A cell membrane is like a cell's gatekeeper. It's the outer layer that surrounds a cell, letting substances in — or keeping them out.
A membrane is a barrier that offers protection, like a rubber membrane that keeps rain from seeping under your roof. A cell membrane surrounds and protects the contents of a cell. It controls which substances can enter and exit the cell. The membrane also gives a cell its shape and enables the cell to attach to other cells, forming tissues.