Multicellular organism

A multicellular organism is made of many cells, but those cells do not all do the same job. An animal needs cells that transmit signals, cells that contract to produce movement and cells that form protective coverings. These different jobs require different cell structures.

Large multicellular organisms therefore need both specialised exchange surfaces and transport systems. Exchange surfaces move substances into or out of the organism efficiently; transport systems carry those substances between the surfaces and distant cells. In animals, blood delivers oxygen and dissolved food molecules and carries wastes away.