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10.8: The Calvin Cycle - Biology

10.8: The Calvin Cycle - Biology


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Learning Objectives

By the end of this section, you will be able to:

  • Describe the Calvin cycle
  • Define carbon fixation
  • Explain how photosynthesis works in the energy cycle of all living organisms

After the energy from the sun is converted and packaged into ATP and NADPH, the cell has the fuel needed to build food in the form of carbohydrate molecules. The carbohydrate molecules made will have a backbone of carbon atoms. Where does the carbon come from? The carbon atoms used to build carbohydrate molecules comes from carbon dioxide, the gas that animals exhale with each breath. The Calvin cycle is the term used for the reactions of photosynthesis that use the energy stored by the light-dependent reactions to form glucose and other carbohydrate molecules.

The Interworkings of the Calvin Cycle

In plants, carbon dioxide (CO2) enters the chloroplast through the stomata and diffuses into the stroma of the chloroplast—the site of the Calvin cycle reactions where sugar is synthesized. The reactions are named after the scientist who discovered them, and reference the fact that the reactions function as a cycle. Others call it the Calvin-Benson cycle to include the name of another scientist involved in its discovery (Figure 1).

The Calvin cycle reactions (Figure 2) can be organized into three basic stages: fixation, reduction, and regeneration. In the stroma, in addition to CO2, two other chemicals are present to initiate the Calvin cycle: an enzyme abbreviated RuBisCO, and the molecule ribulose bisphosphate (RuBP). RuBP has five atoms of carbon and a phosphate group on each end.

RuBisCO catalyzes a reaction between CO2 and RuBP, which forms a six-carbon compound that is immediately converted into two three-carbon compounds. This process is called carbon fixation, because CO2 is “fixed” from its inorganic form into organic molecules.

ATP and NADPH use their stored energy to convert the three-carbon compound, 3-PGA, into another three-carbon compound called G3P. This type of reaction is called a reduction reaction, because it involves the gain of electrons. A reduction is the gain of an electron by an atom or molecule. The molecules of ADP and NAD+, resulting from the reduction reaction, return to the light-dependent reactions to be re-energized.

One of the G3P molecules leaves the Calvin cycle to contribute to the formation of the carbohydrate molecule, which is commonly glucose (C6H12O6). Because the carbohydrate molecule has six carbon atoms, it takes six turns of the Calvin cycle to make one carbohydrate molecule (one for each carbon dioxide molecule fixed). The remaining G3P molecules regenerate RuBP, which enables the system to prepare for the carbon-fixation step. ATP is also used in the regeneration of RuBP.

In summary, it takes six turns of the Calvin cycle to fix six carbon atoms from CO2. These six turns require energy input from 12 ATP molecules and 12 NADPH molecules in the reduction step and 6 ATP molecules in the regeneration step.

Concept in Action

Check out this animation of the Calvin cycle. Click Stage 1, Stage 2, and then Stage 3 to see G3P and ATP regenerate to form RuBP.

Try It

Photosynthesis

The shared evolutionary history of all photosynthetic organisms is conspicuous, as the basic process has changed little over eras of time. Even between the giant tropical leaves in the rainforest and tiny cyanobacteria, the process and components of photosynthesis that use water as an electron donor remain largely the same. Photosystems function to absorb light and use electron transport chains to convert energy. The Calvin cycle reactions assemble carbohydrate molecules with this energy.

However, as with all biochemical pathways, a variety of conditions leads to varied adaptations that affect the basic pattern. Photosynthesis in dry-climate plants (Figure 3) has evolved with adaptations that conserve water. In the harsh dry heat, every drop of water and precious energy must be used to survive. Two adaptations have evolved in such plants. In one form, a more efficient use of CO2 allows plants to photosynthesize even when CO2 is in short supply, as when the stomata are closed on hot days. The other adaptation performs preliminary reactions of the Calvin cycle at night, because opening the stomata at this time conserves water due to cooler temperatures. In addition, this adaptation has allowed plants to carry out low levels of photosynthesis without opening stomata at all, an extreme mechanism to face extremely dry periods.

Photosynthesis in Prokaryotes

The two parts of photosynthesis—the light-dependent reactions and the Calvin cycle—have been described, as they take place in chloroplasts. However, prokaryotes, such as cyanobacteria, lack membrane-bound organelles. Prokaryotic photosynthetic autotrophic organisms have infoldings of the plasma membrane for chlorophyll attachment and photosynthesis (Figure 4). It is here that organisms like cyanobacteria can carry out photosynthesis.

