______ is considered the functional unit of life.
Cell (biology) The cell is the structural and functional unit of all living organisms, and is sometimes called the building block of life.. Some organisms, such as bacteria, are unicellular, consisting of a single cell. Other organisms, such as humans, are multicellular, (humans have an estimated 100 trillion cells; a typical cell size is 10 Âµm, a typical cell mass 1 nanogram). The largest known cell is an ostrich egg.