The structure and function of membranes, with reference to membrane constituents and different types of membrane.
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The structure and function of membranes, with reference to membrane constituents and different types of membrane. Membranes are essential for the compartmentalisation of a cell and therefore it is clear that the cell would struggle to develop any complexity without them. Although there are many different uses for membranes, especially in the eukaryotic cell, they serve a very important energetic purpose. For instance, they can retain a concentration gradient of, for example, Hydrogen ion, which means that the cell can use membranes not only for compartmentalising reactions, but also for compartmentalisation of energy and preventing diffusion. Therefore, without a complex system of membranes, the eukaryotic cell simply would not be able to carry out as many functions as it does. The prokaryotic cell differs in the sense that it does not possess membrane-bound organelles but still has an outer cell membrane. This has meant that it cannot carry out as many functions as the eukaryotic cells but its simplicity means that reproduction is far simpler. Therefore it is the membranes of the eukaryotic cell which are of more interest The basic constituents of all membranes are phospholipids, although there are different types of lipid present that I shall discuss later.
Phosphatidylcholine tend to be on the outside. The flip-flops in the ER occur around 100,000 times faster than in most membranes. The lateral movements of the bilayer are very fast and, in this way, a phospholipid can move across an eukaryotic in about 20 seconds. These movements were revealed by an experiment in which membrane proteins in a mouse cell were tagged with a green fluorescent dye. The membrane proteins in a human cell were then tagged with a red dye and the membranes of the two cells were fused with the Sendai virus. The membrane was left for about an hour and after that time the membrane was no longer half-green and half-red but a brown colour. This indicated that the proteins had completely intermingled due to the lateral movements of both membranes that were obviously very fluid. However, this fluidity is easily affected by temperature and when the membrane cools below a certain phase transition temperature, it becomes more gel-like because flexion is restricted and rotation is much more common.[diag.Lodisch.p601,14-9]For instance, frostbite occurs when the plasma membrane hardens because of the cold and thus causing the cell to die.
However, the function of most glycolipids and glycoproteins is unknown or very difficult to work out especially as they probably perform a wide range of functions. They are integral parts of the membrane and are included in the fluid mosaic model, which is now the accepted structure of the plasma membrane. This takes into account its fluidity, its selective permeability and its function as a receptor. [diag.lodisch.p596] So far, I have described the typical eukaryotic cell membrane. In fact, all membranes consist of the phospholipid bilayer and they differ in the types of phospholipid present and, more importantly, the types of protein associated with the membrane. For instance, the membrane of the lysosome contains many proteins which pump protons inside so that the acidic conditions are maintained. The mitochondrial membrane has many ATP synthase enzymes that span it so ATP can be manufactured. Prokaryotic membranes are generally much more simple for ease of manufacture and also because the organism performs much fewer functions than an eukaryotic cell. However, they do possess ion channels and other pores that allow the entry of nutrients and the exit of waste materials. In conclusion, membranes have so many variables that their characteristics can be changed by the addition of a protein or a different type of phospholipid for example.
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