Just as an internal combustion engine requires fuel to do work, animals need fuel to power their body processes. Animals take in complex molecules as food and break them down to release the energy they contain. This process is called "catabolism." Animals use the energy of catabolism to do work and to assemble complex molecules of their own from simple building blocks, a process called "anabolism." The sum of anabolism and catabolism is "metabolism," a broad term that includes all chemical reactions in the body.

Living Systems Follow the Rules of Thermodynamics

Living organisms are extremely complex. Perhaps this is the reason we often forget that all animals, including people, are made up entirely of chemicals and that these chemicals react with each other according to the same rules that govern chemical reactions in test tubes. Indeed, as...