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Transcript
2.1.5 Biological Membranes What are the functions of the cell surface membrane? 1. 2. 3. 4. 5. 6. The cell surface membrane structure 75% phospholipid with some proteins, cholesterol and polysaccharides. About 7 nm thick. Is a phospholipid bilayer. The phosphate group is hydrophilic (water-loving) and turns towards the solution. The fatty acids are hydrophobic (water-hating) and turn away from the solution. They form a mirror image of each other. (a) Phospholipid molecule enlarged and (b) layer of phospholipids at the surface of water A phospholipid bilayer Week 3 The Fluid mosaic theory Proposed in 1972 by Singer & Nicholson. There are several proteins in the bilayer. Intrinsic proteins span the whole bilayer. Extrinsic proteins are only found on either the inner or outer surface. H bonds between proteins and phospholipids keep the membrane relatively stable. Glycolipids are also present and important in cell-cell recognition. The fluid mosaic model The fluid mosaic model The fluid mosaic model Factors which increase Factors which decrease fluidity of the membrane fluidity of the membrane More unsaturated fatty More saturated fatty acids acids in the membrane in the membrane Shorter fatty acid tails Longer fatty acid tails Less cholesterol in the membrane (plant cells rarely have cholesterol) Increases in temperature More cholesterol in the membrane (animal cells about 20%) Lower temperatures Factors that affect membrane structure - Temperature Factors that affect membrane structure - Solvents The fluid mosaic model Entry into and out of cells is controlled by membrane proteins. Carrier proteins and channel proteins allow the transport of ions and other molecules. They can also regulate concentrations of these on either side of the membrane. Some use active transport (using energy from ATP) to transport against a concentration gradient. The fluid mosaic model The fluid mosaic model There may also be: Receptor proteins, which are important in binding extra cellular molecules such as hormones. Membrane enzymes such as ATPase. Glycoproteins; protein & polysaccharide complexes which act as antigens (cell surface markers). Cell Signalling and Cell Recognition Using OCR BIO AS books P.19-21, ‘Blocking Viruses With Synthetic Receptors’ article and OCR AS Bio sheet on Cell Signalling. Write your own summary on cell signalling and recognition. Explain the term cell signalling and the role of membrane-bound receptors as sites for hormones and drugs to bind with . Carry out Practical Activity 5.2 Factors Affecting Membrane Structure