Question

How many turns of Calvin cycle are required for the formation of three molecules of glucose?

• A. 18
• B. 6
• C. 3
• D. 1

Hint:

Keep a handy note of the overall energetics and stoichiometry for the synthesis of one single molecule of glucose ($\text{C}_6\text{H}_{12}\text{O}_6$) in $C_3$ plants:

• $6 \text{ molecules of } \text{CO}_2 \rightarrow 6 \text{ turns of the cycle}$

• $18 \text{ molecules of ATP consumed}$

• $12 \text{ molecules of NADPH consumed}$
For $n$ molecules of glucose, simply multiply each of these standard baseline values by $n$.

Answer 1

The Correct Option is A

Concept: The Calvin cycle (also known as the $C_3$ pathway or the dark reaction of photosynthesis) occurs in the stroma of chloroplasts. The primary purpose of this cyclic pathway is to fix inorganic carbon dioxide ($\text{CO}_2$) into organic macromolecules like carbohydrates (glucose). The process is divided into three distinct phases:

• Carboxylation: Fixation of carbon dioxide onto RuBP (Ribulose-1,5-bisphosphate).

• Reduction: Utilization of assimilatory power (ATP and NADPH) to create glyceraldehyde-3-phosphate (G3P).

• Regeneration: Reconstruction of the $\text{CO}_2$ acceptor molecule RuBP so that the cycle can continue uninterrupted.

Step 1: Determine the carbon requirement per turn of the Calvin cycle
In a single turn of the Calvin cycle, exactly

one molecule of $\text{CO}_2$ enters the pathway and is fixed by the enzyme RuBisCO. \[ 1 \text{ Turn of Calvin Cycle} = 1 \text{ Carbon atom fixed from } \text{CO}_2 \]

Step 2: Calculate the turns needed for a single molecule of glucose
Glucose is a hexose sugar with the molecular formula $\text{C}_6\text{H}_{12}\text{O}_6$. A single molecule of glucose contains exactly

6 carbon atoms. To synthesize one molecule of glucose, 6 molecules of carbon dioxide must be fixed. Since each turn fixes one carbon atom, we can establish the fundamental ratio: \[ \text{Turns required for 1 Glucose molecule} = 6 \text{ turns} \]

Step 3: Compute the turns required for three molecules of glucose
The problem specifically asks for the number of turns required to manufacture

three molecules of glucose. Using a direct linear proportion: \[ \text{Total turns required} = (\text{Turns per glucose molecule}) \times (\text{Number of glucose molecules}) \] \[ \text{Total turns required} = 6 \times 3 = 18 \text{ turns} \] Thus, 18 complete turns of the Calvin cycle are mandatory to yield three molecules of glucose. This perfectly aligns with option (1).