Question

i also need to know what a protein channel would be, in a cell city

Answer 1

Kinda like the toll booths on a highway.

Answer 2

Probably more like the outer boundaries of the city. Although, there aren't really checkpoints as you leave a city haha. maybe in a warzone? :p

Answer 3

Perhaps like a low bridge crossing the road leading in to the city, only allowing cars under a certain height to enter, since protein channels are generally selective.