Question

A protein channel, like the peripheral and integral proteins in the diagram, is a transport protein that
A. always moves substances against a concentration gradient.
B. always moves substances with a concentration gradient.
C. serves as a tunnel for specific substances.
D. always requires energy from the cell.

i'm thinking A or D but i'm not sure.

Answer 1

A protein channel is a transport protein that helps larger molecules, that would otherwise not be able to defuse throw the membrane of a cell, to move in or out of the cell depending on the concentration gradient.

Answer 2

they're not only for larger molecules but also for very polar molecules. They don't always need energy, they can move substances along, or against, their concentration gradient.