the plasma membrane is composed of a bilayer . This bilayer behaves very much like a fluid, that means the lipids in this bilayer are in constant lateral motions. The phospholipids can laterally exchange their places, however they can not diffuse transversely as shown below
The lipid components which make up the bilayer consist mostly of phospholipids and to a smaller extend of glycolipids . The plasma membrane is assymetric with respect to the lipid distribution. Glycolipids are generally found in the outside layer, whereas phospholipids are predominantly in the inside layer (cytoplasmic side) but they also are components of the outside layer. Glycolipids and glycoproteins embedded in the plasma membrane act as cell identidy markers. Cholesterol is another important component of the animal plasma membrane. Sometimes the content is as high as one molecule cholesterol per one molecule phospholipid. Cholesterol partially immobilizes the fatty acid tails making the membrane less flexible and thus less permeable to smaller molecules.
The other important components of the plasma membrane are proteins. Proteins can be associated with the outside (peripheral) of the membrane or they extend throughout the entire membrane (transmembrane). Mant of the plasma proteins are not fixed into a certain posistion, but can freely move within the membrane. The protein content of differs in membranes of different origin:
plasma membrane 50% 50%
neuron 18% 79%
mitochondrion 80% 20%
liver 70% to 50% 30% to 50%
Proteins in the plasma membrane serve as transporter molecules, i.e. they can transport substances in and out of the cell. They also function as recognition sites for substances such as hormones, transmitting chemical signal to the cell. Some proteins on the periphery of the plasma membrane function as enzymes, catalyzing chemical reactions