Difference between protein channels, protein carriers and protein pumps?

Question

I'm revising for my biology exam and I don't fully comprehend the difference between protein channels, carriers and pumps. I know that:

1. Protein channels do not require ATP (passive transport)
2. The difference between passive and active transport

My questions are:

1. Are protein carriers and pumps the same?
2. I read that protein carriers can perform both active and passive transport, is this true?
3. But protein pumps always require ATP?

Answer 1

1. No, carriers are not the same as pumps. Carriers may or may not carry out active transport and pumps always use energy. Carriers, for example, can make use of the concentration gradient of a certain ion built up by pumps to transport other molecules actively against their gradient. For example, the glucose transporter uses the sodium gradient to transport glucose against the concentration gradient into the cell, while getting the energy by co-transporting sodium into the cell along the gradient. The sodium import yields energy. That in turn is caused by the Na⁺ K⁺-ATPase, which builds up the sodium gradient using ATP. The glucose transporter is called a symporter, because it co-transports sodium into the cell. An antiporter transports another particle in the opposite direction.
2. Yes, true. The glucose transporter as explained above is an example of active carrier-mediated transport. Other carriers just mediate passive diffusion of, for example, large molecules that don't fit through channels and won't readily cross the cell membrane.
3. Yes, pumps always use energy under physiological conditions. The energy source is always ATP.

_{Will add references shortly.}