MCBM Exam #2: Talbot Questions and
Answers (Graded A)
What are the standard accepted "normal" values for ECF ion concentrations? - ANSWER -
What are the standard "normal" values for ICF ion concentrations? - ANSWER -
What direction does sodium want to flow? Potassium? - ANSWER - Sodium: Wants to flow
*into* cell
Potassium: Wants to flow *out* of cell
Which ion has the largest gradient? What direction will it want to flow? - ANSWER - Calcium has
a huge gradient and will go down gradient *into* the cell if possible.
What is a membrane potential? What is the relative charge difference between the outside and
inside of a cell? - ANSWER - Voltage difference across their plasma membranes. The inside is
more *negative* at rest (potassium is leaking out).
What is the voltage influenced by? - ANSWER - 1) The ionic concentration gradients
2) Membrane permeability to these ions
What is the ion responsible for membrane potential? Why? - ANSWER - Potassium. Leaky
channels let potassium leave the cell. When these positively charged ions leave the cell, it
makes the inside of the cell more negative.
Consider the following scenario: *Membrane potential=-60mV*
(first value is concentration inside, second value is concentration outside)
Sodium: 15 mM, 145 mM
Potassium: 120 mM, 4.5 mM
,Chloride: 20 mM, 116 mM
In the previous example, chloride is going out of the cell AGAINST it's concentration gradient. Is
this passive or active transport? - ANSWER - Passive. Even though it is moving against it's
concentration gradient, it is moving DOWN it's electrical gradient.
What does the Goldman-Hodgkin-Katz (GHK) equation measure? How is it different from the
Nernst equation? - ANSWER - GHK allows you to predict membrane potential.
Nernst equation is a theoretical value, GHK is more accurate to the specific membrane you are
studying.
GHK takes into account multiple ions, nernst only considers one.
GHK is comparing relative permeabilities for given ions, nernst only considers concentration
differences.
What is the difference between Membrane and Equilibrium potential? - ANSWER - Membrane:
real value that can be measured. (GHK can estimate this)
Equilibrium: Theoretical value of what the membrane potential will be if an ion is at equilibrium.
What direction will each flow and why? - ANSWER - Sodium: Moves *into* cell. Concentration
going from high to low, voltage going from positive to negative.
Potassium: Can't tell just by looking at this because concentration going from high to low, but
voltage going from negative to positive. Negative charge will want to hold potassium in and we
don't know which gradient will "win"
Chloride: Can't tell just by looking at this. Same reasoning as potassium but reversed.
Bottom line: based off this info alone, we can only predict sodium's movement.
, What does the Nernst equation represent? When paired with the actual membrane potential,
what can these two values tell us? - ANSWER - It predicts what the membrane potential will be
at equilibrium for each ion at the given concentrations.
If we know the equilibrium potential (from Nernst), and the actual membrane potential, we can
predict the direction of flow for a given ion.
Example: A cell has a membrane potential of -88 mV. You use the Nernst equation to calculate
the equilibrium potential of potassium and you get -88 mV. What would this mean for the
movement of potassium? - ANSWER - Potassium would be an equilibrium and net flux would be
zero at this voltage.
Consider the following scenario: *Membrane potential=-60mV*
(first value is concentration inside, second value is concentration outside, third value is
equilibrium)
Sodium: 15 mM, 145 mM, +61 mV
Potassium: 120 mM, 4.5 mM, -88 mV
Chloride: 20 mM, 116 mM, -47 mV
What direction will each flow and why? - ANSWER - *Compare the membrane potential to the
equilibrium potential for each ion*
Sodium: In, going from more positive to more negative
Potassium: Out, going from more positive to more negative
Chloride: Out, going from more negative to more positive
What is conductance? - ANSWER - Refers to the ease of movement of an ion across a
membrane.
What is resistance? - ANSWER - Inverse of conductance (1/G), refers to the resistance of ion
movement.
What is capacitance? - ANSWER - Stores charge separated across the membrane; influences
rate of voltage change.
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