BSCI 426/BIOL 622                         Fall '14                          Problem Set 11

assigned:     12/02/14
due:           12/09/14

Selectivity in Channels

The following table summarizes results that might have been obtained on a hypothetical channel. This channel is ideally selective for cations over anions.

Permeant ion	single-channel conductance in  0.1 M chloride salt	single-channel conductance in  1.0 M chloride salt	Pion/PK (biionic 1.0M, chloride salts)
Li^+	5 pS	50 pS	0.3
Na^+	7 pS	60 pS	0.6
K^+	10 pS	50 pS	1

From these results,

    -Generate an hypothesis that could explain the reasons for the observed changes in conductance with concentration and with the nature of the cation.
    -Does your hypothesis also explain the relationship between the conductance measurements and the permeability ratios?              If not, propose an hypothesis that does?
    -Using your hypothesis, predict what the permeability ratios would be if the reversal potentials were measured using 50 mM salt solutions?
    -What can you say about the nature of the selectivity filter?
    -What experiments, similar to those presented, should be performed to further elucidate the nature of the selectivity filter?

Please answer these questions thoughtfully.  Your answer will reflect your understanding and reasoning ability.