Question

How many ATP and $NADPH$ molecules are required to make one molecule of glucose through the Calvin pathway?

• A. 18 ATP and 12 NADPH
• B. 6 ATP and 12 NADPH
• C. 24 ATP and 18 NADPH
• D. 12 ATP and 18 NADPH

Hint:

Remember the 3:2 ratio. For every single Carbon atom you want to "glue" into a sugar, you need 3 ATP "batteries" and 2 NADPH "reducing agents."

Answer 1

The Correct Option is A

The Calvin cycle (the dark reaction of photosynthesis) uses the energy stored in ATP and $NADPH$ to fix atmospheric $CO_2$ into sugar. To build one molecule of glucose ($C_6H_{12}O_6$), the cycle must turn six times. The energy "cost" breakdown per single $CO_2$ molecule fixed is:
• Reduction Phase: For every $CO_2$ that enters, two molecules of 3-PGA are converted to G3P. This requires 2 ATP and 2 NADPH.
• Regeneration Phase: To keep the cycle going, the $CO_2$ acceptor (RuBP) must be regenerated from G3P. This requires an additional 1 ATP.
• Total per $CO_2$: 3 ATP and 2 $NADPH$. Since one glucose molecule contains six carbon atoms, we multiply these values by six:
• Total ATP: $6 \text{ turns} \times 3 \text{ ATP/turn} = \mathbf{18 \text{ ATP}}$
• Total NADPH: $6 \text{ turns} \times 2 \text{ NADPH/turn} = \mathbf{12 \text{ NADPH}}$