An animal or plant obtain energy form sugars or other organic molecules by allowing the carbon and hydrogen atoms in these molecules to become oxidized—to produce CO2 and H20 respectively—in process called cellular respiration. Plants use Photosynthesis, which uses the energy of sunlight to produce sugars and other organic molecules from the carbon atoms in CO2 in the atmosphere. The plants serve as food for animals, which uses O2 to oxidize food molecules, releasing the same carbon atoms in the form of CO2 back to the atmosphere. These processes are done by the oxidation of organic molecules to produce energy.
