Cell membrane.swf / membran sel.swf

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Every cell has a plasma membrane that encloses it and maintains differences between the cell contents and the outside environment that are crucial to the function of the cell. All biological membranes consist of assemblies of lipid and protein molecules. The lipids are rod-shaped molecules arranged in a double layer so that their hydrophobic ends, which repel water, point inwards and their hydrophilic ends, which attract it, point towards the aqueous environment and the inside of the cell. This lipid bilayer provides the basic structure of the membrane, and forms a barrier that is relatively impermeable to most water-soluble molecules. Proteins are embedded in the bilayer; they also have hydrophobic surfaces in contact with the lipids, and hydrophilic surfaces exposed on either side of the membrane. At physiological temperatures, the lipid bilayer is fluid, and so the proteins are able to move about within the plane of the membrane. The two leaflets of the bilayer contain different lipids, and different proteins are exposed on the two faces of the membrane.

The respiratory gases exchange freely across the membrane, because oxygen and carbon dioxide are soluble in lipid. Apart from this it is the proteins that span the membrane which act as pumps and channels for the exchange of materials between the inside and outside of the cell. They allow entry of nutrients into the cell and the exit of waste products. They are also responsible for generating differences in the ionic composition between the inside and the outside of the cell. Finally, proteins act as molecular sensors (membrane receptors) allowing the cell to change its behaviour in response to external chemical signals. In addition to the plasma membrane, most cells contain a variety of organelles — internal structures that are also surrounded by membranes. These include the nucleus, the endoplasmic reticulum, the Golgi complex, and the mitochondria.

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