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43 Cards in this Set
- Front
- Back
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Why can a PM be considered an electrical capacitor?
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It's an insulator (lipid bilayer) surrounded by two conductors (salt solutions on either side).
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How does a cell's membrane potential usually compare to the outside?
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It's usually negative (-70mV in neurons), and becomes positive when it's employed to carry out some function.
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What are some examples of functions stimulated by a positive swing in a cell's membrane potential?
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Hormonal and neurotransmitter secretion, prevention of polyspermy, immune responses in lymphocytes, muscle contraction, reactions of sensory cells to their optimal stimuli (except rods and cones, who become more negative), generation and conduction of nerve impulses.
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What three things influence whether and how an ion will flow across a PM?
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The ion's concentration gradient, the existing electrical gradient, and the membrane's permeability to the ion (this reflects the number of open ion channels permeable to the ion).
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What is inward current?
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Flow of positive charge into a cell.
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On a graph, which way is inward current plotted?
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Downward.
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What two factors determine a transmembrane voltage gradient?
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Ion concentration gradients and selective ion permeabilities.
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What is Nernst equilibrium?
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The state when there is enough charge buildup to stop diffusion of a permeant ion across a PM.
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What's easier to change--voltage gradient or concentration gradient?
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Voltage gradient is much easier to influence. A 100mV change in a 10 micron cell involves 2 million monovalent ions, which only changes the concentration by a few thousandths of a % of typical K+ concentration.
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What is the Nernst equation and what is its chief limitation?
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E= -(61/z) log (Ci/Co)
E is in mV. This only works for a single permeant ion at a time. |
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What's the usefulness of the Nernst equation?
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Predicts, at a given membrane potential, whether an ion will flow into or out of a cell.
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What is the net driving force?
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The difference between the existing membrane potential and the Nernst potential for a particular ion. NDF=Vm-Eion
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What compartment (inside or outside of cell) is described by the NDF equation?
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Always the inside relative to outside.
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What is more realistic about the GHK equation compared to the Nernst?
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Describes multiple ions with multiple permeabilities.
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What are the assumptions of the GHK equation?
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Ions are maintained in a dynamic steady state, and electrical potential drops linearly across the membrane.
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What do Na/K pumps do?
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Maintain high intracellular K and low internal Na.
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What's the range of "typical" membrane potentials?
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-40 to -90 mV.
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What's more permeable to a PM--Na or K?
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K is about 100 times as permeable as Na.
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Which ion's Nernst potential most closely matches a typical cell's Vrest?
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K. Vrest~Ek.
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What does "dynamic steady state" mean?
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Despite a lack of equilibrium, ionic concentrations are maintained inside a cell.
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What are some extreme examples of ionic concentration gradients that are held out of balance in a cell? How is this sustained?
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Na and Ca. Na is 100x higher outside the cell; Ca is 10,000x higher. Carriers and pumps maintain this huge concentration difference.
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In a resting cell, which ion is actually at Nernst equilibrium?
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Cl-. There is no pump for Cl-.
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How does Cl- concentration outside a cell usually compare to inside?
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It's higher outside a cell.
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What are the main intracellular anions?
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Organic anions--proteins, amino acids, sugars, peptides.
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When do terms for Cl- in the GHK equation become relevant?
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When there are changes in the membrane potential or, in experiments, during changes in the ionic concentrations.
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Which ion dominates the GHK equation at rest? Why?
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K+, because it's most permeable. Permeability weights each concentration in the GHK. At rest, the GHK is actually pretty close to the Nernst for K+.
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At rest, GHK=Nernst for which ion?
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K+
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The GHK only includes monovalent ions? Does this make it ineffective? Why?
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No; Most cells are not very permeable to Ca. During a heart AP however, when many Ca channels are open, the GHK would not be useful. You would need an equation with the NDF.
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Besides membrane potential, the GHK can also be used to calculate ____.
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Reversal potential for an ion channel that is permeant to more than one kind of ion.
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What is reversal potential?
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The voltage at which there is no net ionic flux through an open channel.
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For an ion channel that's permeant to only one ion, reversal potential=____.
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Nernst potential.
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For an ion channel that's permeant to two ions, the reversal potential will be between the ____ for the two ions.
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Nernst potential.
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What's useful about knowing the reversal potential?
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You can predict whether the opening, closing, or pharmacological block of that type of ion channel will tend to depolarize or hyperpolarize a cell membrane.
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What's the normal resting potential for a cell?
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-70 mV
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What's the normal resting potential for a heart cell?
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-85 mV
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The Nernst potential is a(n) ____ phenomenon.
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Ion
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A reversal potential is a(n) ____ phenomenon.
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Channel
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Do carriers and pumps have large effects on membrane potential?
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No, because they operate slowly and selectively.
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Name two specific non-selective cation channels.
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ACh receptor channels and Excitatory glutamate channels.
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Name two examples of inhibitory channels.
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Cl- channels and K+ channels.
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What's the approximate reversal potential for a non-selective cation channel?
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About zero, which is right between the Nernst for Na and the Nernst for K.
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What is the Nernst for Na?
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+60 mV
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What is the Nernst for K?
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-90 mV
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