In biology, the smallest unit that can live on
its own and that makes up all living organisms and the tissues of the body. A
mammalian cell has three main parts: the cell membrane, the nucleus, and the
cytoplasm. The cell membrane surrounds the cell and controls the substances
that go into and out of the cell. The nucleus is a structure inside the cell
that contains the nucleolus and most of the cell’s DNA. It is also where most
RNA is made. The cytoplasm is the fluid inside the cell. The cytoplasm is where most
chemical reactions take place and most proteins get made. The human body has
more than 30 trillion cells1.