How many molecules of CO2 are needed to make one glucose molecule?

• A. Four
• B. Five
• C. Six
• D. Eight

Answer: (C)

The number of CO2 molecules needed matches the number of carbon atoms in glucose. Glucose has six carbon atoms, and each CO2 molecule provides one carbon. So six CO2 molecules are required to build one glucose molecule in photosynthesis (6 CO2 + 6 H2O → C6H12O6 + 6 O2). The other counts would not supply enough or would supply too many carbons.