Question

Describe in general terms how the five-carbon precursor to the production phase of the calvin cycle is regenerated from three carbon products of the cycle

Answer 1

The carbon fixation reaction leads to carbon dioxide reduction and fixation. It is also called the Calvin cycle or reductive pentose phosphate pathway or cycle. Calvin cycle is an anabolic process that occurs during Photosynthesis and leads to the formation of sugars. It is a light-independent cycle that takes place in the stroma of the chloroplast and dependent on the products of the light reaction (ATP and NADPH) for the formation of sugars. NADPH from light reaction gets oxidized and ATP provides chemical energy during the Calvin cycle. Carbon dioxide from the environmententers the cycleand contributes to the formation of sugar. It is a series of biochemical reaction which reduces carbon dioxide to three-carbon sugar. This cycle occurs in the stroma of the chloroplast. The main function of this cycle is to produce the carbohydrate with the help of products produced during light reactions that are ATP and NADPH. But the carbohydrate produced during this reaction is not glucose. It is actually a three-carbon molecule of sugar named glyceraldehyde-3-phosphate. For the net production of one molecule of glyceraldehyde-3-phosphate, three molecules of carbon dioxide are preferred.

The Calvin cycle occurs after the light reaction. In this cycle, there occurs a series of reactions which uses carbon dioxide from the surroundings to produce carbohydrate. The Calvin cycle includes three major steps. These steps are: -

• Carbon dioxide fixation
• Reduction
• Regeneration of RuBP (ribulose-1,5-bisphosphate)

The figure of the Calvin cycle is given below:-

Calvin cycle occurs after light reaction and dependent on the products of light reaction that are ATP and NADPH. The main function of this process is to produce carbohydrate molecules. To produce a six-carbon molecule (glucose), this cycle requires 18 ATP and 12 NADPH.

The first step during the Calvin cycle is carbon dioxide fixation. During carboxylation, carbon dioxide is affiliate into RuBP (ribulose-1,5-bisphosphate) which is a five-carbon molecule. The enzyme that catalyzes this step is RuBisCo (ribulose-1,5-bisphosphate carboxylase oxygenase). The product of this reaction is an intermediate molecule of six carbon which generally splits to form two molecules of 3-phosphoglycerate. Thus, for every molecule of carbon dioxide, six molecules of 3-phosphoglycerate formed.

The carbon dioxide fixation occurs when carbon dioxide affiliates with RuBP (ribulose-1,5-bisphosphate). During carboxylation, carbon dioxide is incorporated into RuBP (ribulose-1,5-bisphosphate) which is a five-carbon molecule. The enzyme that catalyzes this step is RuBisCo (ribulose-1,5-bisphosphate carboxylase oxygenase). The reaction for this is as given below-