Diffusion – Biological Membranes

by Kevin Ahern, PhD

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    00:01 Now one of the things to consider with a lipid bilayer, I've mentioned that it's fairly impermeable, but we can actually measure permeability of ions across a semipermeable membrane, and that's illustrated what we see right here. Membranes are not absolutely impermeable but they're pretty impermeable to a lot of things. When we discuss and think about permeability, we have to think about some considerations about the tendency of molecules. Molecules in solution will tend to move from a high concentration to a low concentration. And if I have a circumstance such as I've set up here, with the chambers on the left, you see that the chambers on the left have been divided in two by a semipermeable membrane.

    00:41 In the left chamber of the vessel on the left we see has a high concentration of molecules, and on the right we see that there is a lower concentration. The molecules can slowly move across that membrane, and if we give things sufficient time, what will happen is that the two concentrations will equal out, that is the high will move to the low.

    01:04 Now the rate of diffusion or the rate of movement of a molecule from the left to the right for example, is related to several things. First it's related to the concentration difference between the two. The greater the concentration difference, the more likely and the easier it is for a molecule to move from the left to the right. Second is charge.

    01:26 Now charge is a little harder to envision, but you could imagine that if something had a great charge difference that there would actually be an electrical potential that would occur across that membrane and that electrical potential might be driving a process to happen and that's exactly what happens when you have a charge difference across the membrane. The third function of course is the permeability of the membrane. The more permeable a membrane is, the easier this process can happen and the faster this process can happen. The less permeable the membrane is, the less likely and the slower that process will occur.

    02:01 Now, membranes like this that are poorly permeable, as I said, can provide a barrier but will not absolutely stop the movement of molecules across it.

    02:10 Now the first movement across membranes that I want to talk about is illustrated by this example here. This movement across membranes is occurring in the process we call facilitated diffusion. Now as its name suggests, diffusion is the movement of molecules from high concentration to low concentration to even each other out, like we saw in the previous illustration.

    02:37 Facilitated part means that something is helping that movement to occur. Well since the lipid bilayer is fairly impermeable, it's not very much allowing these molecules to move across on their own. If the cell has a need to get these molecules across the membrane, the cell will typically have a protein embedded in the membrane that will allow that to occur.

    02:58 Now there's two typical types of proteins that allow this to occur in this process.

    03:04 Now when I emphasize when I say diffusion and I say facilitated diffusion, I'm talking about a process that does not require the input of energy. Because the process is naturally happening as molecules would normally move, from a high concentration to a low concentration, and these proteins that I will describe are simply allowing that process to occur.

    03:28 There are two types of proteins as you can see on here. One is a carrier protein that’s shown in the yellowish color. The carrier protein has a chamber as that you can see, that is allowing the yellow molecules to bind into it and then the carrier is changing shape to allow it to move across. This change as I'm illustrating with my hands, this change that can occur doesn't require any input of energy and is driven simply by the movement of the molecule from high concentration to low. As this occurs over time, then the concentrations of the two will equalize. A good example of this is in our blood cells we have a GLUT, I mentioned the GLUTs earlier, that will allow glucose to move across the membrane of the blood cell according to the concentration difference. So blood cells will have a lower concentration of glucose than the surrounding fluid around them. With the GLUTs, the glucose will move freely into those cells and it doesn't require the expenditure of energy to get the glucose in.

    04:31 Another type of protein that's involved in these exchanges is an ion channel. Now I’ll say more about ion channels later in this talk, but ion channels typically are specific for ions, charged molecules, like sodium or potassium. And the remarkable thing about them, that is the ion channels, is that they're very specific, typically, for one ion and rejecting other ions.

    About the Lecture

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

    Included Quiz Questions

    1. It always requires a transport protein.
    2. It is affected by the concentration of materials on either side of the membrane.
    3. It requires an energy source to move from lower to higher concentrations.
    4. Regulation/control of it is essential for cells.
    5. The charge difference has an effect on movement across the lipid bilayer.
    1. It requires only the energy of diffusion.
    2. It uses ATP or GTP.
    3. It does not require a protein.
    4. It requires antiports.
    5. It moves ions from lower concentrations to higher concentrations.

    Author of lecture Diffusion – Biological Membranes

     Kevin Ahern, PhD

    Kevin Ahern, PhD

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