The Energy Cycle

Living things access energy by breaking down carbohydrate molecules. However, if plants make carbohydrate molecules, why would they need to break them down? Carbohydrates are storage molecules for energy in all living things. Although energy can be stored in molecules like ATP, carbohydrates are much more stable and efficient reservoirs for chemical energy. Photosynthetic organisms also carry out the reactions of respiration to harvest the energy that they have stored in carbohydrates, for example, plants have mitochondria in addition to chloroplasts.
You may have noticed that the overall reaction for photosynthesis:

6CO2+6H2O→C6H12O6+6O2

is the reverse of the overall reaction for cellular respiration:

6O2+C6H12O6→6CO2+6H2O

Photosynthesis produces oxygen as a byproduct, and respiration produces carbon dioxide as a byproduct.

In nature, there is no such thing as waste. Every single atom of matter is conserved, recycling indefinitely. Substances change form or move from one type of molecule to another, but never disappear (Figure 5).

CO2 is no more a form of waste produced by respiration than oxygen is a waste product of photosynthesis. Both are byproducts of reactions that move on to other reactions. Photosynthesis absorbs energy to build carbohydrates in chloroplasts, and aerobic cellular respiration releases energy by using oxygen to break down carbohydrates in mitochondria. Both organelles use electron transport chains to generate the energy necessary to drive other reactions. Photosynthesis and cellular respiration function in a biological cycle, allowing organisms to access life-sustaining energy that originates millions of miles away in a star.

Section Summary

Using the energy carriers formed in the first stage of photosynthesis, the Calvin cycle reactions fix CO2 from the environment to build carbohydrate molecules. An enzyme, RuBisCO, catalyzes the fixation reaction, by combining CO2 with RuBP. The resulting six-carbon compound is broken down into two three-carbon compounds, and the energy in ATP and NADPH is used to convert these molecules into G3P. One of the three-carbon molecules of G3P leaves the cycle to become a part of a carbohydrate molecule. The remaining G3P molecules stay in the cycle to be formed back into RuBP, which is ready to react with more CO2. Photosynthesis forms a balanced energy cycle with the process of cellular respiration. Plants are capable of both photosynthesis and cellular respiration, since they contain both chloroplasts and mitochondria.

A Open Assessments element has been excluded from this version of the text. You can view it online here: pb.libretexts.org/fob2/?p=126

Additional Self Check Questions

1.Which part of the Calvin cycle would be affected if a cell could not produce the enzyme RuBisCO?

2. Explain the reciprocal nature of the net chemical reactions for photosynthesis and respiration.

Answers

1. None of the cycle could take place, because RuBisCO is essential in fixing carbon dioxide. Specifically, RuBisCO catalyzes the reaction between carbon dioxide and RuBP at the start of the cycle.

2. Photosynthesis takes the energy of sunlight and combines water and carbon dioxide to produce sugar and oxygen as a waste product. The reactions of respiration take sugar and consume oxygen to break it down into carbon dioxide and water, releasing energy. Thus, the reactants of photosynthesis are the products of respiration, and vice versa.


Chapter 10 - Photosynthesis Flashcards Preview

Label the indicated parts in this diagram of a chloroplast.

Fill in the blanks in this overview of photosynthesis in a chloroplast. Indicate the locations of the processes c and h.

c. light reactions in thylakoid membranes

An action spectrum shows the relative rates of photosynthesis under different wavelengths of light.

On the following graph, label the line that represents the absorption spectrum for chlorophyll a and the line for the action spectrum for photosynthesis.

Why are these lines different?

The solid line is the absorption spectrum the dotted line is the action spectrum.

Some wavelengths of light, particularly in the blue and the yellow-orange range, result in a higher rate of photosynthesis than would be indicated by the absorption of those wavelengths by chlorophyll a.

These differences are partially accounted for by accessory pigments, such as chlorophyll b and the carotenoids, which absorb light energy from different wavelengths and make that energy available to drive photosynthesis.

Describe the components of a photosystem.

A photosystem contains light-harvesting complexes of pigment molecules (chlorophyll a, chlorophyll b, and carotenoids) bound to particular proteins and a reaction center, which includes two chlorophyll a molecules (P700 or P680) and a primary electron acceptor.

Fill in the steps of electron flow in the following diagram.

Circle the important products that will be used to provide chemical energy and reducing power to the Calvin cycle.

e. P680, reaction-center chlorophyll a

f. primary electron acceptor

g. electron transport chain

h. photophosphorylation by chemiosmosis

j. P700, reaction-center chlorophyll a

k. primary electron acceptor

a. on the diagram in Question 10.5, sketch the path that electrons from P700 take during cyclic electron flow.

b. Why is neither oxygen nor NADPH generated by cyclic electron flow?

c. How, then is ATP produced by cyclic electron flow?

a. Ferredoxin (Fd) passes the electrons to the cytochrome complex in the electron transport chain, from which they return to P700 + .

b. Electrons from P680 are not passed to P700. Without the oxidizing agent P680 + , water is not split. Fd does not pass electrons to NADP + reductase to form NADPH.

c. Electrons do pass down the electron transport chain, and the energy released by their "fall" drives photophosphorylation.

a. In the light, the proton gradient across the thylakoid membrane is as great as 3 pH units. On which side is the pH lowest?

b. What three factors contribute to the formation of this large difference in H + concentration between the thylakoid space and the stroma?

a. In the thylakoid space (pH of about 5)

b. (1) transport of protons into the thylakoid space as Pq transfers electrons to the cytochrome complex

(2) protons from the splitting of water remain in the thylakoid space

(3) removal of H + in the stroma during the reduction of NADP + .

