4 membrane, cell 118

4 membrane, cell 118 - Biology 118 4. CELLS, MEMBRANES AND...

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Biology 118 4. CELLS, MEMBRANES AND MORE What’s all this about cells? THE CELL MEMBRANE: Phospholipids have a water-soluble (polar, or hydrophilic) phosphate ‘head’ and water-insoluble (non-polar, or hydrophobic) fatty acid ‘tails’. In water, phospholipids line up so that the phosphate heads face water, and the fatty acid tails face each other – two such layers make up the cell membrane, the phospholipid bilayer. Cholesterol can be found, squeezed in between the fatty acid tails. The phospholipid bilayer forms the basic structure of the membrane – it allows the passage of some materials (like gases) but not others (like ions), and hence is selectively permeable. The bilayer has proteins embedded within it: Receptors proteins – act to bind specific extracellular chemicals and trigger a response in the cell. Channel proteins - these proteins form a water-filled corridor between the inside and the outside of the cell. They can be specific for given ions – so there are Na + -channels and K + -channels, for instance. Some channels are always open – these are ‘leaky’ channels. Carrier proteins – these proteins are NOT channels. They transport specific substances across the membrane. A famous example is the Na + / K + -pump; it pumps Na + out and K + into the cell. Enzymes – these carry out specific chemical reactions in the membrane. Anchoring proteins – these help keep cells attached to other cells or structures. Recognition proteins – these identify a cell as belonging in the body (as opposed to a bacterial invader, for instance). Such proteins may also have carbohydrates attached to them. MEMBRANE TRANSPORT: Cells must move materials in and out, both passively and actively – this can occur in many ways. Passive transport moves materials across the membrane without any direct energy (ATP) expenditure by the cell. Diffusion – this is the ‘downhill’ movement of a substance in solution, from an area of higher
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This note was uploaded on 11/02/2011 for the course BIO 118 taught by Professor Bilgen during the Winter '08 term at University of Washington.

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4 membrane, cell 118 - Biology 118 4. CELLS, MEMBRANES AND...

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