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The Structure and biological functions of cell membranes

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Andrea McCaffery HEFC Biological Science The Structure and biological functions of cell membranes Cells are the fundamental units of life, because a cell is the simplest unit capable of independent existence. Biological membranes maintain the spatial organisation of life. Cell membranes define the boundaries of living cells and work to shield it from changes in its environment. Essentially, membranes prevent undesirable agents from entering cells and keep needed molecules on the inside. Therefore, the cell membrane controls and regulates everything that passes in or out of the cell. The membrane is essentially made up of a phospholipid bilayer, which forms a boundary enclosing the cell contents and is also folded through the cell, separating compartments for specialised purposes. The structure is flexible and allows for growth and movement as well as for the insertion and operation of protein machinery. The lipid bilayer is two layers of lipid molecules, which are termed amphililic (meaning "loving both"). The head of the molecule, made up of glycerol and phosphate, is hydrophilic ("water loving") and polar and the long hydrocarbon tails, composed of fatty acids, are hydrophobic ("water hating") and non-polar. The hydrophilic head bears electrical charges contributed by the phosphate and some bases. These charges are responsible for the hydrophilicity. No lipid bears a positive charge they are all negative or neutral. Therefore all membranes are negatively charged. They form a bilayer structure with the hydrophilic heads to the external and internal surface and the hydrophobic tails towards each other. ...read more.


There is another, very different, form of transport that takes place within the cell membrane and that is bulk transport, where larger particles need to enter or leave the cell. Membrane transport systems cannot do this type of job, but the cell membrane itself can. Endocytosis is the term used when particles are surrounded by and taken up into the membrane-lined vesicles. This can occur at a relatively large scale, for example, when bacteria are ingested. In this case the process is called phagocytosis -"cell eating" and the alternative process is pinocytosis -"cell drinking". Cells that specialise in this are refered to as phagocytes and a white blood cell is a good example of this. See diagram. Andrea McCaffery HEFC Biological Science Exocytosis is the opposite process which involves the secretion of material through the cell membrane. The Golgi apparratus is a system of tubular structures formed by plasma membrane. The Golgi receives vesicles from the endoplasmic reticulum. These vesicles containing material such as proteins and hormones then divide way from the Golgi body, fuse with the plasma membrane and are exported from the cell where the molecules release into the tissue fluid. For transport reasons, some specialised membranes have different structures according to their functions. The nucleus, which controls the activity of the cell, has a double membrane but this is perforated by many nuclear pores to allow the movement of RNA. ...read more.


These can pass all the way through the membrane (Intrinsic proteins), providing a passage for molecules, or can rotate between the surfaces (Extrinsic proteins), acting as receptors to such things as hormones and to act as recognition markers. As well as diffussion, we have looked at the other more complex mechanisms of transport such as osmosis, facilitated diffusion and active transport which take place across the cell membrane. The other processes, Endocytosis and Exocytosis, involving the Golgi Apparratus are the methods of transport for inporting and exporting even larger molecules through the cell membrane. Using these methods of transport, cell membranes control the inward and outward movement of useful molecules. We have also seen that there are a number of specialised membranes with slightly different structures according to there particular function. For example, we have looked at the way in which the membranes of the Mitochondria and the chloroplast have adapted to best suit there function and how the two have a similar double membrane with multiple folds in the inner membrane. Although for the same fundemental purpose ; to increase the surface area, the inner membrane of the Mitochondria and the chloroplast are the sites for completely different chemical reactions. The structures of biological cell membranes are designed in such a way as to aid their very function. It is the interface between a cell and its environment and a fluid structure which preforms the selective transport of essential materials into and out of the cell. ...read more.

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