple Routes Into the Vacuole
• Biology LibreTexts - Chloroplast
• Khan Academy - Mitochondria and chloroplasts
• National Center for Biotechnology Information - Chloroplasts and Photosynthesis
• Florida State University - Molecular Expressions - Chloroplasts
• Table Of Contents

A chloroplast is an organelle within the cells of  plants  and certain algae  that is the site of  photosynthesis , which is the process by which energy from the Sun is converted into chemical energy for growth. A chloroplast is a type of plastid (a saclike organelle with a double membrane) that contains  chlorophyll to absorb light energy.

Chloroplasts are present in the cells of all green tissues of plants and algae . Chloroplasts are also found in photosynthetic tissues that do not appear green, such as the brown blades of giant kelp or the red leaves of certain plants. In plants, chloroplasts are concentrated particularly in the parenchyma cells of the leaf mesophyll (the internal cell layers of a leaf ).

Why are chloroplasts green?

Chloroplasts are green because they contain the pigment chlorophyll , which is vital for photosynthesis . Chlorophyll occurs in several distinct forms. Chlorophylls  a  and  b  are the major pigments found in higher plants and green algae.

Unlike most other organelles , chloroplasts and mitochondria have small circular chromosomes known as extranuclear DNA. Chloroplast DNA contains genes that are involved with aspects of  photosynthesis and other chloroplast activities. It is thought that both chloroplasts and mitochondria are descended from free-living cyanobacteria , which could explain why they possess DNA that is distinct from the rest of the cell.

chloroplast , structure within the cells of plants and green algae that is the site of photosynthesis , the process by which light energy is converted to chemical energy , resulting in the production of oxygen and energy-rich organic compounds . Photosynthetic cyanobacteria are free-living close relatives of chloroplasts; endosymbiotic theory posits that chloroplasts and mitochondria (energy-producing organelles in eukaryotic cells ) are descended from such organisms.

Chloroplasts are a type of plastid—a round, oval, or disk-shaped body that is involved in the synthesis and storage of foodstuffs. Chloroplasts are distinguished from other types of plastids by their green colour, which results from the presence of two pigments, chlorophyll a and chlorophyll b . A function of those pigments is to absorb light energy for the process of photosynthesis . Other pigments, such as carotenoids , are also present in chloroplasts and serve as accessory pigments, trapping solar energy and passing it to chlorophyll. In plants, chloroplasts occur in all green tissues, though they are concentrated particularly in the parenchyma cells of the leaf mesophyll.

Chloroplasts are roughly 1–2 μm (1 μm = 0.001 mm) thick and 5–7 μm in diameter . They are enclosed in a chloroplast envelope, which consists of a double membrane with outer and inner layers, between which is a gap called the intermembrane space. A third, internal membrane, extensively folded and characterized by the presence of closed disks (or thylakoids ), is known as the thylakoid membrane. In most higher plants, the thylakoids are arranged in tight stacks called grana (singular granum ). Grana are connected by stromal lamellae, extensions that run from one granum, through the stroma, into a neighbouring granum . The thylakoid membrane envelops a central aqueous region known as the thylakoid lumen. The space between the inner membrane and the thylakoid membrane is filled with stroma , a matrix containing dissolved enzymes , starch granules, and copies of the chloroplast genome.

The thylakoid membrane houses chlorophylls and different protein complexes, including photosystem I, photosystem II, and ATP ( adenosine triphosphate ) synthase, which are specialized for light-dependent photosynthesis. When sunlight strikes the thylakoids, the light energy excites chlorophyll pigments, causing them to give up electrons . The electrons then enter the electron transport chain, a series of reactions that ultimately drives the phosphorylation of adenosine diphosphate (ADP) to the energy-rich storage compound ATP. Electron transport also results in the production of the reducing agent nicotinamide adenine dinucleotide phosphate (NADPH).

ATP and NADPH are used in the light-independent reactions (dark reactions) of photosynthesis, in which carbon dioxide and water are assimilated into organic compounds . The light-independent reactions of photosynthesis are carried out in the chloroplast stroma, which contains the enzyme ribulose-1,5-bisphosphate carboxylase/oxygenase (rubisco). Rubisco catalyzes the first step of carbon fixation in the Calvin cycle (also called Calvin-Benson cycle), the primary pathway of carbon transport in plants. Among so-called C 4 plants, the initial carbon fixation step and the Calvin cycle are separated spatially—carbon fixation occurs via phosphoenolpyruvate (PEP) carboxylation in chloroplasts located in the mesophyll, while malate, the four-carbon product of that process, is transported to chloroplasts in bundle-sheath cells, where the Calvin cycle is carried out. C 4 photosynthesis attempts to minimize the loss of carbon dioxide to photorespiration. In plants that use crassulacean acid metabolism (CAM), PEP carboxylation and the Calvin cycle are separated temporally in chloroplasts, the former taking place at night and the latter during the day. The CAM pathway allows plants to carry out photosynthesis with minimal water loss.

