Dark Reaction

The reactions that catalyze the reduction of CO2 to carbohydrates with the help of the ATP and NADPH2 generated by the light reactions are called the dark reaction. The enzymatic reduction of CO2 by these reactions is also known as carbon fixation. These reactions that result in CO2 fixation take place in a cyclic way and were discovered by Melvin Calvin. Hence, the cycle is called Calvin cycle. Fixation of carbondioxide in plants during photosynthesis occurs in three stages – fixation, reduction and regeneration of RuBP.

Fixation

The acceptor molecule of CO2 is a 5C compound called ribulose-1,5¬ bisphosphate (RuBP). Fixation of a molecule of CO2 to RuBP is catalyzed by the enzyme RuBP carboxylase. The resulting 6C compound is highly unstable and gets cleaved to form two molecules of 3C compounds called phosphoglyceric acid (PGA).

Reduction

The two molecules of PGA are further reduced to glyceraldehyde-3¬ phosphates in two steps. First, two PGA molecules are converted to 1,3 -bisphosphoglyceric acids by the enzyme PGA kinase. This reaction consumes two molecules of ATP in the ratio of one ATP for each molecule of 1,3-bisphosphoglyceric acid formed.

In the second step, the two molecules of 1,3-bisphosphoglyceric acid are reduced to glyceraldehyde-3-phosphates by the enzyme glyceraldehyde¬3-phosphate dehydrogenase with the help of the light generated reducing power NADPH2. So, two molecules of NADPH2 will be consumed during this reaction. To reduce one molecule of CO2 upto reduction two ATP and two NADPH2 are consumed.

Regeneration of RuBP

The glyceraldehyde 3-phosphate molecules are converted to RuBP through a series of reactions, which generate 4C, 6C and 7C phosphorylated compounds as intermediates. For better and easy understanding of these reactions, a simplified scheme of Calvin cycle considering three CO2 molecules fixation reactions is shown below.

The reactions of regeneration of RuBP are as follows.

1. Some of the Glyceraldehyde 3-phosphate molecules are converted to dihydroxy acetone phosphates.

2. Glyceraldehyde 3-phosphate combines with dihydroxy acetone phosphate to form fructose1,6-bisphosphate.

3. Fructose 1,6-bisphosphate undergoes dephosphorylation to form fructose 6-phosphate.

4. Fructose 6-phosphate combines with glyceraldehyde 3-phosphate obtained from the fixation of second molecule of CO2 to form Ribose 5-phosphate (R5P) and Erythrose 4-phosphate (Er4P).

5. Erythrose 4-phosphate combines with DHAP obtained from the second CO2 fixation, to form sedoheptulose 1,7-bisphosphate.

6. Sedoheptulose 1,7-bisphosphate undergoes dephosphorylation to form sedoheptulose 7-phosphate.

7. Sedoheptulose 7-phosphate combines with glyceraldehyde 3-phosphate obtained by the third CO2 fixation, to form two molecules of 5C compounds – ribose 5-phosphate and xylulose 5-phosphate (Xy5P).

8. Ribose 5-phosphate and xylulose 5-phosphate molecules are transformed to ribulose 5-phosphate (Ru5P).

9. Ru5P molecules are then phosphorylated by ATP to form RuBP molecules, which again enter into the cycle of CO2 fixation.

In the above illustration, three CO2 molecules are fixed and the net gain is a 3C called DHAP. These triose phosphate molecules combine to form hexose phosphates, which are used to form sucrose. For every carbon fixation 3ATP and 2 NADPH2 are consumed