The plasma membrane of an animal cell is comprised primarily of proteins and lipids. The main lipids are phospholipids, with glycolipids embedded in the external portion of the membrane acting as cell identifiers. Cholesterol in the plasma membrane helps to make it less flexible, and it also enables the cell to be selective regarding what size molecules are allowed to enter the cell.
Many of the proteins in the plasma membrane can move about freely and are not anchored in a fixed position. These proteins serve as transporter molecules and move substances in and out of the cell. Some proteins can also act as enzymes and catalyze certain biochemical reactions.
The plasma membrane separates the cytoplasm and the interior components of the cell from its outside environment, and it functions as a selectively permeable wall that allows certain substances to enter the cell, while keeping others out. The membrane plays a role in cellular adhesion, cell signaling and ion conductivity. It also serves as an attachment surface for various extracellular structures.
The selective permeability of the plasma membrane enables small polar molecules to diffuse through to the interior of the cell, but large polar molecules and certain ions are kept from passing through on their own. In order to cross into the cell, large polar molecules require the plasma membrane proteins to act as transporter molecules. Because of the selective permeability of the plasma membrane, the cell is able to maintain the correct ratio of osmotic pressure between its interior and its outside environment.