Question #3: Membrane
Potential
(permeable to a multiple ion
species)
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In a hypothetical cell the intra and extracellular
environments are as follows:
| |
inside |
outside |
ratio of permeabilities |
| Sodium |
50 |
337 |
.019 |
| Chloride |
41 |
340 |
.381 |
| Calcium |
.04 |
10 |
.002 |
| Potassium |
168 |
6 |
1 |
1. What would the membrane potential be if the membrane were
only permeable to potassium and sodium? Show your work. (-65.79mV)
2. What would the membrane potential be if the membrane were
permeable to all ions? Show your work. (-61.83mV)
3. Does the membrane hyperpolarize or depolarize when the
potassium conductance increases? (hyperpolarize) Why?
4. New bonus questions: What factors contribute the most to
the difference in membrane potential found between the cell in
Question 1 and the cell in question 2? (Chloride
conductance)
5. Second new bonus question: What effect would temperature
have on the membrane potential of the cell described in Question
1? (increases in temperature would hyperpolarize the
cell, decreases in temperature would depolarize the cell)
Questions, Comments:
(4/3/98, 12:36:05 AM) Marla Shu (sms7224) wrote:
Okay, so how do we do this one? I was wondering if the variable "z" is
always one, or if it refers to charge, calcium being +2, chloride being
-1, sodium being +1.....etc...? Also we've tried adding up all of the
individual membrane potentials....for part 2 of question 3...and um, well,
that doesn't work. And then we tried to use the GHK equation. Are we
supposed to assume that permeability=conductance? And what does the variable
"P" stand for, permeability? pressure? wha? who? blah??? could we see the
solutions in addition to the answers? that would clear up a lot.....
thanks.
(4/3/98, 1:56:26 PM) Brad Skow (sbs8650@oberlin.edu) wrote:
In response to the above: I worked it with P being the ratio of
permeability for the ion, and used the GHK equation. I don't think
you can do this one with the Nernst Equation, so the valence ('z')
doesn't come in. But my answers were off by about +2 mV.
(4/5/98, 11:28:51 AM) () wrote:
are we going to get any answers to this before the exam? it would be helpful...
(4/5/98, 6:07:16 PM) AMB answers to above questions (fborroni@oberlin.edu) wrote:
Brad is correct. You would use the GHK equation. The difference in values
that Brad and others may have gotten would be the result of using different
temperatures to calculate the constant K. I used 295 degrees Kelvin for the
temperature
Also, Valence (z) doesn't figure into K (RT/F).
AMB
(4/5/98, 6:10:36 PM) AMB reply to specific part of sms7224's question (fborroni@oberlin.edu) wrote:
The P in the GHK equation stands for permeability. Permeability is proportional
but not exactly equal to conductance. Both conductance and permeability
can be thought of as values that represent the ease in which an ion
flows across the membrane.
(3/8/98, 5:15:44 PM) Emily (seo2327) wrote:
Are you sure you used 295K? I worked these out with standard room
temperature (298K), then 295K and I'm still off by a few mV. Maybe you
used 310K as written on the handout?
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