In the Calvin cycle of photosynthesis, what is the primary role of $C_0{C}_2$ (carbon dioxide)?

• A. It donates high-energy electrons to photosystem II during the light reactions.
• B. It is split to release $O_2$ as a byproduct.
• C. It directly generates ATP by driving chemiosmosis across the thylakoid membrane.
• D. It is fixed by Rubisco and incorporated into organic molecules that can be used to form sugars.

Answer: (D)

Verified step by step guidance

1. Understand that the Calvin cycle is the set of light-independent reactions in photosynthesis where carbon dioxide is converted into organic molecules.
2. Recognize that carbon dioxide (CO\_2) enters the Calvin cycle and is 'fixed' by the enzyme Rubisco, meaning it is attached to an organic molecule to form a stable intermediate.
3. Know that this fixation process incorporates CO\_2 into a 5-carbon sugar called ribulose bisphosphate (RuBP), producing two molecules of 3-phosphoglycerate (3-PGA).
4. Realize that these organic molecules are then used in subsequent steps of the Calvin cycle to eventually form glucose and other sugars.
5. Note that the other options describe processes related to the light reactions or oxygen evolution, which are separate from the role of CO\_2 in the Calvin cycle.