Cell surface membrane

The cell surface membrane is a plasma membrane composed of a phospholipid bilayer. It acts as a hydrophobic barrier that prevents the passive diffusion of hydrophilic species such as glucose and amino acids into the cell. Hydrophilic channel proteins are embedded in the bilayer which provides a route by which polar substances can enter, either down a concentration gradient by facilitated diffusion or against a concentration gradient by active transport. The relative movements of the lipid molecules together with the random arrangement proteins give rise to the term the fluid mosaic model of the cell surface membrane.