Question

Choose the incorrect statement about Calvin cycle:

• A. It begins with the carboxylation of $\mathrm{RuBP}$
  
• B. 3-Phosphoglycerate is the first ${\mathrm{CO}}_2$ fixation product
  
• C. One, out of six $3-\mathrm{PGA}$ molecules, is used for production of glucose
  
• D. PEP carboxylase fixes ${\mathrm{CO}}_2$ in $C_3$ cycle
  

Answer 1

The correct option is D PEP carboxylase fixes ${\mathrm{CO}}_2$ in $C_3$ 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 ${\text{C}}_3$ cycle are required for the production of a single glucose molecule.

Carboxylation: Six molecules of ${\text{CO}}_2$ are integrated into six molecules of $\mathrm{RuBP}$ to form a six-carbon intermediate product with the help of $\text{RuBisCO}$ enzyme. This is immediately broken down into twelve molecules of 3-Phosphoglycerate ($\text{3-PGA}$) which is a three-carbon compound.

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

Regeneration: Out of the twelve molecules of glyceraldehyde-3-phosphate ($\text{G-3-P}$), two molecules form glucose and the remaining ten molecules of this 3-carbon compound are used to generate six molecules of $\mathrm{RuBP}$ (5-carbon compound). As $\mathrm{RuBP}$ 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 $\text{3-PGA}$ (which further gets reduced to form twelve molecules of $\text{G-3-P}$), two molecules are used for the formation of glucose. This is similar to the utilisation of one molecule of $\text{3-PGA}$ for the glucose production, out of the six $\text{3-PGA}$ molecules.

Hence, the first three statements are correct.