Describe the C3 cycle of carbon fixation in plants.

1.	Describe the C3 cycle of carbon fixation in plants.
Answer» (a) C₃ or Calvin cycle proceeds in three stages Carboxylation, Reduction and Regeneration. (b) Carboxylation is the first stage and involves fixation of CO₂ into a stable organic intermediate. (c) In plants, atmospheric CO₂enters the mesophyll layer of leaves by passing through pores on the leaf surface called stomata. It can then diffuse into mesophyll cells, and into the stroma of chloroplasts, where the Calvin cycle takes place. (d) In the first step of the cycle, an enzyme RUBisCo (RuBP carboxylase-oxygenase) catalyzes attachment of CO₂ to a five-carbon sugar called Ribulose bisphosphate (RuBP). (e) The resulting 6-carbon molecule is unstable, however, and quickly splits into two molecules of a three-carbon compound called 3-phosphoglycerate (3-PGA). Thus, for each CO₂ that enters the cycle, two 3-PGA molecules are produced.

Answer»

(a) C₃ or Calvin cycle proceeds in three stages Carboxylation, Reduction and Regeneration.

(b) Carboxylation is the first stage and involves fixation of CO₂ into a stable organic intermediate.

(c) In plants, atmospheric CO₂enters the mesophyll layer of leaves by passing through pores on the leaf surface called stomata. It can then diffuse into mesophyll cells, and into the stroma of chloroplasts, where the Calvin cycle takes place.

(d) In the first step of the cycle, an enzyme RUBisCo (RuBP carboxylase-oxygenase) catalyzes attachment of CO₂ to a five-carbon sugar called Ribulose bisphosphate (RuBP).

(e) The resulting 6-carbon molecule is unstable, however, and quickly splits into two molecules of a three-carbon compound called 3-phosphoglycerate (3-PGA). Thus, for each CO₂ that enters the cycle, two 3-PGA molecules are produced.

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