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Question

Choose the incorrect statement about Calvin cycle:

A
It begins with the carboxylation of RuBP
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B
3-Phosphoglycerate is the first CO2 fixation product
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C
One, out of six 3PGA molecules, is used for production of glucose
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D
PEP carboxylase fixes CO2 in C3 cycle
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Solution

The correct option is D PEP carboxylase fixes CO2 in C3 cycle

Calvin cycle is the carbon reduction cycle and it has three stages:
• Carboxylation
• Reduction
• Regeneration

Since glucose is a six-carbon molecule, six carbon dioxide molecules need to be fixed for the formation of one glucose molecule. Therefore, six turns of C3 cycle are required for the production of a single glucose molecule.

Carboxylation: Six molecules of CO2 are integrated into six molecules of RuBP to form a six-carbon intermediate product with the help of RuBisCO enzyme. This is immediately broken down into twelve molecules of 3-Phosphoglycerate (3-PGA) which is a three-carbon compound.

Reduction: The twelve molecules of 3-phosphoglycerate (3-PGA) are reduced to twelve molecules of glyceraldehyde-3-phosphate (G-3-P) in the presence of ATP and NADPH.

Regeneration: Out of the twelve molecules of glyceraldehyde-3-phosphate (G-3-P), two molecules form glucose and the remaining ten molecules of this 3-carbon compound are used to generate six molecules of RuBP (5-carbon compound). As RuBP was given by the cell, it should be replenished back to the cell for the cycle to keep going.

Hence, out of the twelve molecules of 3-PGA (which further gets reduced to form twelve molecules of G-3-P), two molecules are used for the formation of glucose. This is similar to the utilisation of one molecule of 3-PGA for the glucose production, out of the six 3-PGA molecules.

Hence, the first three statements are correct.

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