Ion Channel – Biological Membranes

by Kevin Ahern, PhD

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    00:01 Ion channel proteins schematically are shown as you see here. They will typically have a chamber that is very selective for specific ions. Okay. Selective permeability means that certain ions will move through them, but the system will reject other ions that are not the same as that. So for example, the sodium system will only allow sodium ions to pass through it, it will not allow potassium ions to past through it.

    00:26 As I said, the ion channels allow for a very high rate of ion movement. Now the gates that control this can open and close, and the ability of the gates to open and close is a very important consideration for the function of an individual cell. There are various things that can regulate the opening and closing of the gates in these ion channel proteins. A manipulation can cause a change, and when this change happens, then the membrane will change its polarization.

    00:53 So for example, a nerve cell that is receiving a signal, if it opens its sodium channels, will now have a very big difference in its sodium concentration as a result of sodium moving through that channel. Now there are different types of gates that respond to different things in these channel proteins. So for example there are gates that are known as voltage-gated.

    01:16 In our nerve cells for example, we can start a signal at one end of the nerve cell, and nerve cells can be very, very long by the way, but that signal has to be propagated all the way along the nerve cell. How does that signal get propagated? One of the ways in which it happens is that along the nerve cell, is that there are gates that are sensitive to very, very small changes in voltage. As the sodium ions come into the nerve cell, near that place where it comes in, the voltage will change very slightly, which means that a gate that is voltage sensitive that is near that, will sense it and open itself, which will result in the propagation of a series of steps of opening, opening, opening, opening, opening, opening, opening all the way along the nerve cell, and remarkably, this can happen very, very rapidly. Sodium and potassium are the most common of these types of gates but there are also calcium gates that can open, as well as others.

    02:14 Now ligand gated, are typically found at the points of junction or synapses between nerve cells, where the information from one nerve cell must be communicated across the gap to another. These ligand gated channels can respond to certain neurotransmitter or other molecules that are released by the first cell into the second cell. So let’s imagine I have a nerve cell for example that has gotten a signal, and the signal has reached the end of that nerve cell. Well we don’t want the signal to die, we want the signal to be transmitted to the next nerve cell, but there's a gap between the two. When that happens, the first nerve cell will release what's called a neurotransmitter, and the neurotransmitter will go across that junction, from the one cell to the other cell, and bind to it. And when it binds to it, what it'll cause the other cell to do is open its gates. Now this process I've just described to you is a ligand gated process, where a molecule, a neurotransmitter for example, is moving across the synapse, and causing a gate to open. The cause of that gate to open now propagates along the next nerve cell as I've just described. There are other gates that other nerve cells can respond to. For example there are gates that respond to light, there are gates that respond to mechanical pressure, and there are also gates that respond to temperature Now a very interesting gate that we talk about and I’ve talked about it also in another lecture in these modules, is that of ATP synthase. ATP synthase actually allows the movement of protons into the mitochondrion. So what we are looking at here is a depiction of the lipid bilayer of the mitochondrion. The mitochondrion has embedded in it a portion of the ATP synthase enzyme. The portion that's relevant for this discussion is the pink part above that's embedded in the lipid bilayer. You can see in this construction that the concentration of protons is greater above, which corresponds to outside the mitochondrion, than it is below, which corresponds to in. The pink portion of the ATP synthase is an ion channel, and the ion channel allows the movement of protons to pass through it. So we see the lipid bilayer, we see what's called the stalk of the ATP synthase right here. We see the rotor.

    04:39 Now the rotor is a part that connects the stalk in the lipid bilayer to the mushroom head at the very bottom. Now this rotor actually rotates, it actually turns. And the way that it turns is as the protons move through the stalk. This is a remarkable process.

    04:56 I’d like to make the analogy of this to a turbine at a dam, where water flows and turns the turbine, and the turbine turns and generates electricity. This ATP synthase has protons moving through its turbine, and the turbine rotates the rotor, and the movement of the rotor causes ATP to be synthesized, a remarkable process. Now we can see this happening here because the protons in the higher concentration, enter the chamber from above, move through the chamber, and come out below. This is a very natural movement for these because again, the concentration of the protons is greater outside of the lipid bilayer than it is inside.

    05:36 ADP enters the mushroom chamber as you can see here, along with phosphate, the mushroom part of the ATP synthase squeezes these together and when it squeezes them together it forms an ATP which it then releases, and this is the way that most of the ATP in our cells is made.

    About the Lecture

    The lecture Ion Channel – Biological Membranes by Kevin Ahern, PhD is from the course Biochemistry: Basics.

    Included Quiz Questions

    1. They rely on diffusion.
    2. They require ATP or GTP.
    3. Individual ion channels allow a wide range of ions to pass through them.
    4. They are found in the cytoplasm of cells.
    5. Most ion channels open spontaneously.
    1. They are electroneutral.
    2. They are gated.
    3. Blockers of their action can have both medicinal and neurotoxic effects.
    4. Opening/closing changes the voltage across a membrane.
    5. Ion channels are specific for the molecules they allow to flow through.
    1. Lidocaine
    2. Acetylcholine
    3. Glutamate
    4. γ-Aminobutyric acid (GABA)
    5. Cyclic nucleotides

    Author of lecture Ion Channel – Biological Membranes

     Kevin Ahern, PhD

    Kevin Ahern, PhD

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