Saturday, October 31, 2009

#8 Membrane carrier vs. channel proteins

GRE-ST #26 question about difference between carrier and channel proteins. I just discovered that I don't even kind of idea of what membrane carrier protein is.
Membrane carrier proteins differ from membrane channel proteins by which of the following characteristics?
(A) Carrier proteins are glycoproteins, while channel proteins are lipoproteins.
(B) Carrier proteins transport molecules down their electrochemical gradient, while channel proteins transport molecules against their electrochemical gradient.
(C) Carrier proteins can mediate active transport, while channel proteins cannot.
(D) Carrier proteins do not bind to the material transported, while channel proteins do.
(E) Carrier proteins are synthesized on free cytoplasmic ribosomes, while channel proteins are synthesized on ribosomes bound to the endoplasmic reticulum.

Show answer
The best answer is provided in MBoC, of course. It is pretty short and uncovered in chapter 11, paragraph "There are two main classes of membrane transport proteins: carriers and channels". Briefly, carriers catch molecule of interest and push it inside or outside of compartment or cell by change in conformation, while channels organize – actually – a channel (often, filled with water or solution) that creates favorable conditions for transporting molecules of interest in- or outside.

Problem of transport through membrane is in its impermeability for charged molecules. That is why transport proteins are crucially important for cell. Transport differ, in common, into two groups: active and passive. It is transport against or down electrochemical gradient of molecules.
What is gradient? This topic is widely covered in calculus, but can be described simply with analogy of hill. If you put piece of rock on side of hill, rock will ran down to decrease its potential energy. Thus rock will roll down potential energy gradient. Same thing happens in the cell. If there are lots of molecules out of the cell and little inside, they will attempt to penetrate membrane. This process is spontaneous and do not need any energy, because it is actually hidden in solution and molecules themself, as energy for rolling is actually potential energy.
Another story with active transport. There we, obviously, need to take energy to drag molecule against its concentration gradient as we need energy to roll rock back on the hill.
Electrochemical gradient referred to two types of gradients. First one is gradient of electric charge. Because particles discussed now are charged, electric forces are involved. It takes some energy to pull positive charge into positively charged solute. We discussed chemical, or concentration, gradient above, it is about forces that drive molecules to occupy the larges volume. When speaking about electrochemical gradient, we are speaking actually about some mixture of those forces.

Now lets continue our talk about transport through membrane.
Important point here is that channels can't participate in active transport, i.e. force molecules against their electrochemical gradient, it was made possible by carrier proteins. As I mentioned, channel proteins create pore with channel, that act as defense from hydrophobic forces from phospholipid tails.

But what is the answer? That is difference in mechanism of transport. Using our brief discussion it is easy to identify false answers and explain their fallibility. Some trouble can be caused by answer A, because it is not clear are those proteins glycosilated or bound by any part to lipids. All – carriers and channel – proteins are membrane proteins, thus are synthesized on ER ribosomes. The difference between carriers and channels are, actually, in their names. As road tunnels don't ever touch cars, but ferries do, carriers closely interact and fix molecules on their surface, while channels just opens free road for rolling down gradient, so the right answer is D.