How many CO2 molecules are required to synthesize one molecule of glucose in photosynthesis?

How many CO2 molecules are required to synthesize one molecule of glucose in photosynthesis?

• A. Six CO2.
• B. Twelve CO2.
• C. Three CO2.
• D. Nine CO2.

Answer: (A)

The main idea is that glucose is a six-carbon sugar, so assembling one glucose molecule in photosynthesis requires six carbon atoms from carbon dioxide. In the Calvin cycle, carbon dioxide is fixed one molecule at a time, producing three-carbon sugars that are later turned into glyceraldehyde-3-phosphate. To build a six-carbon glucose, you need six carbons, which means fixing six CO2 molecules. Two of the resulting three-carbon units exit the cycle to be combined into one glucose molecule, while the rest are used to regenerate the acceptor molecule for the next round. The energy and reducing power needed to drive these steps come from the light reactions, but the stoichiometry for forming one glucose from CO2 is six CO2 molecules fixed.

The main idea is that glucose is a six-carbon sugar, so assembling one glucose molecule in photosynthesis requires six carbon atoms from carbon dioxide. In the Calvin cycle, carbon dioxide is fixed one molecule at a time, producing three-carbon sugars that are later turned into glyceraldehyde-3-phosphate. To build a six-carbon glucose, you need six carbons, which means fixing six CO2 molecules. Two of the resulting three-carbon units exit the cycle to be combined into one glucose molecule, while the rest are used to regenerate the acceptor molecule for the next round. The energy and reducing power needed to drive these steps come from the light reactions, but the stoichiometry for forming one glucose from CO2 is six CO2 molecules fixed.