Question

how could a channel protein regulate what enters the cell?

Answer 1

well, imagine something like a basketball hoop. a basket ball will fit. but will a giant balloon? or that mean fat kid who deserves it like no one else? =) that's the case for water channels, for example.

also, many ion channel proteins work like bouncers. there's a 'face-check' where ions have to bind to specific structures for the channel to let them through (via conformational changes).
others don't let you into the club unless someone's coming out - there's examples of that as well.
again others don't let you in unless you pay them a twenty...well, ATP, you know...opens doors for every one of us.

Answer 2

haha awesome analogies.