Which is true about CO2 usage for glucose synthesis?

Which is true about CO2 usage for glucose synthesis?

• A. Two CO2 molecules are required
• B. Four CO2 molecules are required
• C. Six CO2 molecules are required
• D. Eight CO2 molecules are required

Answer: (C)

This question is about how carbon from carbon dioxide ends up in glucose during photosynthesis. Glucose has six carbon atoms, so six carbon dioxide molecules must be fixed to supply those six carbons. In the Calvin cycle, each CO2 fixed contributes one carbon to the growing sugar skeleton, and the cycle must run six times to provide enough carbons to form one glucose molecule. The energy from the light-dependent reactions (ATP and NADPH) is used to drive the conversions that turn those fixed carbons into a hexose sugar, ultimately yielding glucose from two molecules of glyceraldehyde-3-phosphate. Because you need exactly six carbons for one glucose, six CO2 molecules are required. The other numbers would not supply the correct total of carbons for a single glucose.

This question is about how carbon from carbon dioxide ends up in glucose during photosynthesis. Glucose has six carbon atoms, so six carbon dioxide molecules must be fixed to supply those six carbons. In the Calvin cycle, each CO2 fixed contributes one carbon to the growing sugar skeleton, and the cycle must run six times to provide enough carbons to form one glucose molecule. The energy from the light-dependent reactions (ATP and NADPH) is used to drive the conversions that turn those fixed carbons into a hexose sugar, ultimately yielding glucose from two molecules of glyceraldehyde-3-phosphate. Because you need exactly six carbons for one glucose, six CO2 molecules are required. The other numbers would not supply the correct total of carbons for a single glucose.