what is an ion pump?

Question

I'm unsure about how ion pumps work and what they are, specifically cotransport and glucose moving into epithelial cells.

Answer 1

Hi Kara,

 

This is a great question, and something that confuses a lot of people. I hope my answer is helpful!

 

An ion pump is a membrane protein that pumps ions into and out of the cell, creating a concentration gradient. The most important/well studied ion pump is the sodium potassium pump. Let's look at this one piece at a time.

 

Remember that ions cannot pass through a membrane without help. This is because the inner part of the membrane, which is formed by the fatty acid tails of phospholipids, is non polar and repels ions. An ion can only move across the membrane if it goes through an ion channel or an ion pump. 

 

But what is the difference between these two? To understand that you have to know what a concentration gradient is. A concentration gradient is when there is a higher concentration of an ion (or other particle) on one side of a membrane than on the other. For example, if you have a bunch of sodium ions on the outside of the membrane, but very few on the inside, then you have a concentration gradient. In Biology, everything is trying to reach equilibrium, or balance. That means that the sodium ions will "want" to get to the inside of the membrane so they can even out the concentration difference. An ion channel is simply a membrane protein that forms an ion-friendly passageway through the membrane, so that the ions can get through in the direction they want to go. A pump is different. It actually grabs ions and pushes them through the membrane in a certain direction, whether they want to go that way or not. This can actually create a concentration gradient. For example, you may start with a fairly equal amount of sodium ions on either side of a membrane, but if a sodium pump is pushing ions through to the outside, you will eventually get a lot more sodium ions on the outside than on the inside. Tada! A concentration gradient. 

 

Another difference between an ion pump and an ion channel is that pumps require energy, while channels do not. This is because the pumps are forcing the ions to move, while the channels are simply allowing them to go where they want. When energy is used to get something across a membrane, that is called active transport. When energy is not used, that is called passive transport. Usually the energy that ion pumps use is in the form of ATP.

 

What does this have to do with cotransport and glucose moving into epithelial cells? First you need to know what cotransport means. Cotransport is when two ions or particles move across a membrane at the same time, using the same transport protein (a transport protein is any protein that gets something across a membrane. Ion channels and pumps are both examples of transport proteins.) For example, in the epithelial cells of the intestine, glucose and sodium move into the cell using the same transport protein. Sodium cannot get in unless glucose comes with it. 

 

This is where it gets really interesting! It also gets a little tricky to understand without a picture, so if you have a textbook or some notes, get them out and find a picture to look at while you read my explanation. If you don't have a picture, try to draw one!

 

So in the small intestine, where glucose and other nutrients are transported from the inside (lumen) of the intestine to the blood, the walls are only one cell thick. On the blood side of an intestinal cell, there are lots of sodium potassium pumps, pumping potassium in, and, most importantly, pumping sodium out. On the intestinal lumen side, there are lots of sodium-glucose cotransporters (those proteins that won't let sodium in unless glucose comes with it.) The sodium potassium pumps create a sodium ion concentration gradient-- because sodium is being pumped out, there isn't much left inside the cell. This means that sodium wants to get in. And guess what has to come with it? Glucose! And since so much glucose is coming in with the sodium, glucose builds up inside the cell, creating a glucose concentration gradient. Now the glucose wants to get out, and there is a transport protein on the blood side of the membrane that is happy to let it out. That's how glucose gets transported from the inside of the intestine to the blood. (This is called secondary active transport. Neither of the proteins that let glucose across the membrane use ATP, but the sodium-glucose cotransporter uses the sodium concentration gradient that was created by the active transport of the sodium-potassium pump.)

 

I hope this makes sense and answers your question. Good luck!

 

Rachel