The cell is the smallest living unit of the body — the basic unit that all human structure and function is built from. An adult body is made of somewhere around 30 to 37 trillion of them, each one a tiny, self-contained factory doing a specific job. Every cell is wrapped in a cell membrane, a thin, flexible barrier made mostly of fat molecules that separates the cell’s contents from the fluid around it and controls, with great precision, what is allowed to enter and what is allowed to leave.