No, although many cells use glucose as their primary energy source, lipids and proteins can also be broken down to provide energy as well. Lipids are broken down into simpler and more stable substances (catabolized) into monomers (a structural unit of a polymer), glycerol, and fatty acids, which are then metabolized during cell respiration. Proteins are also catabolized to their amino acid building blocks, which are then fed into the process of glycolysis, also known as the Krebs cycle (for more about the Krebs cycle, see the chapter “Basics of Biology”).