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Question : Describe Calvin cycle. OR C_(3) pathway.

Answer»

Solution :Calvin and his co-workes worked out the whole pathway and showed that the pathway operated in a cyclic manner, the RuBP was regenerated.
Calvin pathway occurs in all photosynthetic plants, it does not matter whether they have `C_(3) or C_(4)` (or any other) pathways.
Calvin cycle can be described under three stages : (1) carboxylation (2) reduction (3) regeneration.
(1) Carboxylation : Carboxylation is the fixation of `CO_(2)` into a stable ORGANIC intermediate.
Carboxylation is the most crucial STEP of the Calvin cycle where `CO_(2)` is utillsed for the carboxylation of RuBP.
This reaction is catalysed by the enzyme RuBP carboxylase which results in the formation of two molecules of 3 - PGA.
Since this enzyme also has an oxygenation activity it would be more correct to call it RuBP carboxylase - oxygenase or RuBisCO.
(2) REDUTION : These are a SERIES of reactions tha lead to the formation of glucose.
The steps invole utilisation of 2 molecules of ATP for phosphorylation and two of NADPH for reduction per `CO_(2)` molecule fixed.
`2PGA + 2ATP + 2NADPH rarr 2` Triose phosphate (2 PGAC)
The fixation of six molecules of`CO_(2)` and 6 turns of the cycle are required for the REMOVAL of one molecule of glucose from the pathway.

Regeneration : Regeneration of the `CO_(2)` acceptor molecule RuBP is crucial if the cycle is to continue uninterrupted. The regeneration steps require one ATP for `PGAL overset(ATP) rarr RuBP`
Hence for every `CO_(2)` molecule entering the Calvin , 3 molecules of ATP and 2 of NADPH are required.
It is probably to meet this difference in number of ATP and NADPH used in the dark reaction that the cyclic phosphorylation takes place.
To make one molecule of glucose 6 truns of the cycle are required.


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