Passive Transport – Transport Across Cell Membranes

by Georgina Cornwall, PhD

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    00:01 So we have a great understanding now of membrane structure and some of the things that we find in the membrane which include proteins.

    00:10 Many of these proteins are involved in transport across cell membranes which is the topic of this lecture.

    00:18 So in this lecture, we will be differentiating between three mechanisms of passsive transport, that is transport that does not require energy.

    00:26 By the end of the lecture, you should be able to diagram the association between primary active transport and secondary active transport.

    00:36 And in addition, you should be able to distinguish between three different modes of bulk passage.

    00:43 So we'll begin by exploring what we mean by passive transport. Passive transport is mainly dependent on concentration gradients.

    00:55 So for example, an area of high concentration and an area of low concentration, we can see movement from high concentration to low concentration just based on movement of particles in the air, atomic movement.

    01:11 Remember when it's warmer, things move faster, when it's cooler, things move slower.

    01:15 So if I were to spray some perfume in this corner of the room, eventually the people over in that corner of the room will smell the perfume. This is simple diffusion.

    01:26 Now when we have simple diffusion occuring, there's no energy required.

    01:32 It would take some energy however if we wanted to put the perfume back in the bottle.

    01:38 So that would be active transport. So passive transport requires absolutely no energy.

    01:44 Things will diffuse through the environment or throughout the cell.

    01:49 Now if we have a cell membrane in the way, can things pass through that cell membrane? And the question comes with what can actually pass through the cell membrane? We already know the middle of the cell membrane, the sandwich. We have hydrophilic outer edges and a large hydrophobic lipid friendly volume in the middle, which is fairly unfriendly if you are a polar molecule.

    02:19 So non-polar molecules can actually squeeze between the hydrophobic heads because they're quite small, very thin layer.

    02:27 And it can make its way if they're small enough, squeeze their way between the lipids.

    02:33 So we'll see often that lipid based or hydrophobic molecules can pass through the cell membrane.

    02:41 Things like steroid hormones will pass through a cell membrane.

    02:45 However, things that are large and polar cannot pass through a cell membrane.

    02:52 So in passive transport, we move things from high concentration to low concentration but what if those molecules are indeed large or hydrophilic molecules? So hydrophilic molecules need a passage to pass through in order to get into the cell.

    03:13 We actually need to provide an aqueous passage. So let's take a look at this a little bit closer.

    03:20 In order to facilitate the diffusion of larger or polar molecules through the cell membrane, we need to have either a channel protein or a carrier protein.

    03:33 Channel proteins act to form, just like they sound, a channel. And still this is passive transport.

    03:39 We've got diffusion down the concentration gradient from high to low concentration but it's just facilitated by a channel protein.

    03:50 This interior column of a channel protein is aqueous continuous with the external environment and the internal environment of the cell. And so hydrophilic molecules can pass through that passage quite easily.

    04:06 These channels can either be open or closed. Sometimes they open passively but other times we require a signal molecule or a gated channel, requires a signal molecule. So signal comes along, binds to it, causes it to open.

    04:25 When that signal molecule falls off, the channel closes and we can no longer move something even down its concentration gradient because there's no channel for it.

    04:36 Then we have the idea of carrier proteins. Carrier proteins are similar to channel proteins in that they involved movement down the concentration gradient. So we go from high to low concentration just like we do with the channel protein.

    04:53 The only real difference here is that we have a confirmational change in that carrier protein, such that when the ligand, the thing that wants to go in the cell binds to it, that triggers opening of the protein and allowing it to carry the molecule through and close.

    05:12 So moving down the concentration gradient, you've got a fair amount of pressure from molecules on the outside here pushing that carrier proteins full when the ligand binds, the molecule wants to enter binds that causes a confirmational change in the carrier protein and it is carried through the membrane and dropped off in the inside of the cell.

    05:35 This still requires no energy because we're moving down the concentration gradient from an area of high concentration outside the cell to an area of lower concentration inside the cells, so no energy required.

    05:49 These two mechanisms, whether we're talking about channel proteins or carrier proteins, allow membranes to be selectively permeable. And it's pretty cool if you think about it because now the protein can choose where to put channels in the membrane or where to put carrier proteins in the membrane.

    06:09 If we want, say sodium, to move in and out of the cell, and we want lots of it to move in and out of the cell, those are gated channels. But we could put lots and lots of those gates into the membrane, so that we can get lots and lots of sodium to move in or out of the cell, depending on the concentration gradient.

    06:29 So this is one of the ways that cells regulate how they're functioning.

    06:34 So proteins of course come from DNA. They're made by ribosomes.

    06:39 And we use that cell membrane transport system, the endomembrane system that we've introduced previously in order to transfer these molecules, the proteins to the cell membrane and allow additional transports.

    06:55 So selectively permeable membranes are what we see in cell membranes.

    About the Lecture

    The lecture Passive Transport – Transport Across Cell Membranes by Georgina Cornwall, PhD is from the course Cellular Structure.

    Included Quiz Questions

    1. False
    2. True
    1. Down the concentration gradient with zero energy expenditure
    2. Down the concentration gradient by using one ATP for each molecule transferred
    3. Up the concentration gradient with zero energy expenditure
    4. In either direction and is independent of a concentration gradient
    5. Up the concentration gradient by using one ATP for each molecule moved
    1. The participation of carrier protein molecules or gated channels
    2. The participation of peripheral proteins
    3. The participation of phospholipids molecules
    4. The participation of hydrophobic molecules
    5. The participation of hydrophilic heads of phospholipids
    1. The rate of facilitated diffusion of glucose across the cell membrane is directly dependent on the number of ATP molecules hydrolyzed during movement of glucose molecules
    2. Facilitated diffusion does not utilize any energy during the transfer of the molecules from the exterior of the cell to the cell interior
    3. The conformational changes in the carrier proteins of gated channels lead to opening and closing of the gated channels
    4. The carrier proteins or gated channels provide the selective aqueous passage for the hydrophilic and larger molecules
    5. During facilitated diffusion, the molecules move down their concentration gradients across the cell membrane

    Author of lecture Passive Transport – Transport Across Cell Membranes

     Georgina Cornwall, PhD

    Georgina Cornwall, PhD

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    By Juan Diego A. on 30. March 2018 for Passive Transport – Transport Across Cell Membranes

    very good, is complicated, but i understand everything, she is a good teacher

    Try to use animations please
    By Tamunotonye T. on 05. February 2018 for Passive Transport – Transport Across Cell Membranes

    couldnt understand a thing. Please try to use animation to explain better