Question

24 PGAL molecules are formed in $C_3$ plant. Calculate the number of $CO_2$ used and the number of G3P used for regeneration of RUBP molecules?

• A. 12 $CO_2$ & 24 G3P
• B. 24 $CO_2$ & 48 G3P
• C. 12 $CO_2$ & 20 G3P
• D. 20 $CO_2$ & 12 G3P

Hint:

Remember the ratio for gross PGAL production: For every 6 PGAL formed $\rightarrow$ 3 $CO_2$ in $\rightarrow$ 5 PGAL regenerate RuBP $\rightarrow$ 1 PGAL net gain.

Answer 1

The Correct Option is C

Step 1: Stoichiometry of the Calvin Cycle:
The standard Calvin Cycle equation for the production of PGAL (G3P) is: \[ 3 CO_2 \rightarrow 6 \text{ PGAL (Total Formed)} \] Out of these 6 PGAL:

• 1 PGAL is the Net Gain (used for glucose synthesis).
• 5 PGAL are used for the **Regeneration** of 3 RuBP molecules.

Step 2: Calculate for 24 PGAL Formed:
The question states that **24 PGAL molecules are formed** (Gross production).

• Scale Factor: $24 \text{ PGAL} / 6 \text{ PGAL (per cycle set)} = 4$.
• $CO_2$ Used: $3 \text{ CO}_2 \times 4 = 12 CO_2$.
• G3P (PGAL) for Regeneration: $5 \text{ PGAL} \times 4 = 20 G3P$.

Step 3: Final Answer:
12 $CO_2$ and 20 G3P. This matches Option (C).