Photosynthesis is the process by which plants use light energy to convert carbon dioxide and water into glucose (sugar) and oxygen. The chemical reaction is: 6CO2 + 6H2O + light energy → C6H12O6 (glucose) + 6O2. Cellular respiration is the process by which organisms break down glucose to release energy. The chemical formula for respiration is: C6H12O6 + 6O2 → 6CO2 + 6H2O + energy. Reactants are the starting substances in a chemical reaction, while products are the substances formed.