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Question
Describe C3 pathway of photosynthesis. How does it differ from C4 pathway?
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Solution
C3 cycle is also known as Calvin cycle as it was discovered by Melvin Calvin while radioactive studies of alga. It occurs in all photosynthetic plants whether they have C3 or C4 pathway.C3 cycle can be described under three stages:- Carboxylation: It is the fixation of CO2 into a stable organic intermediate. In this, CO2 is utilised for the carboxylation of RuBP. This reaction is catalysed by the enzyme RuBisCO and it results in the formation of two molecules of 3-PGA (3-Phosphoglyceric acid).
- Reduction: These reactions lead to the formation of glucose. The steps involve utilisation of two molecules of ATP for phosphorylation and two of NADPH for reduction, per molecule of CO2 fixed. The fixation of six molecules of CO2 and six turns of the cycle are required for the removal of one molecule of glucose from the pathway.
- Regeneration: For the cycle to continue uninterrupted, regeneration of the CO2 acceptor molecule is crucial. This step requires one ATP for phosphorylation to form RuBP. To make one molecule of glucose, six turns of the cycle are required. 18 ATP and 12 NADPH molecules are used to make a molecule of glucose. Hence, for every CO2 molecule entering the Calvin cycle, three molecules of ATP and two molecules of NADPH are required.
- In C3 pathway, the primary acceptor of CO2 is RuBP - a six carbon compound, whereas, in C4 pathway, the primary acceptor of CO2 is PEP - a three-carbon compound.
- In C3 pathway, first, the stable product is 3-phosphoglycerate, whereas in C4 pathway, first, the stable product is oxaloacetic acid.
- C3 pathway occurs in the mesophyll cells of the leaves, whereas C4 pathway occurs in the mesophyll and bundle-sheath cells of the leaves.
- C3 pathway has the slower process of carbon-fixation as compared to C4 pathway.
- In C3 pathway, 3 ATP are consumed to fix one CO2, whereas in C4 pathway, 2 ATP are consumed to fix one CO2.
C3 cycle can be described under three stages:
- Carboxylation: It is the fixation of CO2 into a stable organic intermediate. In this, CO2 is utilised for the carboxylation of RuBP. This reaction is catalysed by the enzyme RuBisCO and it results in the formation of two molecules of 3-PGA (3-Phosphoglyceric acid).
- Reduction: These reactions lead to the formation of glucose. The steps involve utilisation of two molecules of ATP for phosphorylation and two of NADPH for reduction, per molecule of CO2 fixed. The fixation of six molecules of CO2 and six turns of the cycle are required for the removal of one molecule of glucose from the pathway.
- Regeneration: For the cycle to continue uninterrupted, regeneration of the CO2 acceptor molecule is crucial. This step requires one ATP for phosphorylation to form RuBP. To make one molecule of glucose, six turns of the cycle are required. 18 ATP and 12 NADPH molecules are used to make a molecule of glucose. Hence, for every CO2 molecule entering the Calvin cycle, three molecules of ATP and two molecules of NADPH are required.
- In C3 pathway, the primary acceptor of CO2 is RuBP - a six carbon compound, whereas, in C4 pathway, the primary acceptor of CO2 is PEP - a three-carbon compound.
- In C3 pathway, first, the stable product is 3-phosphoglycerate, whereas in C4 pathway, first, the stable product is oxaloacetic acid.
- C3 pathway occurs in the mesophyll cells of the leaves, whereas C4 pathway occurs in the mesophyll and bundle-sheath cells of the leaves.
- C3 pathway has the slower process of carbon-fixation as compared to C4 pathway.
- In C3 pathway, 3 ATP are consumed to fix one CO2, whereas in C4 pathway, 2 ATP are consumed to fix one CO2.
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