Problem Set 9

1. Ion channels are extremely selective for specific ions:

a) Use electronic configurations for each of the ions to explain why it would be logical to think that ion channels that are permeable to potassium ions would also be permeable to sodium ions.

b) Now explain why potassium ion channels do not allow sodium ions to pass. Clearly support this in terms of underlying physical principles.

c) Outline the differences between voltage-gated and ligand-gated ion channels.

2. Calculate the Nernst potential, in mV, for the chloride ion that had extracellular and intracellular concentrations of 120 mM and 4 mM respectively. For a cell with a resting membrane potential of -70 mV, explain what would happen if a chloride channel opened. Would this have an inhibitory or excitatory neural effect?

3. A particular neuron had [K⁺] intracellular and extracellular concentrations of 150 mM and 5 mM respectively and a resting membrane potential of –65 mV. This cell was exposed to a G protein coupled receptor ligand that activated (opened) potassium ion channels.

a) Calculate the change in Gibbs Free Energy associated with the potassium concentration gradient and comment on what would be expected to occur---based only upon concentration gradients--when the potassium ion channels open.

b) If potassium ion concentrations were equal on both sides of the membrane, calculate the change in Gibbs Free Energy required to move a mole of potassium ions into a cell across a –65 mV potential gradient.

c) Using your answers to a and b, explain what effect (stimulatory or inhibitory) the ligands responsible for opening up potassium ion channels would have on this neuron. Clearly provide the rational for your answer to effectively explain the underlying physical basis for your prediction.