In the standard photosynthesis equation, how many water molecules are used with six carbon dioxide molecules to produce sugar?

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

Answer: (A)

The amount of water is tied to the balance of atoms in the photosynthesis equation. In the standard overall reaction, six molecules of carbon dioxide react with six molecules of water to produce one molecule of glucose and six molecules of oxygen. The balanced form is 6 CO2 + 6 H2O -> C6H12O6 + 6 O2. Water donates the hydrogen and oxygen that end up in glucose, and its electrons fuel the reduction of CO2 during the light-dependent steps. If you used fewer or more water molecules, the hydrogen and oxygen atoms wouldn’t balance to form glucose and oxygen, so six water molecules are required.