They use this energy for maintaining their bodies, breathing, digesting, moving, growing, and reproducing. Remember that in the photosynthesis section we saw how plants build themselves by capturing the carbon in carbon dioxide to serve as a building material. Plants take in carbon dioxide gas, which is made up of molecules containing one carbon atom and two oxygen atoms. Plant tissues are classified as carbohydrates because they are made up of a good proportion of various sugar molecules containing carbon atoms. However, animals are made up of mostly of proteins.
Proteins also contain carbon, but they are made up of molecules of amino acids.
When animals eat plants, they take in more carbon atoms than they need to build themselves. Adding oxygen to carbon very quickly is what happens when wood or paper or grass burns in a fire. Animals combine carbon and oxygen more slowly, and at a lower temperature, so they do not burst into flames. Here is a diagram of the respiration process. It looks like the photosynthesis diagram, but the reaction is going in the other direction now.