In the Calvin cycle of photosynthesis, what happens to carbon from $C_0{O}_2$?

• A. It is fixed to ribulose-1,5-bisphosphate (RuBP) by Rubisco and ultimately reduced to form carbohydrates such as glyceraldehyde-3-phosphate (G3P).
• B. It is used to directly produce NADPH by accepting electrons from photosystem I.
• C. It is converted into water and released as a waste product through stomata.
• D. It is split during the light reactions to release $O_2$ and generate ATP.

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 (CO\_2) is incorporated into organic molecules.
2. Recognize that the enzyme Rubisco catalyzes the fixation of CO\_2 by attaching it to a 5-carbon sugar called ribulose-1,5-bisphosphate (RuBP), forming an unstable 6-carbon intermediate.
3. Know that this 6-carbon intermediate quickly splits into two molecules of 3-phosphoglycerate (3-PGA), which are then phosphorylated and reduced using ATP and NADPH produced in the light reactions.
4. Follow that the reduction of 3-PGA leads to the formation of glyceraldehyde-3-phosphate (G3P), a 3-carbon sugar that can be used to synthesize glucose and other carbohydrates.
5. Conclude that the carbon from CO\_2 is thus fixed and converted into carbohydrate molecules through a series of enzymatic steps in the Calvin cycle, rather than being used directly to produce NADPH, converted into water, or split to release oxygen.