A Lipid is a fatty/waxy organic compound that is insoluble to polar solvents, such as water. The main biological function of a lipid is energy storage. Other functions include being a structural component of the cell membrane, and cell signaling. Butter is an example of a lipid that is used for energy storage. Once broken down in the body, the energy obtained can be used when the body needs energy during physical activity or to keep on living.