Plasma membrane

A plasma membrane encapsules each and every cell. The plasma membrane is necessary to allow different concentrations of substances to be maintained between the cell and it's environment. Eukaryotic cells have internal membranes which surround organelles, allowing different constituent concentrations within the cell.

The structure of the plasma membrane is made up of a lipid bilayer embedded with proteins and cholesterol. Most lipid bilayers are comprised of phospholipids, the most common being phosphatidylcholine. The head part of it is made up of a phosphate and a hydrophilic choline, the tails component of the phospholipid are two fatty acid chains, long hydrocarbon structures with chemically active COOH. Since the head is hydrophilic and the tails are hydrophobic, the phospholipid is said to be amphipathic.

The structure of the bilayer is such that the tails are shielded from contact with water, creating a selectively-permeable barrier. 

The function of the plasma membrane is to communicate with the environment and other cells, by means of the glycocalyx. It allows passage of selective molecules in and out of the cell via carrier or channel proteins embedded within the membrane, and it regulates cell growth, shape change, movement and division. 

The membrane is associated with proteins, both integral and peripheral which act as either: transporters, linkers, receptors or enzymes.