Reserves of fat, carbohydrate, or (rarely) protein in cells and tissues that function as an important store of energy that can be released and used in ATP production when required by the organism. For example, in animals fat is stored in adipose tissue, and carbohydrate—in the form of the storage compound glycogen—is stored in liver and muscle cells. In plants starch is a major storage compound, being found in perennating organs (see perennation) and seeds (in which it is mobilized at germination), and oils are important storage materials in some species (e.g. in the seeds of the castor-oil plant).