Why are 6 molecules of CO\(_2\) required during the Calvin cycle to produce one molecule of glucose?

• A. Because six CO\(_2\) molecules are required to split water molecules during photosynthesis.
• B. Because each CO\(_2\) molecule is needed to regenerate ATP for the cycle.
• C. Because six CO\(_2\) molecules are needed to produce two molecules of pyruvate.
• D. Because each CO\(_2\) molecule contributes one carbon atom, and glucose contains six carbon atoms.

Answer: (D)

Verified step by step guidance

1. Understand the structure of glucose: Glucose (C\(_6\)H\(_{12}\)O\(_6\)) is a six-carbon sugar molecule. This means that six carbon atoms are required to form one molecule of glucose.
2. Recognize the role of CO\(_2\) in the Calvin cycle: Each molecule of CO\(_2\) contributes one carbon atom to the process of building glucose during the Calvin cycle.
3. Recall the process of carbon fixation: In the Calvin cycle, CO\(_2\) is fixed by the enzyme RuBisCO and incorporated into a 5-carbon molecule (ribulose-1,5-bisphosphate, or RuBP), forming a 6-carbon intermediate that quickly splits into two 3-carbon molecules.
4. Account for the number of CO\(_2\) molecules: Since each CO\(_2\) molecule provides one carbon atom, six CO\(_2\) molecules are required to provide the six carbon atoms needed to synthesize one glucose molecule.
5. Connect the Calvin cycle to glucose production: The Calvin cycle must run multiple times (specifically six times) to fix six CO\(_2\) molecules and produce the necessary intermediates that will eventually form one glucose molecule.