1 BISC 307, Spring 2011 Exam 1 Answers 1. Which of the following statements about the third situation is true? a. At electrochemical equilibrium, chamber A will be 58 mV more negative than chamber B. b. Na+ ions will diffuse from chamber A to chamber B down their concentration gradient. c. If one re-applies a hydrostatic pressure of 1 atm to chamber A, water will flow from A to B at 20 ml/min. d. If no hydrostatic pressure is applied to chamber A, water will flow from B to A at 10 ml/min. e. There will be no net movement of water. f. None of the above. 2. Write a number in each blank to match each term on the left with its most appropriate definition on the right. A given definition should only be used only once or not at all. ________ a. Na+/K+ ATPase 1. causes cells to be positive inside compared to outside ________ b. Ca2+ ion 2. movement of water down its concentration gradient ________ c. facilitated diffusion 3. secondary active transporter ________ d. Na+/glucose symporter 4. Brownian motion ________ e. CFTR 5. a peripheral membrane protein ________ f. osmosis 6. trigger for exocytosis in many cells 7. passive transport across cell membranes 8. electrogenic ion pump 9. epithelial Cl- channel ANSWER: a-8 , b-6 , c-7 , d-3 , e-9 , f-2 10. epithelial Na+ channel 3. If you wanted to add glucose to a 20 mM solution of KCl in order to make it isoosmotic with a 150 mM solution of NaCl, what would be the final concentration of glucose needed? a. 300 mM d. 130 mM b. 280 mM e. 100 mM When 1 atm of hydrostatic pressure is applied to A, water flows from A to B at 10 ml/min. A BA B1 atm NaCl B Chamber A is drained, then re-filled with an aqueous solution of NaCl with an osmotic pressure of 1 atm. In the diagram on the right, a semipermeable membrane separates chambers A and B, both of which are filled with water. The membrane is permeable to water but impermeable to NaCl. 3 pts. Answer in box. d Answer in box. c 6 pts. 2 pts.2 c. 260 mM f. 80 mM 4. Imagine a membrane that is permeable to Na+ but not to Cl-. This membrane separates two aqueous solutions in which the only solute is NaCl. The concentration of NaCl on side 1 is much higher than on side 2. Which one of the following will occur? a. Na+ will move until the concentration on side 1 is the same as the concentration on side 2. b. Cl- will move until the concentration on side 1 is the same as the concentration on side 2. c. Cl- will move from side 1 to side 2 down its concentration gradient. d. A potential will develop across the membrane, positive on side 1. e. There will be no movements of ions. f. None of the above will occur. 5. The effects of hormones on target cells can be tremendously amplified by intracellular signaling pathways. In the case of the cAMP signaling pathway, which steps are responsible for this amplification? ANS: Amplification occurs at several steps. Binding of one hormone to its receptor can activate several stimulatory G-proteins. Each of these can activate several adenylyl cyclase enzymes, each of which generates many molecules of cAMP. These cAMP molecules in turn activate many protein kinase A molecules, each of which can phosphorylate many target proteins. Some of the target proteins may be enzymes, whose stimulated (or inhibited) activity further amplifies the signal. 6. In general, what roles do Ca2+ ions play in the intracellular signaling mediated by inositol trisphosphate (IP3)? ANS: IP3 causes the intracellular release of Ca2+ stored in the smooth endoplasmic reticulum. These Ca2+ ions can trigger exocytosis, bind to calmodulin and activate the calmodulin-dependent protein kinase, and have other effects. IP3 can also be converted to IP4 within the cell, which can enhance the function of Ca2+ channels. 7. The distribution of ions across the membrane of a particular nerve cell is such that ECa = +100 mV, ENa = +60 mV, ECl = -70 mV, EK = -90 mV, resting potential = -80 mV, and threshold for generating action potentials is -60 mV. This nerve cell receives a variety of different types of synaptic inputs. In the box, write an E or I next to each choice to note whether the synapses described would be excitatory (E) or inhibitory (I). ____________ a. a synapse which increased pNa in the postsynaptic membrane. (E) ____________ b. a synapse which increased pK in the postsynaptic membrane. (I) ____________ c. a synapse which increased pCl in the postsynaptic membrane. (I) 8. The trace below shows a recording of current (on the vertical axis) flowing through a single voltage-gated K+ channel as a function of time (on the horizontal axis). The membrane potential is held at -70 mV, EK is -90 mV, and ENa is +50 mV. Imagine that you suddenly changed the membrane potential to -30 mV. Write “Increase,” “Decrease,” or “No Change” in each of the blanks below, to indicate the changes you would expect to see in these single K+ channel currents. 0 +5 pA Answer in box. f 3 pts. 6 pts. 7 pts. 3 pts. 3 pts.3 ____________ a. Mean time interval between channel openings. (Decrease) ____________ b. Mean channel open time (how long channel stays open before closing). (No Change) ____________ c. Mean amplitude of single channel current. (Increase) 9. Explain the ionic basis of the refractory period that follows an action potential in a typical unmyelinated axon. ANS: The refractory period is caused by two ionic events. The first event is inactivation of voltage-gated Na+ channels. Depolarization has two effects on Na+ channels, opening of the activation gate followed with a delay by closing of the inactivation gate. An inactivated channel cannot re-open to contribute inward current to another action potential until the gates have re-set to their resting configurations. The second event is the delayed opening of voltage-gated K+ channels. The resulting increase in K+ permeability generates an outward current that moves the membrane potential in the negative direction, towards the K+ equilibrium potential (EK). This persistent rise in K+ permeability makes it less likely that any depolarizing influence will bring the membrane to threshold for generation of another action potential, thus it extends the duration of the refractory period. 10. Which of the following statements about acetylcholinesterase at the neuromuscular junction is correct? Read this question carefully! a. Acetylcholinesterase is the