91Ó°ÊÓ

A biological membrane that separates and protects the inside of all cells from the outside environment called cell membrane.

Lipids, proteins, and cholesterol are structured as asymmetric bimolecular leaflets of lipids with embedded proteins in biological membranes. Asymmetry in protein orientation confers distinct features on the two membrane sides. Each kind of integral membrane protein has a single, particular orientation with regard to the cytosolic and exoplasmic faces of a cellular membrane.

Thus, barriers and gatekeepers function as cell membranes. They're semi-permeable, which implies that some molecules can pass through but others can't. Cell membranes, on the other hand, prevent the diffusion of large molecules like carbohydrates and amino acids as well as highly charged molecules like ions.