Calvin Cycle Steps and Diagram

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Calvin Cycle

This is a diagram of the Calvin Cycle.
This is a diagram of the Calvin Cycle, which is the set of chemical reactions that occur without light (dark reactions) in photosynthesis. Atoms are black - carbon, white - hydrogen, red - oxygen, pink - phosphorus. Mike Jones, Creative Commons License

The Calvin cycle is a set of light independent redox reactions that occur during photosynthesis and carbon fixation to convert carbon dioxide into the sugar glucose. These reactions occur in the stroma of the chloroplast, which is the fluid-filled region between the thylakoid membrane and inner membrane of the organelle. Here is a look at the redox reactions that occur during the Calvin cycle.

Other Names for the Calvin Cycle

You may know the Calvin cycle by another name. The set of reactions also is known as the dark reactions, C3 cycle, Calvin-Benson-Bassham (CBB) cycle, or reductive pentose phosphate cycle. The cycle was discovered in 1950 by Melvin Calvin, James Bassham, and Andrew Benson at the University of California, Berkeley. They used radioactive carbon-14 to trace the path of carbon atoms in carbon fixation.

Overview of the Calvin Cycle

The Calvin cycle is part of photosynthesis, which occurs in two stages. In the first stage, chemical reactions use energy from light to produce ATP and NADPH. In the second stage (Calvin cycle or dark reactions), carbon dioxide and water are converted into organic molecules, such as glucose. Although the Calvin cycle may be called the "dark reactions," these reactions don't actually occur in the dark or during nighttime. The reactions require reduced NADP, which comes from a light-dependent reaction. The Calvin cycle consists of:

  • Carbon fixation - Carbon dioxide (CO2) is reacted to produce glyceraldehyde 3-phosphate (G3P). The enzyme RuBisCO catalyzes the carboxylation of a 5-carbon compound to make a 6-carbon compound that splits in half to form two 3-phosphoglycerate (3-PGA) molecules. The enzyme phosphoglycerate kinase catalyzes phosphorylation of 3-PGA to form 1,3-biphosphoglycerate (1,3BPGA).
  • Reduction reactions - The enzyme glyceraldehyde 3-phosphate dehydrogenase catalyzes reduction of 1,3BPGA by NADPH.
  • Ribulose 1,5-bisphosphate (RuBP) regeneration - At the end of the regeneration, the net gain of the set of reactions is one G3P molecule per 3 carbon dioxide molecules.

Calvin Cycle Chemical Equation

The overall chemical equation for the Calvin cycle is:

3 CO2 + 6 NADPH + 5 H2O + 9 ATP → glyceraldehyde-3-phosphate (G3P) + 2 H+ + 6 NADP+ + 9 ADP + 8 Pi (Pi = inorganic phosphate)

Six runs of the cycle are required to produce one glucose molecule. Surplus G3P produced by the reactions can be used to form a variety of carbohydrates, depending on the needs of the plant.

Note About Light Independence

Although the steps of the Calvin cycle don't require light, the process only occurs when light is available (daytime). Why? Because it's a waste of energy because there is no electron flow without light. The enzymes that power the Calvin cycle are therefore regulated to be light dependent even though the chemical reactions themselves don't require photons.

At night, plants convert starch into sucrose and release it into the phloem. CAM plants store malic acid at night and release it during the day. These reactions are also known as "dark reactions."

References

Bassham J, Benson A, Calvin M (1950). "The path of carbon in photosynthesis". J Biol Chem 185 (2): 781–7. PMID 14774424.