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MCB Exam 2 Questions and Answers

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MCB Exam 2 Questions and Answers

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  • June 7, 2024
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MCBM Exam #2: Talbot
What are the standard accepted "normal" values for ECF ion concentrations? -



What are the standard "normal" values for ICF ion concentrations? -



What direction does sodium want to flow? Potassium? - 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? - 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? - Voltage difference across their plasma membranes. The

inside is more *negative* at rest (potassium is leaking out).



What is the voltage influenced by? - 1) The ionic concentration gradients



2) Membrane permeability to these ions

,What is the ion responsible for membrane potential? Why? - 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

What direction will each flow and why? - 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? - 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? - 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? - *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



In the previous example, chloride is going out of the cell AGAINST it's concentration

gradient. Is this passive or active transport? - 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? - 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? - 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.

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