Dark reaction

"Dark reaction" of photosynthesis

"Dark reaction" are enzyme-catalyzed biochemical process, which occur in the stroma of chloroplast. The name "dark reaction" was given in thought, that the process occur in the darkness. But as now, it is established that, the biochemical reactions in the stroma are directly dependent upon the product of light reaction and both the processes are complimentary. Either of the processes can not be operated for longer time in alone. Hence, it is better to call the dark reaction as "carbon reaction" of photosynthesis.
During carbon reaction CO₂ is reduced into carbohydrate ((CH₂O)_n) in the stroma. The reducing equivalent and energy are provided by NADPH and ATP (synthesized on stromal face of thylakoid during light reaction) respectively.

Calvin–Benson cycle is ultimate process for the biological reduction of CO₂ into carbohydrate

In all the green plants CO₂ is reduced into carbohydrate ((CH₂O)_n) through the Calvin-Benson cycle only. The cycle consists of a series of reaction, that was discovered by the M. Calvin, A. Benson, J. A. Bassham and their colleagues during 1950s for which it is also known as "Calvin cycle". In this cycle, the first stable intermediate is a 3-carbon acid (3, phosphoglyceric acid), hence also named as "C3 cycle". During the process, CO₂ is reduced from +4 oxidation state to 0. Hence the process is also known as "reductive pentose phosphate cycle" or "photosynthetic carbon reduction cycle".

The Calvin-Benson cycle or C3 cycle

Calvin–Benson cycle takes place in the stroma (liquid phase) of chloroplast. This cycle has three processes:

• Carboxylation
• Reduction
• Regenereation

Calvin cycle (image source: Taiz, L., Zeiger, E., Møller, I.M. and Murphy, A., 2015. Plant physiology and development (No. Ed. 6). Sinauer Associates Incorporated.)

Carboxylation

Carboxylation is the very first reaction where, CO₂ is added to the acceptor of second carbon of CO₂ Ribulose 1,3, bisphosphate (a five carbon-monosaccharide) to produced two molecules of 3, phosphoglycerate (a three-carbon acid). The reaction is catalyzed by enzyme Ribulose bisphosphate carboxylase oxygenase (RubisCO).
RubisCO is the most abundant enzyme on earth. It has properties of both carboxylation and oxygenation.

Reduction

During reduction 3, phosphoglycerate is reduced to glyceraldehyde 3 phosphate (a three-carbon aldose monosaccharide). This is a two-steps reaction, where, reducing equivalent is provided by the oxidation of NADPH and energy comes from hydrolysis of ATP.

Regeneration

CO₂ acceptor ribulose 1,3, bisphosphate is regenerated from glyceraldehyde 3 phosphate via a series of enzyme-catalyzed reactions. During regeneration process a number of intermediates including 4 to 7 carbon monosaccharides are produced.

Fixation of 1 molecule of CO₂ through Calvin-Benson cycle requires 3ATP and 2 NADPH.

In a balanced Calvin–Benson cycle 3 molecules of CO₂ are accepted by 3 molecules of Ribulose 1,3, bisphosphate to produce 6 molecule of glyceraldehyde 3 phosphate (total 18 carbon). One molecule of glyceraldehyde 3 phosphate leaves the cycle and thus 3 carbon are fixed. Remaining 15 carbon in the form of 5 molecules of glyceraldehyde 3 phosphate converted into 3 molecules of Ribulose 1,3, bisphosphate in regeneration step are ready to proceed another cycle.
Operation of one Calvin–Benson cycle requires 9 molecules of ATP (6 in reduction and 3 in regeneration) plus 6 molecules of NADPH (in reduction). This yield one molecule of triose phosphate (3 carbon of CO₂ are fixed)
Thus, for fixation of one carbon through Calvin-Benson cycle requires 3ATP and 2NADPH.

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First published on 13-04-2021
Last updated on 26-04-2021