Ion Channels and Membrane Potential - Online Tutor, Practice Problems & Exam Prep

Ion channels are gated proteins that facilitate passive transport of ions across cell membranes, responding to voltage changes, ligand binding, or mechanical forces. Membrane potential refers to the charge and concentration differences across the membrane, with resting membrane potential indicating a balance of ion movement. The Nernst equation calculates this potential, influenced by the sodium-potassium pump and potassium leak channels. Techniques like patch clamping allow for the measurement of ion flow through individual channels, enhancing our understanding of cellular activities.

Ion Channels

Hi. In this video, we're going to be talking about ion channels and membrane potentials. So first, let's focus on ion channels. So what are ion channels? They are proteins that form transmembrane pores, which allow for passive transport of small polar molecules. So, ion channels are gated, so what does that mean? Well, that means that they are not continuously open. They can close and open preventing or allowing transport of molecules across the membrane.

Now, there are different types of gates that an ion channel can have. One is called a voltage-gated ion channel, and that is when the channel opens depending on the charge that exists across the membrane. There's a ligand-gated which means that the channel opens in response to binding of a ligand, and then there's mechanically gated, and these channels open in response to a mechanical force. And, I think, this is kind of the hardest one to imagine, but, for instance, there are ion channels that respond to the mechanical vibrations made from sound in your ear. And so those channels can open based on those sound, sort of mechanics vibrating, and opening those channels, I think, is a good example of that one.

So, ion channels are very specific and usually permeable only to a single ion. And so, how they do this is they contain a filter called the selectivity filter, which is inside this narrow pore and ions have to be able to pass this filter, and only ions that can pass this filter will be passed through the channel. So, how they do that is usually ions, because of their charge, are associated with water, usually through weak interactions with water. And so in order to move across the selectivity filter, ions must dissociate from water. Well, the selectivity filter is set up in a way that it will selectively dissociate the ions from the water of the ion that's supposed to pass, but not of ions that aren't supposed to pass. So, only the targeted ion will be able to dissociate from water and cross through the ion channel.

And so, ion channels, just to review, move molecules in response to charges across the membrane. So here we have this, here's an example of a voltage-gated channel, where you can see that there are all these different molecules on this side and not a lot on this side. So then it opens and passes the molecules through, but if it closes, then the molecules can't get through. And then you have your ligand-gated channel, which opens in response to a ligand binding and that transfers molecules across the membrane. So with that, let's now move on.

Membrane Potential

Okay, so now we are going to talk about membrane potential. In membrane potential, the term just refers to the difference in the environment on either side of the cell. So the inside of the cell, intracellular, or outside of the cell. There are lots of differences between the inside and the extracellular environments, but we're only focused on membrane potential which only focuses on a particular type of difference, and that difference is going to be differences in concentration of certain compounds or molecules, and the charge of those compounds and molecules. Membrane potential just refers to concentration and charge differences on the intracellular and the extracellular sides of the cell.

Now, with membrane potential, there's a really kind of buzzword here that you need to know, and that's resting membrane potential. When you hear resting, you might think that, oh, okay. It's not moving, but that is not what resting membrane potential means. What resting membrane potential means is when the amount of positive ions coming in and out of the cell is pretty much equal to the amount of negative ions coming in and out of the cell. This doesn't mean that the positive and negative charges are equal on either side. It doesn't mean that nothing is moving across the membrane. It just means that the negative and positive charges that are moving across the membrane are equal. So a cell or a membrane can have resting membrane potential even if the extracellular environment is more positive than the negative intracellular environment, but they just have to be moving positive and negative ions across the membrane at the same rate. So that's resting membrane potential. Resting membrane potential doesn't mean nothing is moving; it just means it's moving at the same rate. The charge is balanced.

The way that we actually measure this is through an equation called the Nernst equation. Now I'm not going to talk to you about how to calculate the Nernst equation since that's much more chemistry and much less cell biology, but I do want you to know what this has caused because when we start talking more about individual neurons, which we will do, then you're going to see these negative and positive numbers that are calculated using this Nernst equation, and I just want you to know what the equation is called.

