cell membrane

The cell membrane is the thin covering that surrounds every cell. It holds the cell's contents together and, more importantly, it is selectively permeable: it decides what gets in and what gets out, letting water, oxygen and nutrients pass while keeping wastes and unwanted molecules where they are. That job of controlling traffic is the phrase a question here is looking for.

The membrane is easy to confuse with the parts around it. Inside it lies the jelly-like cytoplasm, where most of the cell's chemistry happens, and within that sit the nucleus, which holds the genetic material, and the mitochondria, which release energy from food. A plant cell has one more layer outside the membrane: a rigid cell wall, made mainly of cellulose, which gives the cell its shape and support. An animal cell has no wall, which is the quickest way to tell the two apart under a question.

Many of the membrane's jobs are done by diffusion, the movement of particles from where they are crowded to where they are not, and by osmosis, which is diffusion applied to water across the membrane. Both are passive: nothing is being pumped, and the particles simply spread until they are even.