What is the fate of the carbon in the carbon dioxide used in the dark stage of photosynthesis?

• A. Makes carbohydrate
• B. Becomes carbonic acid
• C. Released as methane
• D. Is stored as starch only

Answer: (A)

In the dark stage, carbon dioxide is fixed by the Calvin cycle to build organic molecules. The carbon from CO2 is ultimately incorporated into carbohydrate molecules—starting from glyceraldehyde-3-phosphate and then forming sugars like glucose. Those sugars can be used to make starch for storage in the chloroplast or to produce sucrose for transport to other parts of the plant. So the fate of the carbon is to become carbohydrate. It isn’t stored as carbonic acid, released as methane, or confined to starch alone.