The Composition and Architecture of Biological Membranes: Proteins and Phospholipids - Pro, Exams of Chemistry

Download The Composition and Architecture of Biological Membranes: Proteins and Phospholipids - Pro and more Exams Chemistry in PDF only on Docsity! Name__________________________________ CHEM 731/Exam 1/Dr. N. Campbell Each question is worth 8 points for a total of 104 points. Please write legibly and neatly. I have to be able to read and understand your writing to give you a grade. 1. The composition and architecture of membranes (a) List the major components of membranes. (b) When a preparation of mitochondrial membranes was treated with high salt (0.5 M NaCl), it was observed that 40% of the total protein in this preparation was solubilized. What kind of membrane proteins are in this soluble extract, and what forces normally hold them to the membrane? (c) What kind of proteins constitutes the insoluble 60%, and what forces hold these proteins in the membrane? Ans: (a) phospholipids, sterols, proteins (integral and peripheral); (b) peripheral membrane proteins, which are associated with the membrane through ionic and hydrogen bonds between their charged and polar side chains and the charged head groups of phospholipids; (c) integral membrane proteins (which are held to the membrane by hydrophobic interactions between their nonpolar side chains and the hydrophobic fatty acyl chains of phospholipids), and those peripheral membrane proteins that are held to the membrane by a covalent lipid anchor. 2. The composition and architecture of membranes What are the principle features of the fluid mosaic model of membranes? Ans: The principle features of the fluid mosaic model of membranes include: (1) a lipid bilayer in which individual lipids are free to move laterally but not across the bilayer; (2) integral membrane proteins, which penetrate or span the bilayer, associating with lipid acyl chains by hydrophobic interactions and exhibiting lateral mobility; (3) peripheral membrane proteins, which associate noncovalently with the lipid head groups and protruding domains of integral membrane proteins, and which are sometimes tethered to the membrane by a covalent lipid anchor 3. The composition and architecture of membranes (a) Explain why phosphoglycerides are capable of spontaneously assembling into the bilayer structure found in biological membranes but triacylglycerols are not. (b) What are the forces that drive bilayer formation? Ans: (a): Triacylglycerols have three fatty acyl groups in ester linkage with glycerol; they are very hydrophobic because the carboxyl groups, which are involved in the ester linkages, cannot ionize. Phosphoglycerides have a polar region at their head group, where a phosphate in a phosphodiester linkage bears a full negative charge. The head group itself (serine, ethanolamine, choline, etc.) may also be charged and is polar in any case. Thus, the phospholipid is amphipathic, having both polar and nonpolar regions, and it forms lipid bilayers spontaneously in water. (b) These lipid bilayers are stabilized by the energy gained from burying hydrophobic groups out of contact with water. A hydrophobic chain in water forces the formation of a cage of immobilized water molecules around it. When several hydrophobic regions cluster in a bilayer, the surface area exposed to water decreases, and the water molecules in the cage are released, accompanied by a gain in entropy that drives the formation of the bilayer. 4. The composition and architecture of membranes A protein is found to extend all the way through the membrane of a cell. Describe this protein in terms of the location of particular types of amino acid side chains in its structure and its ability to move within the membrane. Ans: This integral membrane protein associates with the lipid bilayer through hydrophobic interactions between domains containing many hydrophobic amino acids and the fatty acyl chains of membrane lipids. Polar and charged residues are located on portions of the protein that protrude out of either face of the membrane. The protein is free to diffuse laterally in the plane of the membrane, but cannot move across the lipid bilayer. 5. Membrane dynamics The bacterium E. coli can grow at 20 °C or at 40 °C. At which growth temperature would you expect the membrane phospholipids to have a higher ratio of saturated to unsaturated fatty acids, and why? Ans: At 40 °C, the membranes of E. coli will contain more saturated fatty acids than at 20 °C. The cell regulates fatty acid composition to achieve the same fluidity in its membranes, regardless of growth temperature. Saturated fatty acids counterbalance the fluidizing effect of high temperature 6. Membrane dynamics A plant breeder has developed a new frost-resistant variety of tomato that contains higher levels of unsaturated fatty acids in membrane lipids than those found in standard tomato varieties. However, when temperatures climb above 95 °F, this frost-resistant variety dies, whereas the standard variety continues to grow. Provide a likely explanation of the biochemical basis of increased tolerance to cold and increased susceptibility to heat of this new tomato variety. Ans: More unsaturated fatty acids will cause an increase in membrane fluidity because unsaturated fatty acids contain “kinks” and cannot pack as tightly as saturated fatty acids. At cold temperatures, the fluidity increase from the extra unsaturated fatty acids counterbalances the tendency of lipids to solidify at low temperature. At high temperatures, the fluidizing effects of the extra unsaturated fatty acids add to the fluidizing effect of higher temperature, and the membrane of the new plant loses its integrity. 7. Solute transport across membranes Distinguish between simple diffusion (SD), facilitated diffusion (FD), and active transport (AT) across a membrane. (Items to include are energy dependence, carrier protein(s), and concentration gradient). Ans: Simple diffusion: The movement of lipophilic molecules across the lipid bilayer from high concentration to low concentration. Simple diffusion does not establish a concentration gradient. Because the lipophilic molecule can “dissolve” in the lipid bilayer and the movement is from high concentration to low concentration, this type of movement does not require energy input or carrier proteins. Facilitated Diffusion: The movement of polar or charged molecules across the lipid bilyer from high concentration to low concentration. Facilitated diffusion does not establish a concentration gradient. Because this type of movement is from high concentration to low concentration, there is no requirement for energy input. However, this type of movement involves the crossing of the lipid bilayer by polar or charged molecules; therefore, specific channels (carrier proteins) are formed in the membrane to assist the movement of the polar or charged molecules across the lipid bilayer.