Question

Dissect the Calvin cycle and tell its energy story.

Answer 1

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 , carbon dioxide and water are converted into organic molecules , such as glucose. Although the celvin cycle may be called the dark reactions, these reactions dont actually occur in the dark or during night time. The reactions require reduced NADP, which comes from a light dependent reaction. The Calvin Cycle consist of :

1) Carbon fixation - Carbon dioxide is reacted to produce glyceraldehyde 3-phosphate(G3P). The enzyme RuBisCO catalyzes the carboxylation of a 2-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-biphosphate(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 :

3CO₂ + 6 NADPH + 5 H₂O + 9 ATP -------> glyceraldehyde-3-phosphate (G3P) + 2 H⁺ + 6 NADP⁺ +9ADP + 8pi

(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 plants.