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The Cell Membrane.

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The Cell Membrane All cells are separated from their environments by a plasma membrane. It has little mechanical strength but plays a vital role in controlling which materials pass into and out of the cell. Although basically a double layer of phospholipid molecules arranged tail to tail, the membrane is a complex structure studded with proteins. These can be embedded in the membrane or they can penetrate the bilayer forming pores aiding in the transport of certain substances. (Boyle et al, 1999). In 1972, Singer and Nicolson put forward the fluid mosaic model of membrane structure. It is a dynamic structure in which a mosaic of protein molecules floats in a fluid bilayer - some moving freely while others are fixed in position anchored to organelles within the cell. Lipids also move about. The fluid mosaic model (Adds et al, 2000) The main structural component of the cell membrane is a double layer of phospholipids molecules - a lipid bilayer, as proposed by Davson and Danielli in 1935 and is about 7.5nm thick. The two layers of phospholipids are arranged with the fatty acid chains pointing inwards towards one another. ...read more.


The glucose is released and the protein reverts back to its original shape and position in the membrane. These are known as carrier proteins. Another type of protein responsible for facilitated diffusion is the channel protein which are which are fixed shape molecules. It is selective about which ions can pass through and controls the passage of selected charged particles. These are not active processes as no energy is required. Another form of transport is osmosis. Water diffuses from a region of high concentration to a region of lower concentration through a partially permeable membrane. If a human or animal cell is placed in distilled water, it absorbs water by osmosis and swells up. As cell surface membranes have virtually no physical strength the cell often bursts. Hypotonic plasma causes red blood cells to swell and burst leaving 'ghosts' of the membrane. This process is known as haemolysis and is a passive process which requires no energy. (Boyle et al, 1999). Active transport is the energy consuming transport of molecules or ions across a membrane against a concentration gradient. It is driven by metabolic energy derived from ATP. ...read more.


(Boyle et al, 1999). The golgi complex also plays an important role in the formation of lysosomes. Lysosomes are membrane bound organelles containing digestive enzymes called lytic enzymes and is bounded by a single membrane. If they were not enclosed by a membranous sac they would attack the other cell organelles. One use of lysosomes is they supply the enzymes which destroy old or surplus organelles. They also digest material taken into the cell. After a white cell has engulfed a bacterium, for example, lysosmes discharge enzymes into the vacuole and digest the organism. This process is called phagocytosis. (Boyle et al, 1999). Mitochondria are large, individual organelles which occur in large numbers in most cells. Each mitochondrion is bounded by a double membrane with the outer layer being a smooth continuous boundary. The inner membrane is extensively folded in to form partitions called cristae and encloses the mitochondrial matrix. Mitochondria are the sites of the aerobic respiration within cells. In conclusion, the structure and function of the cell membrane is of great importance in order for the cell to operate efficiently. The most important property of the cell membrane is its partial permeability. Because of this property, the cell membrane plays a key part in determining the composition of the cytoplasm by controlling what gets in and out. ...read more.

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