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

Step 1 : Understanding the Question
This question focuses on the bioenergetics of the Calvin cycle, specifically the total amount of energy (ATP) and reducing power (NADPH) required to synthesize one molecule of glucose.
Step 2 : Key Formulas and approach
The approach is based on the stoichiometry of carbon fixation. To produce one molecule of glucose ($C_6H_{12}O_6$), six molecules of $CO_2$ must be processed. Total Requirement = $6 \times (\text{Requirement per } CO_2 \text{ molecule})$.
Step 3 : Detailed Explanation

Per CO$_2$ Fixed: To fix a single molecule of $CO_2$, the Calvin cycle consumes 2 molecules of ATP and 2 molecules of NADPH during the reduction phase.

Regeneration Phase: An additional 1 molecule of ATP is required to regenerate the $CO_2$ acceptor (RuBP) so that the cycle can continue.

Single Turn Total: Total per $CO_2 = 3 \text{ ATP and } 2 \text{ NADPH}$.

Calculation for Glucose: Glucose is a 6-carbon sugar. To synthesize one glucose molecule, the cycle must turn six times to fix 6 $CO_2$ molecules.

Final Totals: Total ATP = $6 \times 3 = 18$ ATP. Total NADPH = $6 \times 2 = 12$ NADPH.

Step 4 : Final Answer
The synthesis of one glucose molecule requires 18 ATP and 12 NADPH molecules. Thus, the correct option is (A).