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

Step 1: Read the demand carefully.
We must find how many turns of the Calvin cycle are needed to build three molecules of glucose.
Step 2: Recall what one turn fixes.
In one single turn of the Calvin cycle, exactly one molecule of carbon dioxide is fixed, which means one carbon atom is added to the pool.
Step 3: Count carbons in glucose.
Glucose has the formula $C_6H_{12}O_6$, so it carries 6 carbon atoms.
Step 4: Turns for one glucose.
Since each turn brings in 1 carbon and glucose needs 6 carbons, we need 6 turns to make one molecule of glucose.
Step 5: Scale up to three glucose molecules.
For three glucose molecules we simply multiply: $6 \times 3 = 18$ turns.
Step 6: State the final number.
Therefore 18 complete turns of the Calvin cycle are required, which matches the first option.
\[ \boxed{18} \]