The chloroplast genome typically is circular (though linear forms have also been observed) and is roughly 120–200 kilobases in length. The modern chloroplast genome, however, is much reduced in size: over the course of evolution , increasing numbers of chloroplast genes have been transferred to the genome in the cell nucleus . As a result, proteins encoded by nuclear DNA have become essential to chloroplast function. Hence, the outer membrane of the chloroplast, which is freely permeable to small molecules, also contains transmembrane channels for the import of larger molecules, including nuclear-encoded proteins. The inner membrane is more restrictive, with transport limited to certain proteins (e.g., nuclear-encoded proteins) that are targeted for passage through transmembrane channels.

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Photosynthesis Worksheets

Did you know the planet Earth would be lifeless without photosynthesis? Do you know how plants make their own food? Find answers to these and many such queries with our printable photosynthesis worksheets for students of grade 3 through grade 7. Featured here are vibrant charts illustrating the photosynthesis process, the equation of photosynthesis, precise and apt definitions of key terms in photosynthesis and activities like label the process, complete the paragraph, differentiate between light and dark reactions and much more. Our free photosynthesis worksheets are definitely worth a try!

Photosynthesis Basic Chart

This photosynthesis basic chart facilitates grasping and retaining the process of photosynthesis with ease. Equip children of grade 3 and grade 4 with this chart to familiarize them with the concept.

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Photosynthesis Equations Chart

The photosynthesis equation is presented in a visually appealing way in this chart. The word equation states the reactants - Co 2 and H 2 O and the products glucose and O 2 , followed by a balanced chemical equation.

Photosynthesis Equations Worksheet

Follow-up the equation chart with this complete the photosynthesis equations activity pdf, for children of grade 5 and grade 6 to reaffirm the concepts. Word equation and balanced chemical equations are included here.

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Photosynthesis Process | Descriptions

This printable handout illustrates the process of photosynthesis with concise descriptions. Comprehend the vital components involved and the products of photosynthesis with easy-to-understand descriptions.

Photosynthesis | Basic Vocabulary

This basic photosynthesis vocabulary worksheet simplifies the process of photosynthesis. The concept is broken into simple chunks for a vivid understanding of the terms and process involved in photosynthesis.

Photosynthesis | Advanced Vocabulary

Comprehend the process of photosynthesis with this vocabulary worksheet. Definitions of terms associated with the light and dark reactions of photosynthesis have been included here.

What is Photosynthesis?

Observe the photosynthesis process diagram presented in this printable 4th grade and 5th grade worksheet and plug in the words from the word box to complete the paragraph and answer the question "What is photosynthesis?".

Label the Photosynthesis Diagram

Help students transit from passive listeners to active participants with this labelling the photosynthesis process worksheet pdf. Direct the 3rd grade and 4th grade students to use the words from the word box to label the diagram.

Label the Reactants and Products of Photosynthesis

Refine the knowledge of students with this worksheet. They comprehend the major reactants and products and label them by recollecting the vocabulary words.

Describe the Reactants and Products

Brainstorm children of what they know and initiate discussions about each reactant and product with this worksheet. Instruct students to identify the reactants and products and describe them in a sentence.

Photosynthesis Process | Activity

Recapitulate the process of photosynthesis with this cut and paste activity worksheet pdf. Snip the reactants and products involved in the process of photosynthesis and glue them in the appropriate boxes.

Fill in the blanks

Test comprehension of 6th grade and 7th grade students with this fill in the blanks worksheet, that includes subtle details of the photosynthesis process. Read the sentences and plug the missing term(s).

Photosynthesis | Matching Activity

Employ this photosynthesis-matching-activity to assess the knowledge of students. Make one-to-one correspondence between the photosynthesis vocabulary words and their descriptions.

Structure of the Chloroplast | Chart

Develop an in-depth understanding of the process of photosynthesis with this printable chart. The labeled structure of the chloroplast chart indicates the exact location of the two processes within the leaf.

The Two Phases of Photosynthesis | Chart

The light-dependent and light-independent reactions of photosynthesis have been clearly illustrated in this chart. The reactants and products of each stage are labeled as well.

Label the Light and Dark Reactions

Actively involve students with this label-the-photosynthesis-reactions activity. Reiterate the Light reactions and the Calvin-Benson Cycle by labeling the two reactions along with their reactants and products.

