6.3 The Light-Independent Reactions of Photosynthesis: The Calvin Cycle

Natasha Ramroop Singh, PhD

6.3 The Light-Independent Reactions of Photosynthesis: The Calvin Cycle

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 for long-term energy storage. 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, a waste product of respiration in microbes, fungi, plants, and animals. The Calvin cycle, or the light-independent reactions, 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 (Figure 6.13).

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. The most outdated name is “dark reaction,” because light is not directly required. However, the term dark reaction can be misleading because it implies incorrectly that the reaction requires an absence of light, which is why most scientists and instructors no longer use it.

The Calvin cycle reactions (Figure 6.14) can be organized into three basic stages: fixation, reduction, and regeneration.

This illustration shows a circular cycle with three stages. Three molecules of carbon dioxide enter the cycle. In the first stage, the enzyme RuBisCO incorporates the carbon dioxide into an organic molecule. Six ATP molecules are converted into six ADP molecules. In the second stage, the organic molecule is reduced. Six NADPH molecules are converted into six NADP+ ions and one hydrogen ion. Sugar is produced. In stage three, RuBP is regenerated, and three ATP molecules are converted into three ADP molecules. RuBP then starts the cycle again. — **Figure 6.14** The Calvin cycle has three stages. In stage 1, the enzyme RuBisCO incorporates carbon dioxide into an organic molecule. In stage 2, the organic molecule is reduced. In stage 3, RuBP, the molecule that starts the cycle, is regenerated so that the cycle can continue.

Stage 1: CO₂ fixation. In the stroma, in addition to CO₂, two other chemicals are present to initiate the Calvin cycle: an enzyme called ribulose-1,5-bisphosphate carboxylase/oxygenase (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 molecules of three-carbon 3-phosphoglyceric acid (3-PGA). Each turn of the cycle involves only one RuBP and one carbon dioxide and forms two molecules of 3-PGA. This process is called carbon fixation, because CO₂ is “fixed” from its inorganic form into organic molecules.

Stage 2: Reduction. ATP and NADPH use their stored energy to convert 3-PGA, into another three-carbon compound called glyceraldehyde 3-phosphate (G3P). This type of reaction is called a reduction reaction, because it involves the gain of electrons by 3-PGA. The molecules of ADP and NADP⁺, resulting from the reduction reaction, return to the light-dependent reactions to be re-energized.

Stage 3: Regeneration. One of the G3P molecules leaves the Calvin cycle for the cytoplasm to contribute to the formation of other carbohydrate molecules, 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 six ATP molecules in the regeneration step, Figure 6.15.

**Figure 6.15** The Calvin cycle has three stages. In stage 1, the enzyme RuBisCO incorporates carbon dioxide into an organic molecule, 3-PGA. In stage 2, the organic molecule is reduced using electrons supplied by NADPH. In stage 3, RuBP, the molecule that starts the cycle, is regenerated so that the cycle can continue. Only one carbon dioxide molecule is incorporated at a time, so the cycle must be completed three times to produce a single three-carbon G3P molecule, and six times to produce a six-carbon glucose molecule. (Credit: Rao, A., Ryan, K., Tag, A., Fletcher, S. and Hawkins, A. Department of Biology, Texas A&M University)

CONCEPT IN ACTION

QR Code representing a URL

The following is a link to an animation of the Calvin cycle. Click Stage 1, Stage 2, and then Stage 3 to see G3P and ATP regenerate to form RuBP.

EVOLUTION IN ACTION

Photosynthesis

During the evolution of photosynthesis, a major shift occurred from the bacterial type of photosynthesis that involves only one photosystem and is typically anoxygenic (does not generate oxygen) into modern oxygenic (does generate oxygen) photosynthesis, employing two photosystems. This modern oxygenic photosynthesis is used by many organisms—from giant tropical leaves in the rainforest to tiny cyanobacterial cells—and the process and components of this photosynthesis remain largely the same. Photosystems absorb light and use electron transport chains to convert energy into the chemical energy of ATP and NADPH. The subsequent light-independent reactions then 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 6.16) 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.

This photo shows a cactus. — **Figure 6.16** Living in the harsh conditions of the desert has led plants like this cactus to evolve variations in reactions outside the Calvin cycle. These variations increase efficiency and help conserve water and energy. (Credit: Piotr Wojtkowski)

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 6.17). It is here that organisms like cyanobacteria can carry out photosynthesis.

This illustration shows a green ribbon, representing a folded membrane, with many folds stacked on top of another like a rope or hose. The photo shows an electron micrograph of a cleaved thylakoid membrane with similar folds from a unicellular organism — **Figure 6.17** A photosynthetic prokaryote has infolded regions of the plasma membrane that function like thylakoids. Although these are not contained in an organelle, such as a chloroplast, all of the necessary components are present to carry out photosynthesis. (Credit: scale-bar data from Matt Russell)

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:

6 CO₂ + 6 H₂O → C₆H₁₂O₆ + 6 O₂

is the reverse of the overall reaction for cellular respiration:

C₆H₁₂O₆ + 6 O₂ → 6 CO₂ + 6 H₂O

Photosynthesis produces oxygen as a by-product, and respiration produces carbon dioxide as a by-product.

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 6.18). CO₂ is no more a form of waste produced by respiration than oxygen is a waste product of photosynthesis. Both are by-products 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. 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.

This photograph shows a giraffe eating leaves from a tree. Labels indicate that the giraffe consumes oxygen and releases carbon dioxide, whereas the tree consumes carbon dioxide and releases oxygen. — **Figure 6.18** In the carbon cycle, the reactions of photosynthesis and cellular respiration share reciprocal reactants and products. (Credit: modification of work by Stuart Bassil)

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.

Exercises

Glossary

Calvin cycle: the reactions of photosynthesis that use the energy stored by the light-dependent reactions to form glucose and other carbohydrate molecules

carbon fixation: the process of converting inorganic CO₂ gas into organic compounds

Media Attributions

Figure 6.15 by Rao, A., Ryan, K., Tag, A., Fletcher, S. and Hawkins, A. Department of Biology, Texas A&M University
Figure 6.16 by Piotr Wojtkowski
Figure 6.17 scale-bar data from Matt Russell
Figure 6.18 modification of work by Stuart Bassil

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