 This is the simplest version of photosynthesis and respiration that we can come up with. This is a plant. It stands in for all the green plants. And it takes CO2 from the air, and it takes water, and it puts those together, but that requires energy from the sun to make the chemical bonds, which give us plant, as shown here as CH2O. They're just sort of the formula of plant, and it releases oxygen to the air. Now animals and bacteria and fungi and all these other things take plant, and they take oxygen, and they burn them to get energy so they can run around and do things, and that releases the CO2 and H2O. Fires also do this, but they don't do it in quite so controlled a manner.