Question

In a freshwater teleost, potassium passes from the environment into the mitochondrial rich cells of the gills by means of NA+, K+, 2Cl- cotransporter. The main reason a simple K+ channel on the apical membrane wouldn't work for K+ is because

A. K+ leak channels are found only on neurons

B. K+ channels can only be used for efflux

C. the concentration gradient opposes K+ import

D. the charge gradient across the apical membrane opposes K+ import

I feel like the answer is between C and D, but I am not sure what the difference is.

Answer 1

C. the concentration gradient opposes K+ import

Cotransporter acts as a protein pump to supply the ions against the concentration gradient. In the case of K+ ions the ions are enough inside. So the concentration gradient bis high but in need of extra k+ ions, cotransporter transport the k+ into the cell. Simple K+ channel doesn't works beacause k+ channels only transport ions according to the concentration gradient. Charge gradient depends upon the net charge. It won't influence the transport of a particular ion accross a membrane. For a particular ion it will be transported in response to its own concentration gradient.