The dark reaction of photosynthesis | Calvin Cycle

The dark reaction of photosynthesis

(Calvin Cycle) 

Dark reaction is the second phase of photosynthesis. It is a light-independent reaction and can occur day or night.

A very important cycle takes place there which uses the products of light reaction and forms glucose. This cycle is called the Calvin cycle.

The Calvin cycle has three phases:

1. Carboxylation
2. Reduction
3. Regeneration

1. The first step of the Calvin cycle is carboxylation. Carboxylation is the addition of carbon. A molecule of carbon dioxide comes from the atmosphere and adds with RUBP which is ribulose 1,5 bis-phosphate. This RUBP is a 5-carbon compound. This will result in to form of PGA which is phosphoglyceric acid. This PGA is 3 carbon compound. Two molecules of PGA are produced in carboxylation.
2. Now, the second step is the reduction of two PGA molecules. NADPH and ATP molecules are used in the reduction process. NADPH is converted into NADP and ATP is reduced to ADP for the reduction of PGA and we get a triose phosphate, which is a carbon sugar compound. Two PGA molecules undergo reduction, so two NADPH and ATP are used to form two molecules of triose phosphate.
3. Now, the last step is regeneration is taking place. In this step, RUBP regenerates itself from a sugar compound that is triose phosphate. In this step, an ATP molecule is reduced to ADP. One carbon is removed and the remaining 5 carbon from the sugar compound is changed into 5-carbon RUBP.

This cycle just fixed a single carbon and we noticed that three molecule of ATP and two molecules of NADPH is used for the fixation of a single carbon into sugar.

We want to prepare a glucose molecule in the Calvin cycle and we know that glucose is 6 carbon compound. So for this purpose, 6 molecules of carbon dioxide bind with RUBP and form PGA molecules. 

For the fixation of one carbon dioxide, one RUBP is reacting and for the fixation of 6 molecules of carbon dioxide, 6 RUBP will react, which means 6 molecules of 5 carbon compounds have 30 carbons with them. Total PGA molecules have 36 carbons in them.

This same calculation is applied in the case of reduction and regeneration. The numbers of ATP and NADPH have also increased accordingly to the fixation of 6 molecules of carbon dioxide and forming a single glucose molecule.

18 ATP molecules and 12 NADPH molecules are required for a single glucose molecule.

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