In the photosynthesis equation, how many carbon dioxide molecules are used?

• A. One
• B. Six
• C. Twelve
• D. Three

Answer: (B)

The number of carbon dioxide molecules used is tied to the carbon atoms in the sugar produced. In the standard balanced equation for photosynthesis, one molecule of glucose (which has six carbon atoms) is built from six carbon atoms supplied by six carbon dioxide molecules. So six CO2 molecules are consumed for each glucose formed. The other numbers wouldn’t provide the six carbon atoms needed for one glucose, which is why they don’t fit the equation’s stoichiometry.