Molecular Biology of the Cell

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Unformatted text preview: STRUCTURE OF CELLULAR MEMBRANES MEMBRANES Outline Outline • Membrane Biogenesis • Membrane Carbohydrates • Membrane Proteins Integral Integral Peripheral Peripheral • Thought Questions Membrane biogenesis. • A. phospholipids originate in ER, A. 1. plasma membrane a. transferred to Golgi via transition vesicles a. b. then exported to plasma membrane via b. then secretory vesicles which fuse with the plasma membrane plasma 2. nuclear membrane is connected to ER 3. mitochondria & chloroplasts much of the lipid is transferred to much mitochondria & chloroplasts via soluble cytoplasmic proteins called lipid transfer proteins or exchange proteins. Membrane biogenesis Membrane • B. Proteins B. a. proteins bound for plasma membrane, a. mitochondria & chloroplasts are made in the rough ER. b. The terminal end of the mRNA codes for a b. "signal mechanism" that is the "address" for the proteins be they bound for the p. membrane or organelle. membrane Membrane Carbohydrates Membrane Glycocalyx: carbohydrate-rich peripheral zone at cell surface Glycolipids: carbohydrate covalently joined to lipid in carbohydrate extracellular face extracellular Glycoprotein: carbohydrate covalently joined to side chains carbohydrate of protein; exposed on extracellular face of Membrane Proteins Membrane Functions of Membrane Protein Functions Transport needs to be unique for every cell and organelle within a cell cell Proteins Proteins • Two main classes of membrane proteins peripheral & integral Integral- imbedded in hydrophobic interior - mono layer or bilayer (transmembrane) peripheral held in place by hydrogen bonds On outside surface of membrane Does span into the hydrophobic interior Freeze-Fracture Splits Membrane, Proteins Associate With E or P Face Face Protein Arrangements in Membranes Membranes Sequence of peptide determines where in the membrane it will be found Determines shape Membrane spanning region Fig 10-17, MBOC4 Structure of a typical peripheral Amino acid sequence acts as and anchor Integral Membrane Proteins Integral • Peptide bonds are polar • Interior of lipid bilayer is Interior non-polar (hydrophobic) non-polar • How can proteins cross How the hydrophobic environment within the lipid bilayer? lipid Fig 10-19, MBOC4 Structure of a typical integral protein Structure primary structure: hydrophobic aa are primary clustered in segments containing 20-25 residues residues Can form alpha-helices or beta sheets Can alpha-helices (barrels) (barrels) Can form pore protein Proteins Often Form Alpha-Helix to Cross Lipid Bilayer Cross the alpha-helices have the nonpolar middles and polar ends ends Fig 10-19, MBOC4 β Barrels Cross Lipid Bilayers Integral proteins can form pores that allow molecules to move in and out of the cell Fig 10-21, MBOC4 Groups of Alpha-Helixes Cross Lipid Bilayer Integral proteins can form pores that allow molecules to move in and out of the cell Lateral Mobility of Proteins Can Be Restricted Be Fig 10-43, MBOC4 Some Proteins Restricted to Apical or Basal Domain of Cells Basal Fig 10-41, MBOC4 Thought Question Thought • Properties of a Membrane are determined by the structure of the lipid Properties molecules. Predict what would happen to the lipid bilayer if the following were true (or happened) were 1. Phospholipids (PL) have had only one hydrocarbon (HC) chain instead of 1. two two 2. HC chains were shorter (10 carbons) 3. All HC were saturated 4. All HC were unsaturated 5. Bilayer contained a mixture of two kinds of lipid molecule: One with two 5. saturated HC tails and one with two unsaturated HC tails saturated 6. Each lipid molecule is covalently linked 6. To molecule next to it To the molecule in the opposite monolayer (by the end carbon) • • • • • • Acholeplasma laidlawii is a small bacterium-like organism that is unusual because it uses whatever fatty acids are available in the environment for constructing its plasma membrane. When grown in a medium enriched with saturated fatty acids, A. laidlawii is forced to construct membranes with an abnormally high content of saturated fatty acids. When grown in a medium enriched with unsaturated fatty acids, A. laidlawii constructs membranes predominately of unsaturated fatty acids. PROBLEM. An experiment was conducted by growing A. laidlawii on two different media. Group S was grown on a medium rich in stearate, a saturated fatty acid; Group O was grown on a medium rich in oleate, an unsaturated fatty acid. Figure 2 is a graph of data collected from differential scanning calorimetry of the plasma membranes from the two colonies. Which of the peaks represents Group S and which peak represents Group O. Explain your answer. unsaturated fatty acid saturated fatty acid Relative Permeability of Synthetic Lipid Bilayer Synthetic Fig 11-1, MBOC4 Concentration and Charge Differences Affect Movement of Molecules Molecules Fig 11-4, MBOC4 1. In simple diffusion and facilitated diffusion, molecules move from an area of high concentration (outside the cell) to an area of low concentration (inside the cell). It is implied there is greater energy outside of the cell. Why is there greater energy outside the cell? (Or: why is there greater energy when there is a greater concentration?) there ...
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This note was uploaded on 04/20/2010 for the course BIOL 4064 taught by Professor Dr.reyna during the Fall '09 term at Ouachita Baptist.

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