What provides the carbon atoms that are incorporated into sugar molecules during the Calvin cycle?

• A. Water (\(\mathrm{H_2O}\))
• B. Oxygen (\(\mathrm{O_2}\))
• C. Glucose (\(\mathrm{C_6H_{12}O_6}\))
• D. Carbon dioxide (\(\mathrm{CO_2}\))

Answer: (D)

Verified step by step guidance

1. Understand the Calvin cycle: The Calvin cycle is a series of biochemical reactions that occur in the stroma of chloroplasts during photosynthesis. Its primary function is to synthesize sugar molecules using carbon atoms from carbon dioxide (\(\mathrm{CO_2}\)).
2. Identify the source of carbon atoms: During the Calvin cycle, carbon dioxide (\(\mathrm{CO_2}\)) is the molecule that provides the carbon atoms. These carbon atoms are incorporated into organic molecules through a process called carbon fixation.
3. Examine the role of carbon fixation: In the first step of the Calvin cycle, an enzyme called RuBisCO catalyzes the reaction between carbon dioxide (\(\mathrm{CO_2}\)) and ribulose-1,5-bisphosphate (RuBP), forming two molecules of 3-phosphoglycerate (3-PGA). This is the step where carbon atoms from \(\mathrm{CO_2}\) are integrated into the cycle.
4. Understand why other options are incorrect: Water (\(\mathrm{H_2O}\)) provides electrons during the light-dependent reactions, not carbon atoms. Oxygen (\(\mathrm{O_2}\)) is a byproduct of the light-dependent reactions and does not contribute carbon. Glucose (\(\mathrm{C_6H_{12}O_6}\)) is the product of photosynthesis, not the source of carbon atoms.
5. Conclude the correct answer: Carbon dioxide (\(\mathrm{CO_2}\)) is the correct answer because it is the molecule that supplies the carbon atoms used to build sugar molecules during the Calvin cycle.