In the Calvin cycle, what is the basic role of CO2 in photosynthesis?

• A. It provides the carbon atoms that are fixed into organic molecules (e.g., sugars) during carbon fixation.
• B. It is split to release O2 during the light reactions at photosystem II.
• C. It donates high-energy electrons directly to NADP+ to form NADPH.
• D. It acts as the final electron acceptor in the thylakoid electron transport chain.

Answer: (A)

Verified step by step guidance

1. Understand that the Calvin cycle is the set of light-independent reactions in photosynthesis where carbon dioxide (CO2) is incorporated into organic molecules.
2. Recognize that CO2 enters the Calvin cycle during the process called carbon fixation, where the enzyme RuBisCO attaches CO2 to a 5-carbon sugar called ribulose bisphosphate (RuBP).
3. Know that this fixation of CO2 results in a 6-carbon intermediate that quickly splits into two 3-carbon molecules, which are then used to build sugars and other organic compounds.
4. Distinguish that CO2 is not involved in the light reactions where oxygen (O2) is released, nor does it donate electrons directly to NADP+ or act as an electron acceptor in the electron transport chain.
5. Conclude that the basic role of CO2 in photosynthesis is to provide the carbon atoms that are fixed into organic molecules during the Calvin cycle.