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Plasma Membrane Structure.

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PLASMA MEMBRANE STRUCTURE The plasma membrane is found around the outside of the cell and separates the inside of the cell from the outside world. One of its main functions is to act as a selective barrier controlling the movement of substances in and out of the cell. Other functions of a cell membrane are to: * Detect chemical messengers arriving at the cell surface * Link adjacent cells together by membrane junctions * Acts as receptor site for recognition The cell membrane is made up of a phospholipid bilayer with proteins embedded and other components inside. A phospholipid is made up of a glycerol head part and two fatty acid tails, one saturated and one unsaturated. Diagram 1 shows the layout of the phospholipids in the membrane. ...read more.


In a plasma membrane there is approximately 1 cholesterol molecule per phospholipid. Cholesterol has the same properties as a phospholipid in amphipathic (being polar and non-polar) so it arranges itself polar end outwards and tail inwards. They are found between the phospholipids on each side of the membrane (see diagram 3). Cholesterol packs in the phospholipids in the membrane and pulls the fatty acid tails together, restricting movement but not making them solid, which in turn the membrane it stability and rigidity. Cholesterol also prevents crystallization of hydrocarbons (the tails) prevent the membrane becoming solid. (http://cellbio.utmb.edu) Proteins make the membrane selectively permeable; some are fixed in place while others are moving about in the phospholipid. There are two main types of protein in a membrane, one is an integral protein and the other is a peripheral protein. ...read more.


(Boyle, Ingham, Senior, 1999). Peripheral proteins are found on the membrane surface and are attached to integral proteins and are usually found on the cytoplasm side of the membrane. They are associated with cell shape and give structure and motility. Proteins that pass through the membrane but not completely are called extrinsic. Another constitute of membranes are polysaccharides. Simple sugars attach themselves to the extrinsic proteins or lipids to form Glycoproteins or Glycolipids, which are antigens. These polysaccharides stick out of the cell surface "like antennae" (Boyle, Ingham, Senior, pg 75) and are sites of recognition for the cell. They send out information to tell surrounding cells what they are so that they are not confused with invading bacteria. Another functions is that similar cells will know what the cell is and can bind with it to form tissues. They also act as the basis of the immune system as the Glycoproteins are antigens. (www.revisionnotes.co.uk) Question 2 Rob Hunt Page 1/2 ...read more.

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