MCB110 FINAL May 19, 2003 . Your name and student ID QUESTION POINTS 1 (20 points) 2 (30 points) 3 (35 points) 4 (20 points) 5 (20 points) 6 (25 points) 7 (15 points) 8 (10 points) 9 (15 points) 10 (25 points) 11 (15 points) 12 (25 points) 13 (25 points) 14 (30 points) 15 (20 points) TOTAL (300 points) WARNING: Your exam will be taken apart and each question graded separately. Therefore, if you do not put your name and ID# on every page or if you write an answer for one question on the backside of a page for a different question you are in danger of irreversibly LOSING POINTS!Student Name and ID # 1(a) - Which of the following organisms do you think must have the highest proportion of unsaturated fatty acids in their membranes and why (10 p): (a) Antarctic fish (b) Cactus (c) Bacteria from thermal hot springs (d) Humans Antarctic fish (a), as they will need the unsaturated fatty acid chains in order to lower the critical temperature for the transition from a liquid crystal (fluid) to a gel phase (rigid) below the temperatures in the Antarctic. 1(b) - Describe the differences in self-association of lipids in monolayers, micelles and liposomes (10p). In monolayers the hydrophilic lipid heads face the water at the air-water interface, with the fatty acid chains facing the air. In micelles the lipid tails face each other in the inside of a small sphere, with the hydrophilic heads facing the water on the outside. In liposomes the lipids form a bilayer and give rise to a large sphere with an aqueous core. 2(a) - What are the most common arrangements of secondary structure elements in the lipid-interacting section of integral membrane proteins? Could you predict them having knowledge of the protein sequence but lacking direct structural data? (10 p) Transmembrane regions in integral membrane proteins are an alpha helix (single pass) or alpha helical bundle (several passes) and beta barrels. Transmembrane helices can be predicted from hydropathy plots by identifying hydrophobic regions in the sequence spanning about 20-30 a.a. This method will not be able to predict beta barrels. 2(b) – During purification of membrane proteins, what different methods would you use to separate from the membrane integral membrane proteins, peripheral proteins and lipid-anchored proteins (10 p). Integral membrane proteins will have to be solubilized using non-ionic detergents; peripheral proteins can be release from the membrane with high salt or alkaline pH; phospholipases (or enzyme that break covalent bonds between the protein and its linked hydrocarbon chain) will be needed to release the lipid-anchored proteins.Student Name and ID # 2(c) - You are using FRAP to study an integral membrane protein resident in the ER. What would be your conclusion if you see a fast rate or recovery? Would you expect a faster or slower recovery if you lower the temperature of your cells? (10 p) The recovery after photobleaching means that the protein is free to diffuse within the ER membrane. Lower temperature will reduce the fluidity of lipids and thus the diffusion rate of the protein, so recovery will be slowed down. 3(a) - The distribution of K+ across and artificial membrane was measured and equal concentrations were found on both sides. Which of the following statements is true and why (15 p): (a) K+ must be at equilibrium across the membrane (b) K+ cannot be at equilibrium across the membrane (c) There cannot be a membrane potential under these circumstances (d) More information is required to determine if K+ is at equilibrium (d) We need to know what the electric potential is across the membrane to calculate the electrochemical G for K+. Only if the electrochemical potential is zero will K+ be at equilibrium (G = 0). The fact that K+ concentration is the same on both sides does not determine the electric potential, as we have to take into account other possible ions. 3(b) – The concentration of chloride ions in the resting neuron is 120 mM outside the cell and 12 mM inside. If there were chloride leaking channels, which way will chloride move during the resting state? (resting potential –70 meV; 2.3RT = 1.4 kcal/mol; Faraday constant 23 kcal/mol.V). If a certain type of voltage-gated chloride channel were to open at a voltage of +50 meV which way would ions move through the channel? (20 p) Gresting = 1.4 log [Ci]/[Co] + z 23 (-70) = -1.4 + 1.61 = 0.21 kcal/mol (moving out) G50 = -1.4 - 1.15 = -2.65 (moving in)Student Name and ID # 4(a) - Arrange the statements below in the proper sequence of events that occur in an action potential (10 p): (a) The resting potential is reestablished (b) A stimulus depolarizes the membrane to a threshold (c) K+ gates open and K+ rushes out of the cell (d) Na+ gates open and Na+ rushes into the cell (b), (d), (c) and (a) – An initial depolarization causes the opening of the Na+ channels, the movement of Na+ into the cell, and further depolarization. This results in the opening of K+ channels. The movement of K+ out of the cell then restores the resting potential (in fact, causing a momentary hyperpolarization). 4 (b) - A neurotransmitter that binds to a postsynaptic cell and opens K+ channels: (a) Excites the postsynaptic cell (b) Inhibits the postsynaptic cell (c) Depolarizes the postsynaptic cell (d) Must be acetylcholine Why? (10 p) It will inhibit the postsynaptic membrane (b) as the K+ ions will rush out of the cell hyperpolarizing it. 5 – What are the differences between facilitative transporters, active pumps, and cotransporters? Can you give an example of the action of such three types of transport in the brush border epithelial cells of the intestine (20 p). Facilitative transporters move small molecules down their concentration gradient, pumps, using energy (most commonly ATP hydrolysis) move ions and small molecules against their electrochemical gradient, while in cotransport the movement of an ion down an electrochemical gradient is coupled to the movement of a small molecule against its concentration gradient. In the brush border cell the glucose-Na+ cotransporter present in the apical membrane facing the intestinal lumen pumps glucose against a concentration gradient by using the Na+ gradient generated by the Na+/K+ ATPase, thus creating a high concentration of glucose inside these cells. On the basal membrane in
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