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?
  • $\begingroup$ @Christiaan Thanks for the comment, but I'm specifically confused about the proteins, not active and passive transport. $\endgroup$ – city7lights Dec 20 '15 at 13:22
  • $\begingroup$ Usually, when introductory textbooks and tests ask about pumps, they are usually ion pumps, such as Proton pumps or Sodium - Potassium pumps, etc. Carriers usually are used to transport hydrophilic molecules such as glucose through the membrane. Carriers can be both. Passive usually requires another substance, such as sodium where there is a gradient. The binding of the transported molecule will cause a conformation change in the carrier protein that transport the molecule to the other side of the membrane. Have you talked to classmates about their understanding? Study groups are a good idea. $\endgroup$ – AMR Dec 20 '15 at 22:24
  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.

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    $\begingroup$ Good summary. A couple of comments to consider. First, there is not a single type of glucose transporter, which isn't clear from this answer. The one you describe is used in intestinal and renal epithelia, but most mammalian cells use a simple uniporter for glucose uptake. Second, I think that many would consider the proton transport out of the inner mitochondrial membrane, needed for oxidative phosphorylation, to be pumping, but the energy sources there include NADH and oxygen, not ATP. $\endgroup$ – EdM Dec 21 '15 at 17:56
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    $\begingroup$ A small note: There are also carriers that use ATP by their ATP Binding Cassette domain (ABC transporters). The mechanism, however is different from that of the phosphorylation dependent pumps. $\endgroup$ – WYSIWYG Dec 22 '15 at 5:10

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