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Although biological membranes contain various types of lipids and proteins, their distribution between the two different sides of the bilayer is asymmetric. As a general example the outer surface of the bilayer is enriched in phosphatidylethanolamine, whereas the intracellular surface is enriched in phosphatidylcholine. Carbohydrates, whether attached to lipid or protein, are almost exclusively found on the external surfaces of membranes. The asymmetric distribution of lipids and proteins in membranes results in the generation of highly specialized sub-domains within membranes. In addition, there are highly specialized membrane structures such as the endoplasmic reticulum (ER), the Golgi apparatus and vesicles. The most important vesicles are those that contain secreted factors. Membrane bound proteins (e.g. growth factor receptors) are processed as they transit through the ER to the Golgi apparatus and finally to the plasma membrane. As these proteins transit to the surface of the cell they undergo a series of processing events that includes glycosylation.
The vesicles that pinch off from the Golgi apparatus are termed coated vesicles. The membranes of coated vesicles are surrounded by specialized scaffolding proteins that will interact with the extracellular environment. There are three major types of coated vesicles that are characterized by their protein coats. Clathrin-coated vesicles contain the protein clathrin and are involved in transmembrane protein, GPI-linked protein and secreted protein transit to the plasma membrane. COPI (COP = coat protein) forms the surface of vesicles involved in the transfer of proteins between successive Golgi compartments. COPII forms the surface of vesicles that transfer proteins from the ER to the Golgi apparatus. Clathrin-coated vesicles are also involved in the process of endocytosis such as occurs when the LDL receptor binds plasma LDLs for uptake by the liver. The membrane location of these types of receptors is called a clathrin-coated pit.
In addition, certain cells have membrane compositions that are unique to one surface of the cell versus the other. For instance epithelial cells have a membrane surface that interacts with the lumenal cavity of the organ and another that interacts with the surrounding cells. The membrane surface of cells that interacts with lumenal contents is referred to as the apical surface or domain, the rest of the membrane is referred to as the basolateral surface or domain. The apical and basolateral domains do not intermix and contain different compositions of lipid and protein.
Most eukaryotic cells are in contact with their neighboring cells and these interactions are the basis of the formation of organs. Cells that are touching one another are in metabolic contact which is brought about by specialized tubular particles called junctions. Mammalian cells contain three major types of cell junctions called gap junctions,tight junctions, and adherens junctions.
Gap junctions are intercellular channels designed for intercellular communication and their presence allows whole organs to be continuous from within. One major function of gap junctions is to ensure a supply of nutrients to cells of an organ that are not in direct contact with the blood supply. Gap junctions are formed from a type of protein called a connexin.
Tight junctions are primarily found in the epithelia and are designed for occlusion. Tight junctions act as barriers that regulate the movement of solutes and water between various epithelial layers. At least 40 proteins have been found to be involved in the formation of the various tight junctions. These proteins are divided into four major categories; scaffolding, regulatory, transmembrane, and signaling.
Adherens junctions are composed of transmembrane proteins that serve to anchor cells via interactions with the extracellular matrix. The proteins of adherens junctions are members of the various cadherin and integrin protein families. Related to the adherens junctions are the desmosomes and hemidesmosomes that are also involved in membrane anchoring functions.
Given the predominant lipid nature of biological membranes many types of molecules are restricted in their ability to diffuse across a membrane. This is especially true for charged ions, water and hydrophilic compounds. The barrier to membrane translocation is overcome by the presence of specialized channels and transporters. Although channels and transporters are required to move many types of molecules and compounds across membranes, some substances can pass through from one side of a membrane to the other through a process of diffusion. Diffusion of gases such as O2, CO2, NO, and CO occurs at a rate that is solely dependent upon concentration gradients. Lipophilic molecules will also diffuse across membranes at a rate that is directly proportional to the solubility of the compound in the membrane. Although water can diffuse across biological membranes, the physiological need for rapid equilibrium across plasma membranes has led to the evolution of a family of water transporting channels that are calledaquaporins (see section below).