Label the three phases (a through c) and the key molecules (d through o) in this diagram of the Calvin cycle.

c. regeneration of CO2 acceptor (RuBP)

e. ribulose bisphosphate (RuBP)

l. glyceraldehyde-3-phosphate (G3P)

n. glucose and other organic compounds

What are two possible explanations for photorespiration, a process that can result in the loss of as much as 50% of the carbon fixed in the Calvin cycle?

Photorespiration may be an evolutionary relic from the time when there was little O2 in the atmosphere and the ability to rubisco to distinguish between O2 and CO2 was not critical.

Photorespiration appears to protect plants from damaging products of the light reactions that build up when the Calvin cycle slows due to a lack of CO2.

a. Where does the Calvin cycle take place in C4 plants?

b. How can C4 plants successfully perform the Calvin cycle in hot, dry conditions when C3 plants would be undergoing photorespiration?

c. C4 photosynthesis requires more ATP than does C3 photosynthesis. Why?

a. in the bundle-sheath cells

b. Carbon is initially fixed into a four-carbon compound in the mesophyll cells by PEP carboxylas. When this compound is broken down in the bundle-sheath cells, CO2 is maintained at a high enough concentration that rubisco does not accept O2 and cause photorespiration.

c. ATP is used to convert pyruvate, returning from the bundle-sheath cells, to PEP in te mesophyll cells.

Create a diagram of the key events of photosynthesis.

Trace the flow of electrons through photosystems II and I, the production of ATP and NADPH by the light reactions and their transfer into the Calvin cycle, and the major steps in the production of G3P.

Note where these reactions occur in the chlorplast.

Create a concept map to confirm your understanding of the chemiosmotic synthesis of ATP in photophosphorylation.

Which of the following is mismatched with its location?

b. electron transport chain—thylakoid membrane

d. ATP synthase—double membrane surrounding chloroplast

e. splitting of water—thylakoid space

d. ATP synthase—double membrane surrounding chloroplast

Photosynthesis is a redox process in which

a. CO2 is reduced and water is oxidized.

b. NADP + is reduced and RuBP is oxidized.

c. CO2, NADP + , and water are reduced.

d. O2 acts as an oxidizing agent and water acts as a reducing agent.

e. G3P is reduced and the electron transport chain is oxidized.

a. CO2 is reduced and water is oxidized.

Which of the following statements is false?

a. When isolated chlorophyll molecules absorb protons, their electrons fall back to ground state, giving off heat and light.

b. Accessory pigments, cyclic electron flow, and photorespiration may all contribute to photoprotection, protecting plants from the detrimental effects of intense light.

c. In the cyclic electron flow of purple sulfur bacteria, the electron transport chain would pump H + across the plasma membrane from inside to outside the cell.

d. In both photosynthetic prokaryotes and eukaryotes, ATP synthases catalyze the production of ATP within the cytoplasm of the cell.

e. In sulfur bacteria, H2S provides the hydrogen (and thus electron) source for photosynthesis.

d. In both photosynthetic prokaryotes and eukaryotes, ATP synthases catalyze the production of ATP within the cytoplasm of the cell.

A spectrophotometer can be used to measure

a. the absorption spectrum of a substance.

b. the action spectrom of a substance.

c. the amount of energy in a photon.

d. the wavelength of visible light.

e. the efficiency of photosynthesis.

a. the absorption spectrum of a substance.

Accessory pigments within chloroplasts are responsible for

a. driving the splitting of water molecules.

b. absorbing photons of different wavelengths of light and passing that energy ro P680 or P700.

c. providing electrons to the reaction-center chlorophyll after photoexcited electrons pass to NADP + .

d. pumping H + across the thylakoid membrane to create a proton-motive force.

e. anchoring chlorophyll a within the reaction center.

b. absorbing photons of different wavelengths of light and passing that energy ro P680 or P700.

The following diagram is an absorption spectrum for an unknown pigment molecule. What color would this pigment appear to you?


Watch the video: Photosynthesis: Light Reaction, Calvin Cycle, and Electron Transport (May 2022).