BIOLOGY W3004 EXAM #1 NAME:



BIOLOGY W3004 EXAM #1 NAME: .

1. (5 PTS) Why is the K Equilibrium Potential -90mV and the Na Equilibrium Potential +55mV when both are positive ions?

2. (8 PTS) For a mammalian neuron at 370 C. you find that [K]out=20 mM, [K]in=400 mM, [Na]out=400 mM, [Na]in=40mM. You then measure the permeabilities and find that the membrane is 10 times more permeable to K than Na ions. Ignoring Cl, compute the resting potential of the neuron.

What value would the membrane potential reach if PNa/PK were equal to 1?

3. (4 PTS) In the Nernst equation the number 58 seems to be a magic value. Write down the terms that add up to "58" and identify which relate to chemical potentials and which to electrical potentials.

4. (8 PTS)The resting membrane potential was predicted by the Nernst equation to change by 58mV for each 10 fold change in K concentration. However the experiments of Hodgkin and Katz showed a deviation from this behavior, as seen in the graph below. They reasoned that this was probably due to what factor? Why is the deviation from the Nernst prediction greater at lower concentrations of external K?

5. (16 PTS) It is said that in life timing is everything. In neurobiology too. For graded changes in membrane potential the time course (dVm/dt) is determined primarily by what feature of the cell membrane?

(Nobody should get this wrong, please)

In a spherical cell, a step of 100 pA of positive current causes the cell to depolarize by 20 mV. The membrane time constant, tau, of the cell is 15 msec. What is the membrane resistance, Rm? What is the total membrane capacitance, Cm?

What would be the capacitance of 1 square centimeter of this membrane?

5. (continued)

Now for the tough part:

Threshold for action potential generation is 30 mV above (depolarized from) the resting potential. How close together in time would two pulses of the 100 pA current have to occur in order for the cell to reach threshold?

Here's how to proceed: Make a drawing of the current and voltage responses, write down the equation describing the exponential time course of the voltage change (for this you get some credit). Now think about what must happen for the membrane potential to reach threshold (for this you get most of the rest of the credit) and the rest is just math (for this your calculator gets the credit).

(5 PTS) What information about channel properties can you obtain from the slope and intersection (on the V axis) of its I-V relation?

(4 PTS) For the following kinetic model of channel activation what is the Po when K1 = k-1.?

(6 PTS) What kind of information do you get from a hydropathy plot? How would such a plot look for a channel protein with three transmembrane segments?

(12 PTS) Draw a schematic picture of the molecular structure of single subunit of a typical K channel, labeling the pore region, the voltage sensor and the inactivation "ball and chain". How many of these subunits are thought to be required to make a functional ion channel

(4 PTS) What are the four properties of ion channels?

11. (8 PTS) In the squid giant axon Na+ channels exist in a feedback relation to the membrane potential, K+ channels exist in a feedback relation, and the voltage clamp is an example of a feedback process. Can you use this information to provide an explanation of the diagram below:

12. (12 PTS)The voltage trajectory of an action potential vs. time is drawn below.

- Carefully re-draw the trace in your blue book. Then draw, superimposed on this trace, curves representing the activity of the Na and K conductances (not currents). Label them clearly.

- Give reasons for the absolute and relative refractory periods following the action potential, and indicate these periods with brackets on the figure above.

- Re draw the action potential under the following conditions:

1 mM TTX

20 mM TEA

13. (8 PTS) Why doesn't the peak of the action potential reach the Equilibrium potential for Na? Conversely why is it that the undershoot of the action potential does reach the K Equilibrium potential? (this requires some careful thought).

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