Light Reaction Vs Dark Reaction

The differences between the light and dark reactions are stated vividly using a versatile graphic organizer, the T-chart. Integrate the T-chart to help students of grade 6 and grade 7 comprehend the differences between the light and dark reactions of photosynthesis.

Photosynthesis Review Questions

Posing questions is an effective teaching technique. The questions in this pdf worksheet are constructed to summarize the two stages in the process of photosynthesis and elicit responses in the form of definitions and comparisons.

The story of how humans and animals live is breathtaking. What's even more breathtaking is how plants get their energy on an everyday basis. Read this printable passage, and answer questions that ask you to eliminate the wrong title, write a short note and more.

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Photosynthesis Diagram – Process of Energy Transformation

What is Photosynthesis?

Photosynthesis is a process where the living organisms, typically the plants, take in the Sunlight, consume it, process it, and then utilize it as a fuel to function correctly. In other words, the plants use photosynthesis to prepare food for themselves to feed on, where the source of energy remains the light, typically that is emitted from the Sun.

Why Photosynthesis Is Important

Although primarily used by plants and other herbal organisms, photosynthesis, directly and indirectly, affects all living beings that the planet is populated with. Listed below are some of the major contributions of photosynthesis in maintaining the lifecycle and the environment of Earth:

Photosynthesis is used by plants and trees that further produce oxygen that is vital for all types of lives on the planet.

Carbon Cycle

Since the Sunlight, carbon dioxide, and water are the ingredients of photosynthesis, the well-balanced consumption of all three maintains a decent cycle to keep the Earth, its oceans, and all the other living beings in the right order.

If any of the ingredients discussed above go missing, the life on planet Earth wouldn’t exist. Therefore, photosynthesis, directly and indirectly, plays a major role in keeping all living beings on the planet alive

Photosynthesis works as a primary source of energy that is vital to keep things alive.

How to Create a Photosynthesis Diagram

Creating a photosynthesis diagram is fairly simple and straightforward as long as you understand how the process works. Things become even simpler if you have an efficient and robust tool like EdrawMax to help you out. The step-by-step instructions on how to create a photosynthesis diagram in EdrawMax Online are given below:

Step 1: Download and Install EdrawMax

Launch your favorite web browser, go to https://www.edrawsoft.com/edraw-max/ and download EdrawMax into your computer. Then install and open EdrawMax.

Step 2: Get to the Biology Category

Ensure that New is selected from the left pane, select Science and Education from the list of categories in the center, and click the Biology tile from the upper area of the right window.

Step 3: Create Photosynthesis Diagram

Click the Photosynthesis template icon from the right pane to automatically create a new document and add the photosynthesis diagram in the template to it. Alternatively, you can also click the + tile from the right window to create a blank document, and then use the available tools in the symbol library to start drawing your custom illustration from scratch.

With all the advanced technologies and the applications that are available these days, it is advisable to use a pre-built template to illustrate any complex process such as that of photosynthesis. Since such diagrams have been created by the professionals who have decades of experience in the industry, you can expect the representations to be detailed and accurate.

Free Photosynthesis Diagram Templates

Photosynthesis can be demonstrated in various ways where each illustration may explain how the process works in different types of organisms, and what impact does it make on the environment. As suggested earlier, rather than drawing a diagram from the start, it is always a good idea to use an existing template that can be downloaded for free from the Internet.

Some decent and informative photosynthesis diagram templates that you can use for both educational and research purposes are listed below:

The template illustrates how photosynthesis works in the trees. Although entirely customizable, this photosynthesis diagram shows the input ingredients and what the trees produce after photosynthesis is done.

Click here to download, edit, and print this template right now.

This template illustrates how photosynthesis works in a leaf. The diagram shows how a leaf takes in carbon dioxide, light, and water, and emits sugar and oxygen that are the essential elements for life.

3. Photosynthesis and Animals

This illustrative template shows how plants are helpful in maintaining proper balance between all kinds of lives on the planet. The image explains how the animals take in oxygen the plants generate, and how all the ingredients work together to keep the plants alive so the animals can feed themselves.

4. Photosynthesis and Respiration

This one explains how a leaf uses the required ingredients, i.e. the light, carbon dioxide, and water, does photosynthesis, and then emits the element vital for respiration for every life on the planet, i.e. oxygen.

5. Photosynthesis for Kids

This template is primarily prepared for kids to make them understand how the entire process works. The photosynthesis diagram is well-labelled, and clearly illustrates everything in a colorful manner that makes it both attractive and informative.

Although the process of photosynthesis is extremely complex when it comes to understanding it for research purposes, the basic methodology can be easily explained with simple diagrams and the way the objects in it are labelled. In a nutshell, how plants take in natural resources like light, carbon dioxide, and water, and produce oxygen, sugar, and other elements that are important for life is what photosynthesis is all about.