Ion Channels and Pumps

by Jason Amores Sumpter
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in order for ions to get from one side of the membrane to the other, they're gonna need ion channels, these air just protein channels that essentially form a poor through the membrane and allow for the passage of a specific ion. And that's pretty key to note here that thes ion channels, they're going to be selective for specific ions. Now, these iron channels are both critical in establishing membrane potentials and in the transmission of electric signals and neurons, which we're gonna learn about in just a second, which are called action potentials. Now, there are these, uh, special type of ion channels that I want to briefly mentioned called leak channels. They're called leak channels because they actually basically let potassium ions leak out of the inside of the cell, which, hopefully remember, is where they're very highly concentrated. And this is in order to help maintain the negative resting potential or the negative potential of the membrane when it's at rest of neurons. So basically, thes leak channels are going to be like a special case of ion channel that allow these potassium ions to leak out of the interior of the cell. And it's just to help maintain the negative, the negative potential in there. Now, the rial stars of the show are the gated ion channels. These are the guys who are going to be critical Thio neurons sending electric signals and these air ion channels that open or closed in response to stimuli. So their gated by some signal that they have to receive. And we're going to see two major types ligand gated ion channels which open in response to ligand binding. So, like a ligand binds to a receptor and that causes an ion channel to open or voltage gated. This is, um, you know, kind of crazy stuff here, but basically these iron channels will open in response to specific membrane potentials. And these air gonna be the particular types of gated ion channels that air crucial to sending electric signals through. Uh, but the Exxon will through neurons in general. So we're really gonna be focusing on two types here sodium channels, which you can see here in blue, and you just write it and read to be crystal clear. We have potassium channels over here. Now, what's the difference? Well, sodium channels will actually have an extra sort of state of being that we won't see in these potassium channels. Basically, with the potassium channels, they're either open or they're closed. And if they're open, the ions can move through them. If they're closed like you see here, passage is blocked. The ions will just bounce off of them. Know ions, air getting through the membrane. So so sodium channels just like that. They haven't a closed state and an open state, but they also have this special state we call inactivated. And usually it's, uh, this is thought of, as they call it, the ball and chain model. Basically, you have this chain with a ball on the end that is attached to the the ion Channel and given a certain, uh, a certain condition, let's say that ball will actually plug up the ion channel like you see here, and that's going to cause it. Thio not allow any ions through. So three states closed open inactivated ions. Onley will flow through in the open state, and the significance of the inactivated state will become clear in just a moment when we actually talk about steps of the action potential. So the other uhh ion mover shall we say that is critical for maintaining and establishing membrane potentials. These sodium potassium pumps or an ACA at P aces. I like to call it now. These are pumps that, uh hopefully this isn't the first time you're seeing these. We've talked about them before. They will use ATP to actively pump. That's why so, actively, their meaning, ATP is consumed and they're gonna pump three sodium ions out of the cell, and they're gonna bring to potassium ions into the cell. And this is with each cycle. You can't get rid of the sodium ions and not bringing potassium ions. ITT's like three go out to come in one cycle. Repeat so you can see a model of that happening here. I don't actually care that you know the specific steps of what's going on. I just want you to know that ATP gets consumed. We have three sodium ions that leave the cell. This is out. This is in, and we have gonna jump out of the way here. These two potassium ions that come into the cell. Okay, so there is a special type of potential which we call equilibrium potential. This is going to get back to those leak channels. Basically, it's the membrane potential, at which point there's no net movement oven ion in or out of the cell. So basically, while you know some ions are still moving in and out of the cell, the concentrations in and out of the cell are static there. There's no change. There's no net movement here now. Why is this important? Well, remember that there's both concentration Grady INTs and these electric potential Grady INTs, that air acting on the ions right now. Those leak channels we mentioned before they are going to allow potassium toe leak out of the cell along its concentration radiant. But at some point it will reach equilibrium potential because it will have sort of competing forces. So there's going to be a concentration radiant that causes sodium. I'm sorry potassium to want to move out of the cell, but remember that the interior of the cell is negative exterior positive. So at some point, the potassium is going to stop moving out of those leak channels because it's going to reach its equilibrium potential, which we would write Oops, we would rate as E with little K under there it's going to reach its equilibrium potential, where basically the force driving it to move along, its concentration radiant, will be balanced with the force trying. Thio drive it inward with against its concentration ingredient but with its electrical radiant. So those leak channels actually will get potassium Thio hit a point of equilibrium potential when the cell is at rest. All right with that, let's go ahead and flip the page.