This table is a hypothetical listing of the most abundant ions and their concent

Question

This table is a hypothetical listing of the most abundant ions and their concentrations inside of, and

surrounding, a cell. Use this information to answer the questions below concerning membrane potentials.

Ion Cation A Cation B Cation C Anion D (Fixed anions) (And many less abundant ions) Extracellular Intracellular 120 mM 10 mM 1 mM 130 mM 12 mM 100 mM 5 mM 20 mM 110 mM 1. Given the ion gradients above, if the membrane were selectively permeable to A, would the inside of the membrane be relatively (+) or relatively (-)? Assume that no other ions can cross the membrane 2. OK, so what would the membrane potential be if the membrane were sufficiently permeable to A" that it came to equilibrium? Assume no other ions cross the membrane. Remember that the sign on a membrane potential corresponds to the inner side of the plasma membrane (a -70 mV membrane potential tells you that the inner surface of the membrane has a net negative charge relative to the outer surface of the plasma membrane) 3. If you could add a leak channel for any SINGLE ION on the list, which leak channel(s) would you expect to cause the membrane to be more (-) on the inside? Assume that no other ions pass through the membrane. List a ll possibilities

Explanation / Answer

1. If the membrane were selectively permeable to A+, the inside of the membrane would be relatively (+).

Cause: The concentration of A+ in the extracellular fluid is high in comparison to intracellular fluid, thus allowing entry of positive ions to the inside of the cell, increasing the positivity of inside.

2. If the membrane were sufficiently permeable to A+and it comes to equilibrium, then the potential is called as equilibrium potential for that particular ion.

The equilibrium potential for an ion is calculated by the following formula:

ENa= 61.5 log (A+o/ A+i)      

                 = 61.5 log (120/12)

                 = 61.5 mV

So, the membrane potential would be 61.5 mV.

3. If I could add a leak channel for any SINGLE ION on the list, I would expect B+ leak channel to cause the membrane to be more (-) on the inside.

Cause: Since the B+ ion is much more in intracellular than extracellular fluid, it will leave the inside of the cell, causing the inside to be more negative.

The D- channel would not allow it to enter the cell, as the intracellular anion concentration is much higher inside than outside.

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