Calvin cycle

The Calvin cycle (also known as the Benson-Calvin cycle) is the set of chemical reactions that take place in chloroplasts during photosynthesis.

The Calvin cycle.

The cycle is light-independent because it takes place after the energy has been captured from sunlight.

The Calvin cycle is named after Melvin Calvin, who won a Nobel Prize in Chemistry for finding it in 1961.^[1] Calvin and his colleagues, Andrew Benson and James Bassham, did the work at the University of California, Berkeley.^[2]

Context

Using the radioactive carbon-14 isotope as a tracer, Calvin, Andrew Benson and their team mapped the complete route that carbon travels through a plant during photosynthesis. They traced the carbon-14 from soaking up its atmospheric carbon dioxide to its conversion into carbohydrates and other organic compounds.^[3][4] The single-celled algae Chlorella was used to trace the carbon-14.^[5]

The Calvin group showed that sunlight acts on the chlorophyll in a plant to fuel the manufacture of organic compounds, not directly on carbon dioxide as previously believed.

Steps

The steps in the cycle are as follows:^[6]

1. Grab: A five-carbon carbon catcher called RuBP (Ribulose bisphosphate) catches one molecule of carbon dioxide and forms a six-carbon molecule.

2. Split: the enzyme RuBisCO (with the energy of ATP and NADPH molecules) breaks the six-carbon molecule into two equal parts.

3. Leave: A trio of carbons leave and become sugar. The other trio moves on to the next step.

4. Switch: Using ATP and NADPH, the three-carbon molecule is changed into a five-carbon molecule.

5. The cycle starts over again.

The product

The carbohydrate products of the Calvin cycle are three-carbon sugar phosphate molecules, or 'glucose triose phosphates' (G3P). Each step of the cycle has its own enzyme which speeds up the reaction.