There are some important pumps, transporters, and channels you need to know about, that often facilitate this resting membrane potential. That includes the sodium-potassium pump, which is a transporter. This is going to create a gradient, so a concentration gradient, meaning that on one side of the membrane there's going to be more sodium, and on the other side of the membrane, there's going to be more potassium. And because that's a difference in concentration, it's going to be measured by the membrane potential. So the sodium-potassium pump is going to make that large concentration gradient across the membrane. You also have potassium leak channels, which we're going to talk about when we talk about neurons more. These channels are always open, and they allow potassium to flow to restore the membrane potential either back to resting or back to wherever it's supposed to be, which depends on the cell type and what we are talking about.

So, how do scientists know all of this information? Right? Like, they can't really visualize this easily, so how do we know about it? Well, there's a technique called the patch clamp technique. The patch clamp technique can actually measure a single ion channel. So when we talk about potassium leak channels, what we think of is we think of, oh, there's 100 of these in a cell. But the patch clamp technique can actually just focus down on 1 and watch the flow of ions in and out of this one channel. How does it do this? Well, it takes a micropipette, which is an extremely tiny pipette, usually made out of glass or a similar material and is put down straight onto a membrane with a single ion channel in it. Here we see our pipette, and we can see that it has isolated this one ion channel. So as stuff either leaves or enters the neuron, this micropipette is going to send signals to a computer, monitoring how many ions or molecules are passing, how many of them are going to come into the cell, and how many of them are going to be leaving the cell. So, it's just analyzing that flow of ions in and out of that channel. By looking at individual ion channels, then we can really start to get a picture of what all of these individual proteins do during various activities of a cell or neuron.

Okay. So with that, let's turn the page.

Which of the following is not considered a type of gated ion channel?

• A. Voltage gated
• B. Ligand gated
• C. Mechanically gated
• D. Electrical gated

Ion channels require energy to transport substances across a membrane.

• A. True
• B. False

K+ leak channels are important for doing what?

• A. Keeping a steady level of K+ inside the cell
• B. Keeping a steady level of K+ outside the cell
• C. Maintaining a strict concentration gradient across the membrane
• D. Maintaining membrane potential

When the concentration of ions is exactly the same on either side of the membrane, then the cell is at a resting membrane potential.

• A. True
• B. False

Here’s what students ask on this topic:

What are ion channels and how do they function?

Ion channels are proteins that form transmembrane pores, allowing for the passive transport of ions across cell membranes. They are gated, meaning they can open or close in response to specific stimuli. There are three main types of gated ion channels: voltage-gated (responding to changes in membrane potential), ligand-gated (responding to the binding of a specific molecule), and mechanically-gated (responding to mechanical forces). Ion channels are highly selective, often allowing only specific ions to pass through. This selectivity is achieved through a structure called the selectivity filter, which ensures that only the targeted ions can disassociate from water and pass through the channel.

What is membrane potential and how is it measured?

Membrane potential refers to the difference in charge and concentration of ions across the cell membrane. It is crucial for various cellular processes, including the transmission of nerve impulses. The resting membrane potential is the state where the movement of positive and negative ions across the membrane is balanced. This does not mean that the charges are equal on both sides, but rather that the rate of ion movement is equal. The Nernst equation is used to calculate the membrane potential, taking into account the concentration and charge differences of specific ions across the membrane.

What is the role of the sodium-potassium pump in maintaining membrane potential?

The sodium-potassium pump is a crucial transporter that helps maintain the membrane potential by creating a concentration gradient. It actively transports three sodium ions out of the cell and two potassium ions into the cell, consuming ATP in the process. This creates a higher concentration of sodium outside the cell and a higher concentration of potassium inside the cell. The resulting gradient is essential for various cellular functions, including maintaining the resting membrane potential and enabling action potentials in neurons.

How does the patch clamp technique work in studying ion channels?

The patch clamp technique is a powerful method used to study the activity of individual ion channels. It involves using a micropipette to isolate a small patch of the cell membrane containing a single ion channel. The micropipette forms a tight seal with the membrane, allowing researchers to measure the flow of ions through the channel. This technique provides detailed information about the ion channel's properties, such as its conductance, gating mechanisms, and response to different stimuli. It has been instrumental in advancing our understanding of cellular electrophysiology.

What is the selectivity filter in ion channels and how does it work?

The selectivity filter is a crucial component of ion channels that ensures only specific ions can pass through. It is located within the narrow pore of the ion channel and is designed to selectively disassociate the targeted ions from their associated water molecules. This selectivity is achieved through specific interactions between the ions and the amino acids lining the filter. Only ions that can successfully interact with the selectivity filter will be able to pass through the channel, ensuring the channel's specificity and proper cellular function.