Answer to Homework 1

Jan. 30, 2009 Version 1.0 1 MCDB 3120 Problem set 1: Membrane Organization, transport and interaction with Cytoskeleton 1. You have a population of red blood cels (RBCs) and you want to extract both peripheral and integral membrane proteins but not cytoplasmic proteins. Which of the following statements describes the correct experimental procedures for collecting both peripheral and integral membrane proteins? I) treat the RBCs with high salt or low pH I) place the RBCs in a hypotonic solution II) treat the RBCs with detergent IV) treat the RBCs with lipases and lectins a) I ? IV b) I II c) I ? I? II d) IV I I? II e) I?II 2. The length of faty acid tails and the degre of saturation are two factors that afect membrane fluidity. What?s the role of cholesterol in maintaining membrane fluidity in Antarctic fish cels? a) Cholesterol increases the membrane fluidity by reducing the chance that fatty acid tails wil hydrogen bond. b) Cholesterol decreases membrane fluidity by interacting with faty acid tails c) Cholesterol has no efect on membrane fluidity. d) Cholesterol replaces proteins in the membrane alowing more membrane fluidity. 3. Cel membranes are dynamic because phospholipids and proteins that make up the membranes can move within the plane of the membranes. The rate of membrane protein movement is much slower than that of the membrane phospholipids. What properties contribute to this diference? a) The amphipathic nature of lipids fres them from tethering by cytoskeletal proteins. b) Membrane proteins can be anchored to cytoskeletal proteins that increase their difusion rates. c) Lipid microdomains limit their mobility. d) Phospholipids are anchored to cytoskeletal proteins that tether them in place. Jan. 30, 2009 Version 1.0 2 4. How do channel proteins increase the rate of transport of solutes across a membrane? a) They bind the solute. b) They are gated. c) They lower the activation energy for transport. d) They facilitate active transport. 5. The asociations of diferent proteins in red blood cels plasma membrane can be identified in several ways. One general method is to use antibodies that are specific for individual proteins. A mixture of two proteins is incubated together, and then an antibody specific for one protein is added. The resulting antibody-protein complexes are precipitated and analyzed. The technique, when applied on pair-wise mixtures of spectrin, ankyrin, band 3, and actin, yields the results summarized in Table 1. From the information in the table, deduce the most probable asociation betwen these proteins. Note: ?+? indicates the asociation betwen two proteins. Table 1 Precipitation of red blood cel plasma membrane proteins by antibodies specific for individual proteins. Protein mixture Antibody specificity Protein in pelet 1. band 3 + actin actin actin 2. band 3 + spectrin spectrin spectrin 3. band 3 + ankyrin ankyrin band 3 + ankyrin 4. actin + spectrin spectrin actin + spectrin 5. actin + ankyrin ankyrin ankyrin 6. spectrin + ankyrin spectrin Spectrin + ankyrin a) band 3 + ankyrin + spectrin + actin b) band 3+ ankyrin + actin + spectrin c) ankyrin + band 3 +spectrin d) spectrin + ankyrin + band 3 6. In the membrane of human red blood cel the ratio of the mas of protein (average molecular weight 50,000) to phospholipid (average molecular weight 800) to cholesterol (molecular weight 386) is about 2:1:1. Approximately how many lipid molecules are there for every protein molecule? a) 84 b) 168 c) 42 d) 64 Note: there are 84 lipid molecules (phospholipid + cholesterol) for every protein molecule [2 x 50,000/(800+386)=84]. Jan. 30, 2009 Version 1.0 3 7. You are interested in studying lipid rafts. You have devised a method to create an artificial lipid bilayer that contains smal patches of lipids that are straighter and longer than those found in the rest of the bilayer. How would you expect the fluidity of these smal patches to compare with the rest of the lipid bilayer? a) The patches would be more fluid. b) The patches would be les fluid. c) The fluidity would be the same. d) Not enough information to determine. 8. Plasma membranes from red blood cels have been isolated and reasembled into smal vesicles. Using fluorescently labeled lectin, some of the vesicles are fluorescently labeled and some are not. Recal that lectin binds to carbohydrates. Furthermore, the labeled lectin cannot permeate membranes. Which population of vesicles has a surface similar to that of the cel? a) Unlabeled vesicles because glycoproteins are found on the cytoplasmic side of the plasma membrane. b) Labeled vesicles because glycoproteins are found on the extracelular side of the plasma membrane. c) Both because the plasma membrane has extensive areas both with and lacking glycoproteins. d) Labeled vesicles because the outside of the cel is positively charged. 9. In what way(s) are carier proteins like enzymes? a) They lower the activation energy for transport. b) They bind the substrate to be transported. c) They undergo a change in conformation when they function. d) Al of the above. Jan. 30, 2009 Version 1.0 4 10. You have just completed an experiment and obtained the kinetics of uptake of a drug in the absence of ATP by heart muscle cels in tisue culture (se diagram). Which of the following membrane transport supports your kinetics data? a) Simple difusion b) Facilitated difusion c) Active transport d) Not enough information to determine 11. What is the imediate functional consequence of acetylcholine binding to its receptor at the neuromuscular junction? a) It activates pathways to cause the influx of ions. b) It allows sodium ions to flow into the cel. c) It causes the release of Ca 2+ from the cel to propagate the action potential. d) It initiates signal transduction from the muscle cel membrane to the nucleus. 12. Cels with a specific mutation in Na + -K + ATPase are no longer able to pump K + but otherwise normal in function. Which of the following is true about these mutant cels? membrane potential? a) It is the same as wild type (normal) cels. b) It is more negative than wild type (normal) cels. c) It is les negative than wild type (normal) cels. d) Can?t determine based on the information provided. Joy Power Microsoft Word - Problemset1v.1.0_013009.doc