1. Using the Nernst equation, calculate the equilibrium potential for Ca21 and for Cl2 from the...

1. Using the Nernst equation, calculate the equilibrium
potential for Ca21 and for Cl2 from the following sets of data:

a. Given [Ca^{2+}]_{o} = 1 mM, [Ca^{2+}]_{i}
= 100 nM, find E_{Ca}^{2+}

b. Given [Cl^{-}]_{o} = 110 mM, [Cl^{-}]_{i}
= 10 mM, find E_{Cl}^{-}

2. One of the important uses of the Nernst equation is in
describing the flow of ions across plasma membranes. Ions move under the influence
of two forces: the concentration gradient (given in electrical units by the
Nernst equation) and the electrical gradient (given by the membrane voltage).
This is summarized by Ohm’s law:

I_{x} = G_{x} (V_{m }- E_{x})

which describes the movement of ion x across the membrane. I
is the current in amperes (A); G is the conductance, a measure of the
permeability of x, in Siemens (S), which is DI/DV; Vm is the membrane voltage;
and Ex is the equilibrium potential of ion x. Not only does this equation tell
how large the current is, but it also tells what direction the current is
flowing. By convention, a negative value of the current represents either a
positive ion entering the cell or a negative ion leaving the cell. The opposite
is true of a positive value of the current. a. Using the following information,
calculate the magnitude of I_{na}^{+}.

b. Is Na^{+} entering or leaving the cell?

c. Is Na1 moving with or against the concentration gradient?
Is it moving with or against the electrical gradient?