Membrane Proteins

with hydrophilic parts on the outside of the membrane and hydrophilic parts on the inner aspect of the membrane. The carbohydrate groups sticking out of the membrane are made in the Golgi apparatus.

Membrane proteins are not able to move in and out of the lipid bilayer but they can diffuse laterally because of the fluid mosaic model. This has been proven by fusing two cells together and observing the hybrid cell that will contain aspects of both pre-hybrid cells covering the entire surface of the hybrid cell. Some proteins, however, are restricted because they are associated with the cytoskeleton, which is attached to the cell membrane in order to form the structure of the cell. This is especially true in cells that are polar, such as epithelial cells that have an apical portion and a basolateral portion. There are proteins that exist just on the apical side of the cell and not on the basolateral part of the cell, and vice versa. Tight junctions are part of the mechanism that make sure the proteins stay where they are supposed to.

As mentioned, the outer portions of the plasma membranes usually have sugar or carbohydrates attached, making the proteins called glycolipids. This leads to an outer surface of the cell membrane being called a glycocalyx. It is made from oligosaccharides attached to various proteins in and on the surface of the cell. The glycocalyx protects the cell surface and participate in cell to cell interactions.

MEMBRANE PROTEINS

There are several different kinds of membrane proteins. There are membrane receptor proteins that send signals from the outside of a cell to the inside of the cell and vice versa. There are transport proteins that move ions and small molecules across the membrane. There are several membrane enzymes. Finally, there are cell adhesion proteins that allow cells to identify one another.

Integral membrane proteins are specific types of membrane proteins that can be transmembrane proteins. There are two types of integral membrane proteins. These are integral polytopic proteins (also called transmembrane proteins) and integral monotopic proteins.

Most integral membrane proteins are transmembrane proteins, which exists throughout the membrane. They can pass through the membrane just once or can weave in and out of the membrane, being multi-pass membrane proteins. Integral monotopic proteins, on the other hand do not span the entirety of the lipid bilayer. Many transmembrane proteins attach to the cell membrane and cannot move out of the membrane. Most are transport proteins that act as ion or small molecule channels. They require detergents to be extracted from the cell in most cases.

The two main types of transmembrane proteins are alpha-helical and beta-barrel proteins. The alpha-helical proteins are found in prokaryotes and eukaryotes; they can be found in the inner or outer membrane leaflets. Beta-barrels are only seen in prokaryotes or in the outer membranes of chloroplasts and mitochondria. Types I through IV are single-pass molecules, while other types are transmembrane proteins are multi-pass molecules.

Peripheral membrane proteins only adhere temporarily to the biomembranes. They are attached to integral proteins or penetrate the outer lipid layer to a slight degree. These molecules include regulatory subunits of other transmembrane proteins. They are largely water soluble. Those that are called amphitropic proteins have hydrophobic parts that can slightly penetrate the lipid bilayer. Others can do both—can penetrate the lipid bilayer and can attach to transmembrane proteins.

The connection between the lipid bilayer and a protein may involve changes in the structure or shape of the protein so that it can bind to the bilayer. The protein may need to rearrange itself into a specific shape in order to connect to the lipid bilayer. The different functions of these proteins include membrane anchoring, enzymatic activities of lipids, and the transfer of small nonpolar substances across the membrane.