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.
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