Zool 3200: Cell Biology Exam 4 Part I 2/3/15

Transcription

1 Name: Key Trask Zool 3200: Cell Biology Exam 4 Part I 2/3/15 Answer each of the following questions in the space provided, explaining your answers when asked to do so; circle the correct answer or answers for each multiple choice question and circle either True or False to indicate whether each true/false statement is correct or not. (38 points) Which of the following phenomena will be observed if a cell s membrane is pierced? (1 point) a.) the membrane reseals b.) the membrane collapses c.) the membrane beings to rip d.) the membrane expands A bacterium is suddenly expelled from a warm human intestine into the cold world outside. Which of the following adjustments might the bacterium make to maintain the same level of membrane fluidity? (1 point) a.) Produce lipids with hydrocarbon tails that are longer and have fewer double bonds. b.) Produce lipids with hydrocarbon tails that are shorter and have more double bonds. c.) Decrease the amount of cholesterol in the membrane. d.) Decrease the amount of glycolipids in the membrane. There are two properties of phospholipids that affect how tightly they pack together: the length of the hydrocarbon chain and the number of double bonds. The degree of packing, in turn, influences the relative mobility of these molecules in the membrane. Which of the following would yield the most highly mobile phospholipid (listed as number of carbons and number of double bonds, respectively; 1 point)? a.) 24 carbons with 1 double bond b.) 15 carbons with 2 double bonds c.) 20 carbons with 2 double bonds d.) 16 carbons with no double bonds Some lipases are able to cleave the covalent bonds between the glycerol backbone and the attached fatty acid. What final products do you expect to accumulate through the action of an enzyme, diacylglycerol lipase? (1 point) a.) phosphoglycerol and free fatty acid b.) sterol and glycerol c.) free phosphate and glycerol d.) monoacylglycerol and free fatty acid 1

2 True / False: A. Lipid-linked proteins are classified as peripheral membrane proteins because the polypeptide chain does not pass through the bilayer. (1 point) True / False: A protein that relies on protein protein interactions to stabilize its membrane association is classified as a peripheral membrane. (1 point) Which of the two proteins described in the True / False questions above could be purified from other membrane components using buffers with high salt concentrations? (1 point) The peripheral membrane protein described in the 2 nd T/F question would be separated from other membrane components by high-salt buffers. Plasma membranes are extremely thin and fragile, requiring an extensive support network of structural proteins that underlies them. This network is called the cell cortex. (1 point) Which of the following statements about the carbohydrate coating of the cell surface is false? (1 point) a.) It is not usually found on the cytosolic side of the membrane. b.) It can play a role in cell cell adhesion. c.) The arrangement of the polysaccharide side chains is highly ordered, much like the peptide bonds of a polypeptide chain. d.) Specific polysaccharides can be involved in cell cell recognition. True / False. Transport by carrier proteins can be either active or passive, whereas transport by channel proteins is always passive. (1 point) In the absence of K +, the Na + /K + ATPase pump would: (1 point) a.) be incapable of transporting Na+ across the cell membrane. b.) function to move Na +, as though K + was available. c.) be capable of transporting only a small amount of Na +. d.) be unable to hydrolyze ATP. If the plasma membrane of an animal cell was made permeable to Na + and K +, the Na + /K + ATPase pump would: (1 point) a.) be completely inhibited. b.) begin to pump Na + in both directions. c.) begin to synthesize ATP instead of hydrolyzing it. d.) continue to pump ions and to hydrolyze ATP but the energy of hydrolysis would be wasted as an ion gradient would fail to form. e.) continue to pump ions but not hydrolyze ATP. 2

3 If the extracellular K + concentration were rapidly decreased to 1/10 th of normal, the voltage across the membrane would be expected to: (1 point) a.) move farther from 0 millivolts. b.) move closer to 0 millivolts. c.) remain unaffected. In resting cells, the concentration of Na + ions: (1 point) a.) is higher inside the cell than outside. b.) is higher outside the cell than inside. c.) is equal on both sides of the membrane. d.) varies with one s diet. e.) is impossible to predict. What is responsible for initiating the rising phase of the action potential? (1 point) a.) Closing of the K + gates. b.) Opening of the Na + activation gates. c.) Closing of the Na + inactivation gates. d.) Opening of the Na + inactivation gates. e.) Both a. and c. Using microsurgial techniques, you isolate a neuronal axon that is approximately 6cm long from an earthworm; you place the isolated axon in physiologic saline solution. You then stimulate the axon to depolarize by giving it an electric shock near the middle about 3cm from either end. The electric shock induces opening of voltage-gated sodium channels such that the signal can be transmitted down the length of the axon. Which direction does the signal travel? Explain your answer. (3 points) The signal will be transmitted in both directions, because the axon had not yet been stimulated be for I did it with an electric shock. Prior to the shock, all of the Na + channels were in the closed conformation; they were therefore able to be stimulated to open by the electrifying shock. Upon opening, Na + would rush into the axon and diffuse in both directions, resulting in more voltage-gated Na + channels opening to depolarize the membrane in both directions. None of the Na + channels in this scenario had previously been activated such that their inactivation gates were closed. Resident mitochondrial proteins are synthesized by ribosomes.where? (2 points) In both the cytoplasm (and transported to the mitochondria) or in the mitochondrial matrix (by resident mitochondrial ribosomes). 3

