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Cell Surface Membrane

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Introduction The cell surface membrane (formerly called the plasma membrane) surrounds the cytoplasm of eukaryotic cells. The membrane forms a selectively permeable barrier, controlling the substances that enter and leave the cell and therefore enables the cell to regulate its internal environment. Structure The cell surface membrane is approximately 7.5 nm thick and composed of lipids (mainly phospholipids), proteins and carbohydrates (usually attached to proteins or lipids). (See figure below). THE FLUID MOSAIC MODEL In 1972, Singer and Nicolson put forward the 'fluid mosaic' model of membrane structure in which protein molecules float about in a fluid phospholipid bilayer. The scattered protein molecules resemble a mosaic but, since the phospholipid bilayer is fluid, the proteins form a fluid mosaic pattern. Why fluid mosaic? Fluid - because the protein molecules float about in a fluid phospholipid bilayer. The membrane is held together mainly by hydrophobic interactions between the phospholipids and between proteins and phospholipids. These weak interactions allow the molecule to move. Phospholipid molecules move in the plane of the membrane. Proteins are much larger and move more slowly - imagine protein molecules moving about like icebergs in a 'sea' of lipid. Mosaic - because the membrane is made of different types of molecules arranged in a mosaic pattern. ...read more.


For example, hormones are chemical messengers which circulate in the blood but only bind to Neurotransmitters, the chemicals which enable nerve impulses to pass from one nerve cell to the next, also fit into specific receptor proteins in nerve cells. 4. Enzymes can be embedded in the inner membrane of a cell or organelle Proteins sometimes act as enzymes, for example the microvilli on epithelial cells lining some parts of the gut contain digestive enzymes in their cell surface membranes. For example, ATPase. 5. Proteins in membrane also help strengthen the membrane. There are hydrogen bonds between the protein and the hydrophilic heads of the phospholipids. As in lipids, intrinsic proteins have a hydrophobic and hydrophilic region and the interaction between these regions confers stability on the membrane. Glycoproteins - (also found in the outer layer) are proteins with attached polysaccharides of short, branched chains of monosaccharides. Glycoproteins have a variety of specific shapes due to the different branching patterns of the monosaccharides. These allow different cells to recognise each other. For example, some glycoproteins are antigens - they are recognised by white blood cells, which start an immune respond. Extrinsic proteins - those which are entirely outside of the membrane (lying on the surface of membrane), but are bound to it by weak molecular attractions (ionic , hydrogen , and/or Van der Waals bonds) ...read more.


diabetes mellitus. Excess glucose in the bloodstream, caused by eating a meal rich in carbohydrates, is usually taken up by myocytes (muscle cells) and adipocytes (fat cells). The glucose transporter GluT4 is normally present in the cell membrane in small amounts. The presence of insulin (a hormone secreted by the pancreas in response to high glucose levels) causes more GluT4 transporters to be exposed, increasing uptake of glucose into the cell. In type II diabetes there is resistance to the metabolic effects of insulin, either at the cell membrane or in post-receptor signaling systems. This means that little glucose can be taken up by myocytes and adipocytes, and high blood glucose levels are the result. Conclusion The biological membrane is a collage of many different proteins embedded in the fluid matrix of the lipid bilayer. The lipid bilayer is the main fabric of the membrane, and its structure creates a semi-permeable membrane. The hydrophobic core impedes the transport of hydrophilic structures, such as ions and polar molecules but enable hydrophobic molecules, which can dissolve in the membrane, cross it with ease. Proteins determine most of the membrane's specific functions. The plasma membrane and the membranes of the various organelles each have unique collections of proteins. For example, to date more than 50 kinds of proteins have been found in the plasma membrane of red blood cells. ...read more.

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