The Calvin cycle or the C3 cycle is a step in the process of photosynthesis that converts carbon dioxide into glucose. It is also called the Calvin-Benson-Bassham cycle, discovered by Melvin Calvin, James Bassham, and Andrew Benson at the University of California, Berkeley in 1950, who also won a Nobel Prize for it in 1961.

It is a light-independent reaction and hence often known as a dark reaction. However, it does not happen in the dark, and it can happen both in the presence and absence of sunlight. It is the next step, after the light-dependent reactions, and uses its products ATP and NADPH for further chemical reactions.

The cycle takes place in the stroma of the chloroplast. The CO2 enters the leaves via the stomata and diffuses to the stroma through the mesophyll cells. The Calvin cycle is a series of biochemical redox reactions which uses energy in the form of ATP and NADPH to fix carbon dioxide and produce glucose. It is a three-step process of fixation, reduction and regeneration.

Let us look at the three steps in detail.

Steps of Calvin Cycle

  1. Carbon Fixation

The stroma hosts the most important enzyme responsible for the light-independent reaction, that is, RuBisCO (Ribulose bisphosphate carboxylase). A molecule named Ribulose phosphate (RuBP) resides in the stroma of the leaves that is made up of five carbon atoms with two flanking phosphates.

CO2 from the light-dependent reaction enters the site of light-independent reaction; here, it reacts with the RuBP molecule in the presence of RuBisCO to produce two molecules of 3-phosphoglyceric acid (3-PGA) that is made up of three carbons each. Three molecules of CO2 are fixed with three molecules of RuBP to produce a total of 6 3-PGA in one cycle. This is the carbon fixation step of the Calvin cycle, the most important one. 

  1. Reduction

This is the second stage of the Cavin cycle that utilises energy produced from the light-dependent reactions to produce glyceraldehyde-3-phosphate(G3P). The 3-PGA molecule gains electrons or gets reduced to form G3P molecules. 6 molecules of ATP and NADPH are utilised in this reaction to produce 6 molecules of G3P. The ATP is utilised in the release of the terminal phosphate atom and released as ADP, and the NADPH is utilised in the release of the hydrogen atom and released as NADP+

  1. Regeneration

It is a complex step where the whole cycle is balanced to keep the process going. It also uses ATP in the form of energy. One of the G3P molecules leaves the chloroplast and enters the cytoplasm to form the carbohydrate molecule, while the other G3P molecules remain in the stroma to regenerate RuBP to continue the cycle of carbon fixation.

In conclusion, it takes 6 cycles to fix six molecules of CO2. Five out of the six G3P produced are retained by the stroma to regenerate RuBP and continue the cycle. The whole cycle consumes 18 molecules of ATP and 12 molecules of NADPH. This sums up the steps in a Calvin cycle. To know more about plants, photosynthesis, and other plant-related activities, visit BYJU’S Biology.

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