Name: ___________KEY______________ 1 2006 7.012 Problem Set 6 KEY ** Due before 5 PM on WEDNESDAY, November 22, 2006. ** Turn answers in to the box outside of 68-120. PLEASE WRITE YOUR ANSWERS ON THIS PRINTOUT. 1. You create an artificial cell in a test tube. This artificial cell consists of a “cytoplasm” (i.e. the inside of the cell), a lipid bilayer, and the outside. The “cytoplasm” contains 150 mM NaCl, whereas the outside solution contains15 mM NaCl. The only kind of proteins present in the membrane are Cl– ion channels, but they are closed. (a) Are any ions flowing across the cell membrane? If so, which ions are flowing and are they flowing inward or outward? None are flowing. The only ion channels present in the membrane are Cl- channels, but they are closed. Ions cannot pass through the lipid bilayer membrane without an open channel because the are charged and the inside of the bilayer is nonpolar. (b) Is there a membrane potential across the cell membrane? If so, is the membrane potential positive on the inside or negative on the inside? There is no membrane potential across the cell membrane. The charge on each side of the membrane is balanced. On the inside of the membrane, 150mM Na+ is balanced by 150mM Cl–. On the outside of the membrane 15mM Na+ is balanced by 15mM Cl–. (c) Are there concentration gradients across the cell membrane? If so, for which ions are there concentration gradients, and are those ions at higher concentrations or lower concentrations inside the cell? There are concentrations gradients for both Na+ and Cl-. Both Na+ and Cl– are at higher concentrations inside the cell. Now assume that, all of the sudden, the Cl– channels in the cell membrane open. (d) Are any ions flowing across the cell membrane? If so, which ions are flowing and are they flowing inward or outward? Cl– ions are flowing outward. The Cl– channels are now open so Cl– will move down its concentration gradient. (e) Is there now a membrane potential across the cell membrane? If so, is the membrane potential positive on the inside or negative on the inside? There is a membrane potential that is positive on the inside. Since Cl– ions have moved to the outside of the cell membrane, the Na+ ions inside the cell are no longer balanced by Cl– ions, causing a positive membrane potential inside the cell.Name: ___________KEY______________ 2 (f) Are there concentration gradients across the cell membrane? If so, for which ions are there concentration gradients, and are those ions at higher concentrations or lower concentrations inside the cell? There are concentrations gradients for both Na+ and Cl–. Both Na+ and Cl– are at higher concentrations inside the cell. Even though the Cl– channels are open, very few ions (~ one millionth of them) leave the cell before the concentration gradient and electric potential are in equilibrium, so the concentration gradient of Cl– remains virtually unchanged. You now create a different artificial cell in a different test tube. This artificial cell consists of a “cytoplasm” (i.e. the inside of the cell), a lipid bilayer, and the outside. The “cytoplasm” contains 150 mM NaCl, whereas the outside solution contains15 mM NaCl. The only kind of proteins present in the membrane are H+/Na+ pumps, which simultaneously pump one H+ outside the cell for every one Na+ it pumps inside the cell, but these pumps are inactive. (Assume there is an excess of ATP both inside and outside of the cell that allows the pump to function. Also assume the NaCl is dissolved in water, which is where the H+ ions are coming from.) Now assume that, all of the sudden, the pumps in the cell membrane become active. (g) Are any ions flowing across the cell membrane? If so, which ions are flowing and are they flowing inward or outward? Yes, H+ is moving outward and Na+ is moving inward. This is because the pump is actively transporting these ions in these directions. (h) Is there a membrane potential across the cell membrane? If so, is the membrane potential positive on the inside or negative on the inside? No. For each Na+ moved out of the cell, an H+ is moved into the cell. This leads to a conservation of charges inside and outside the cell. (i) Are there concentration gradients across the cell membrane? If so, for which ions are there concentration gradients, and are those ions at higher concentrations or lower concentrations inside the cell? Yes, both Na+ and Cl– have concentration gradients that are higher inside the cell. H+ has a concentration gradient that is higher outside the cell.Name: ___________KEY______________ 3 2. A normal action potential can be graphed as follows: Draw what an action potential would look like if you inhibited the following voltage-gated channels in the following graphs. A normal action potential is drawn into each graph lightly, for comparison. (a) … voltage-gated K+ channels (b) … voltage-gated Ca++ channels Voltage-gated Ca++ channels are not involved in the actions potentials we have talked about. For part (a), voltage-gated K+ channels are normally involved in repolarization, so repolarization will not happen so quickly. It will still happen, though, just more slowly, because the resting K+ channels that exist in the membranes of all cells are still open. Thus those channels will allow for eventual repolarization of the cell. +50mV –70mV +50mV –70mV +50mV –70mVName: ___________KEY______________ 4 (c) …voltage-gated Na+ channels If the Na+ channels do not open then there will be no depolarization of the cell and no action potential. (d) After depolarization occurs during a normal action potential, is the concentration of sodium inside the cell high or low with respect to the concentration of sodium outside the cell? The concentration of sodium will be low inside. The Na+ concentration will have barely changed. So few sodium ions actually move into the cell during an action potential that the concentration will change only slightly, but not enough to even out or reverse the gradient. (e) Do you think that action potentials in a neuron would cease immediately after one exposes the neuron to a drug that inhibits the Na+/K+ pump? No. Many action potentials could happen because the concentrations gradients of ions barely change with each action
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