Can you please describe the dark reaction of photosynthesis ?
Dear Student,
Dark reaction is also called as Calvin cycle, light-independent reactions and bio synthetic phase.
This reaction does not require direct light and occurs in the stroma of chloroplasts. Actually, calling it a dark reaction is a misnomer, but to differentiate it from the light reaction, it is conventionally called as dark reaction.
During this phase, ATP and NADPH2 (formed during light reactions) are utilized for the reduction of CO2 to carbohydrates (glucose).
The path of carbon in the Dark reaction was traced by Melvin Calvin using radioactive carbon (14C). The Calvin pathway occurs in all photosynthetic plants, irrespective of whether they have C3 or C3 pathway.
The primary acceptor of CO2 was found to be a 5-carbon ketose sugar called Ribulose bisphosphate (RuBP). This was surprising as it was believed that for the formation of a 3-carbon product, CO2 would have to be accepted by a 2-carbon compound.
RuBP is used in a cyclic manner (regenerated) and a sugar is synthesised.
The three stages of Calvin cycle: Carboxylation, Reduction and Regeneration.
Carboxylation :
Ribulose 1, 5-bisphosphate combines with CO2, and fixes it to a stable organic intermediate (3-phosphoglycerate – 2 molecules).
Reaction catalysed by the enzyme RuBisCO (RuBP Carboxylase-Oxygenase)
3 PGA is the first stable product of this cycle.
Reduction :
Here, 2 molecules each of ATP and NADPH are required for fixing 1 molecule of CO2.
This stage contains a series of reactions.
Glucose is formed as a result of this series of reactions.
Regeneration :
RuBP regenerates to enable the cycle to continue uninterrupted.
1 ATP molecule is required.
Relation between Light reactions and Calvin cycle:
Photosynthesis depends on an interaction between two sets of reactions: the light reactions and the Calvin cycle.
Chlorophyll and the other molecules responsible for the light reactions are built into the thylakoid membranes.
The enzymes that catalyze the Calvin cycle are located in the stroma.
Beginning with the absorption of light by chlorophyll, the light reactions convert light energy into chemical energy in the form of ATP and NADPH.
The ATP provides the energy, and the NADPH supplies the electrons for the Calvin cycle, which converts carbon dioxide to sugar.
The ADP and NADP+ that result from the Calvin cycle shuttle back to the light reactions, which regenerate ATP and NADPH.
Regards