Cellular Diffusion All cells have an outer protective casing, the cell membrane. It protects the cell from the outside world, but is also vital in the transfer of particles in and out of the cell. The cell membrane controls which particles can enter, and which particles can not, because it is selectively permeable. The membrane is made up of a bi-layer of lipid molecules, which have very small spaces between each other, and if a particle is small enough to fit in-between these gaps, then it can pass through the membrane. There are also proteins in the membrane that span its width, and form channels. These channels allow larger molecules to pass through. The size of the partic