4 Indicate whether each of the following transport processes occurs via the mechanisms described as gated transport (G), transmembrane transport (T), or vesicular transport (V). Your answer should be a four letter string composed of letters G, T, and V only, e.g. VTTTG. Write your answer in the box. (2 points) ( ) Export from nucleus ( ) Import into mitochondria G T V T ( ) Return from Golgi to ER ( ) Return from ER to cytosol Which of the following statements about transport into mitochondria and chloroplasts is false? (1 point) a.) The signal sequence on proteins destined for these organelles is recognized by a receptor protein in the outer membrane of these organelles. b.) After a protein moves through the protein translocator in the outer membrane of these organelles, the protein diffuses in the lumen until it encounters a protein translocator in the inner membrane. c.) Proteins that are transported into these organelles are unfolded as they are being transported. d.) Signal peptidase will remove the signal sequence once the protein has been imported into these organelles. Imagine a protein that has been engineered to contain a nuclear localization signal, a nuclear export signal, a C-terminal peroxisomal targeting sequence, and an amino-terminal endoplasmic reticulum (ER) signal sequence. With all of these signals, where would you expect to find the protein after its synthesis? (1 point) a.) Cytosol b.) Nucleus c.) Shuttling between the cytosol and the nucleus d.) Peroxisomes e.) Endoplasmic reticulum f.) Proteosome Sort the following events to reflect the order in which they occur during the formation of vesicles from the ER destined for the Golgi apparatus. Your answer would be a four-letter string composed of letters A to D only, e.g. DACB. Write your answer in the box. (2 points) A) Sar1 GTP hydrolysis B) Sar1 GTP binding C) Sar1 Sec23 binding B D C A D) Sar1 membrane association 4

5 You have introduced high concentrations of a non-hydrolyzable form of GTP into the cytoplasm of cells such that it is 10x more concentrated than normal cytoplasmic GTP. What is the most likely effect of this treatment on vesicular transport through the constitutive secretory pathway? Explain your answer. (2 points) I would expect an increase in the production of buds from the ER, but no fusion with the Golgi membrane. Sar1-GTP is needed for assembly of the COP proteins/coat at the ER membrane. If Sar1 is unable to hydrolyze GTP, then it would remain in the GTP-bound form, inserted in the ER membrane (due to exposure of its hydrophobic tail) and likely actively recruiting Sec proteins (that make the COP coat). The bud would form and leave the ER, but because Sar1 could not hydrolyze the GTP, it would never release from the vesicle membrane and therefore neither would the COP coat. Disassembly of the coat is required for vesicular fusion, so without this disassembly, no fusion would be able to occur. The cell would have lots of coated vesicles floating around with un-delivered cargo meant to go to the Golgi. What is the function of Rab proteins in intracellular protein trafficking? (2 points) Rab proteins on the vesicle bind to Rab proteins on the target membrane to ensure that vesicles bind to their correct target membrane. Rab interactions are responsible for vesicle docking during which the vesicle is tethered to the target membrane prior to it fusion. Describe how SNARE proteins are involved in vesicular transport. (3 points) V-SNARE proteins on the vesicle bind to T-SNAREs on the target membrane. When these matching V- and T- SNAREs bind to one another, they intertwine such that the two membranes are brought in close proximity to one another. This proximity forces the exclusion of water, which ultimately enables fusion of the vesicle to the target membrane. Fusion results in deliver of the vesicles cargo to the target location. 5

6 Rank the following types of cell signaling from 1 to 4, with 1 representing the type of signaling in which the signal molecule travels the least distance before it encounters a responsive cell and 4 the type of signaling in which the signal molecule has the ability to travel the longest distance. (2 points) 3 paracrine signaling 1 juxtacrine signaling 2 synaptic signaling 4 endocrine signaling What type of activity would a cytosolic receptor be expected to have if it typically elicits a very rapid cellular response after binding to its chemical signaling molecule? (1 point) It is likely that the cytosolic receptor has some sort of enzymatic activity (like the guanalyl cyclase activity described in class for NO) because a rapid response would be elicited when the receptor were bound by a signaling molecule that could pass through the membrane. Upon the signal molecule s binding to the cytosolic receptor, the receptor would change shape and alter its activity (either up or down) to alter cellular behavior rapidly. All members of the steroid hormone receptor family: (1 point) a.) are cell-surface receptors. b.) do not undergo conformational changes upon steroid hormone binding. c.) are found exclusively in the cytoplasm. d.) interact with signal molecules that diffuse through the plasma membrane. e.) result in very rapid changes in cellular behavior upon steroid hormone binding. Nitric Oxide (NO) released from endothelial cells can result in the relaxation of underlying smooth muscle cells in vascular tissue. NO stimulates the intracellular enzyme guanylyl cyclase by: (1 point) a.) activating a G protein. b.) activating a receptor tyrosine kinase. c.) diffusing into cells and stimulating the cyclase directly. d.) activating an intracellular protein kinase. 6