Give a detialed account of the various steps involved in Calvin cycle?

Answer 1

In plants, after the light reactions convert solar energy to chemical energy in the form of ATP and NADPH, the anabolic Calvin cycle occurs. Carbon, which enters the Calvin cycle as CO2, leaves in the form of sugar. Also, the cycle consumes NADPH as reducing power to make sugar by adding high-energy electrons and spends energy in the form of ATP. The Calvin cycle consists of three main phases: carbon fixation, reduction, and regeneration of the CO2 acceptor.

In carbon fixation, the enzyme rubisco catalyzes the incorporation and attachment of each CO2 molecule one by one to ribulose biphosphate (RuBP), a five-carbon sugar.

This resulting six-carbon intermediate is short-lived because it is unstable; it splits in half, immediately forming two molecules of 3-phosphoglycerate for each CO2. Note that it is important to keep track of the number of molecules, and counting the total carbons is one good way to keep track. Originally there are 3 molecules of the 5-carbon RuBP (total: 15 carbons). If 3 CO2 molecules enter one at a time, then there are three 6-carbon intermediates (total: 18 carbons-15 carbons from the original RuBP + 3 carbons each CO2 molecule) and then six 3-phosphoglycerate molecules (total: 18 carbons).

Then reduction occurs as 6 ATP provide the six 3-phosphoglycerate molecules with another phosphate group. As a result, all of the six 3-phosphoglycerate molecules are now six 1,3 biphosphoglycerate. Each 1,3 biphosphoglycerate has 3 carbons, so there are still 18 total carbons. Next, the six 1,3 biphosphoglycerate molecules are reduced to six glyceraldehydes-3-phosphates by six pairs of electrons donated from six NADPH. To be even more detailed, the carboxyl group of the 3-phosphoglycerate (G3P) is reduced by the NADPH electrons to G3P's aldehyde group, which stores more energy. There is a net output of one G3P molecule, which becomes the starting material from which other organic compounds such as glucose are synthesized through various metabolic pathways. Note that G3P is the same 3-carbon sugar formed by the splitting of glucose in glycolysis. Also, there is now a total of 15 carbons in the Calvin cycle (18 carbons - 3 carbons that left in the outputted G3P).

Lastly, the carbon skeletons of the five remaining G3P molecules are rearranged into 3 molecules of 5-carbon RuBP with the help of three more molecules of ATP. The CO2 acceptor RuBP is regenerated and ready to receive CO2 once more as you can see from the total of 15 carbons in the cycle, and the Calvin cycle can now begin again. Also note that the Calvin cycle consumes 6 NADPH molecules and 9 ATP molecules in all for the net synthesis of one G3P molecule.