Molecular Cell Biology
Chemical Composition of Cell Surface Membrane: Role is various cellular processes
The structure of a typical animal cell surface membrane is best explained using the Fluid Mosaic Model, according to which the membrane is a phospholipid bilayer. With phospholipids, cholesterol, glycosylated surface proteins and transmembrane proteins embedded within but able to move within their plane, it forms a selectively permeable barrier through which materials going in and out of the cell must pass. While the basic function of the plasma membrane is enclosure of the cellular contents, its purpose is multifarious as it plays fundamental roles in transport of materials across the membrane, structural support, cell signaling, cell recognition, cell adhesion and catalyzing membrane-associated reactions. Such versatility of membrane functions is achieved through variability in the chemical composition of the plasma membrane.
Chemically, the plasma membrane is composed of a wide variety of proteins, lipids in the form of cholesterol, phospholipid and sphingolipid and carbohydrates in the form of glycoproteins. Between various types of cells, there may not only be differences in the type and amount of lipids, carbohydrates and proteins but the amount of these chemicals may vary between each monolayer of the plasma membrane. For instance in the human red blood cell, the outer monolayer has a higher proportion of choline-rich lipid molecules (PtdCho) while most of phospholipid molecules at the cytosolic monolayer contain phosphatidylethanolamine (PtdEtn) and phosphatidylserine (PtdSer) (Steck et. al, 1974). The higher proportion of phosphatidylserine in the inner monolayer also contributes towards the asymmetry of charge distribution across the membrane.
Schematic diagram of RBC membrane
The asymmetry of chemical composition across the membrane is directly related to the function of the membrane of the particular cell type and is well exploited by cells e.g. in inducing apoptosis. During apoptosis PtdSer is rapidly exported to the extracellular monolayer. The sudden increase in the concentration of PtdSer is used as a signal to trigger macrophages into digesting the dying cells.
Proteins attach to the plasma membrane in various ways. Transmembrane proteins are amphipathic, having regions that are hydrophobic and others that are hydrophilic, while others may be peripheral or integral proteins. Peripheral proteins attach to the inner or the outer periphery of the cell and form a whole family of enzymes. Carotenoid oxygenase, which is involved in the cleavage of retinol or Vitamin A (Guiliano et. al, 2003), is one such example of a peripheral protein functioning as an enzyme. The integral proteins forming transmembrane ATPases catalyze the decomposition of ATP into ADP and a free phosphate ion along with energy. Not only do they play a role in energy production, but they also import metabolites for cell metabolism and expel toxins and wasteful solutes. Another example of a membrane protein showing enzymatic activity is that of Phopholipase C, which cleaves a phosphatidylinositol 4,5 bisphosphate (PIP2) molecule into diacyl glycerol (DAG) and inositol 1,4,5 trisphosphate (IP3) (Smrcka et. al, 1991). IP3 is soluble and is imported into the cytosol, where it causes the concentration of Ca2+ ions to increase, which in turn initiates a cascade of intracellular changes.
Cell-cell communication is also achieved by the receptors embedded in the plasma membrane. For instance, the acetylcholine receptor (AchR) is an integral membrane protein, which responds to the binding of the neurotransmitter acetylcholine (Ach). There are two types of AchRs; nicotinic AchR (nAchR) and muscarinic AchR (mAchR) (Thunnissen 2009). nAchR is a pentameric structure and forms a channel through the membrane, which opens in response to the binding of Ach (Brejc et. al, 2001). Upon binding, Na+ and K+ flow into the channel, depolarizing the membrane. For recovery, the Ach is hydrolyzed by the esterase activity of the receptor, forming acetate and choline and terminating the effect generated by binding of Ach.
Another cellular process namely cell adhesion is also mediated by transmembrane proteins. This is achieved through cell adhesion molecules (CAMs), which are proteins located on the extracellular monolayer of the plasma membrane. One such molecule, VCAM-1, belonging to the Immunoglobulin superfamily (IgSF) of CAMs, also known as cluster of differentiation, mediates the adhesion of lymphocytes, monocytes, eosinophils and basophils to vascular endothelium. Similarly cadherins, which support calcium-dependent adhesion, have members that are found in the plasma membranes of various cell types (Brackenbury et. al, 1981). Examples include CDH1 in epithelial cells, desmogleins in desmosomes or CDH11 in osteoblasts.
As detailed above, membranes play a pivotal role in catalysis of reactions, cell-signaling, cell communication and cell adhesion. Their role in cell-recognition and hence the immune system is also important. This can be illustrated with the example of antigen-antibody interaction in blood transfusion. Membranes use glycoproteins as a primary means of cell-cell recognition. In the red blood cell bilayer are embedded macromolecules, which act as cell-surface antigens. While these could be proteins, glycolipids or glycoproteins in nature, most are glycoproteins with their specificity determined by the oligosaccharide sequences with which they are glycosylated (Daniel, 2010). In the event of the introduction of foreign antigens, the antibodies within the blood plasma will interact with the antigens and produce a defensive and destructive effect.
In conclusion, as illustrated by the above examples, cell surface membranes are embedded with lipids, carbohydrates and proteins, which singly or in combination with each other produce a variety of agents which facilitate the cell to carry out processes such as cell signaling, recognition, communication and adhesion.
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