The Structure and Function of the Cell Membrane

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The cell (or plasma) membrane is the cell’s most important organelle – it controls how substances can move in and out of the cell and is responsible for many of the cell’s properties. The plasma membrane is composed of phospholipids, carbohydrates and proteins arranged in a fluid-mosaic structure. The individual phospholipid molecules can move relative to each other this gives the membrane a “fluid” structure that constantly changes shape, the proteins that are embedded in the phospholipid bilayer vary in shape, size and pattern in the same way as tiles in a mosaic. The phospholipids are arranged in a bilayer, with their polar, hydrophilic phosphate heads facing inwards (which interact with the water in the cytoplasm) on one layer and facing outwards (these interact with the water that surrounds the cell) on the other layer, and their non-polar, hydrophobic fatty acid tails of both phospholipid layers point into the centre of the membrane towards each other in the middle of the bilayer (so they are protected from the water on both sides). Lipid-soluble material moves through the membrane via the phospholipids; so they allow lipid-soluble substances to enter and leave the cell, prevent water-soluble substances entering and leaving the cell. Different kinds of membranes can contain phospholipids with different fatty acids, which affect the strength and flexibility of the membrane, and animal cell membranes. They also contain cholesterol which links the fatty acids together which stabilises and strengthens the membrane. The proteins in the membrane either sit on one of the layers (extrinsic proteins) or proteins that span the bilayer from one side to another (intrinsic proteins). The extrinsic proteins on the outside surface of the membrane either give support to the membrane or in conjunction with the glycolipids act as receptors for molecules such as hormones
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