A hormone (from Greek ὁρμή, “impetus”) is a chemical released by a cell, a gland, or an organ in one part of the body that affects cells in other parts of the organism. Only a small amount of hormone is required to alter cell metabolism. In essence, it is a chemical messenger that transports a signal from one cell to another. All multicellular organisms produce hormones; plant hormones are also called phytohormones. Hormones in animals are often transported in the blood.