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Osmosis – Transport Across Cell Membranes

by Georgina Cornwall, PhD
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    00:00 Then we could have a situation where the proteins are not embedded in the membrane. And you have a high concentration of stuff on one side of the membrane and a low concentration of stuff on the other side of the membrane. What happens when this is a situation and this stuff, the molecules cannot pass through the membrane because there are no channel proteins or carrier proteins. They want to. Everything in life wants to come to equilibrium.

    00:31 So here's a U tube. There is a membrane in the middle. And in this tube we have a lot of solute on one side of the membrane. And then we have less solutes on the other side.

    00:46 The solutes are sitting in a solvent. The solvent here is just an aqueous environment like we would see in a cell. So when water can pass through a cell membrane, you'd think well, should it be able to? Wait a moment. Why would water be able pass through the cell membrane.

    01:04 Isn't water a polar molecule? Don't we have that huge area of hydrophobicity, right? With the hydrophobic tails of the phospholipids. So water happens to be so small that it can actually sneak in between those somehow. There is some other mechanisms that we're learning about now that help water pass through. Aquaporines and such. We're not going to dig into those quite yet.

    01:31 So either way let's just assume for simplicity that water can so small it can just sneak right between the phospholipids. Not exactly true but it can. So what's going to happen to bring everything into equilibrium. Water is then going to pass through the membrane to the area of high concentration of solutes in order to make it equal with the concentration of solutes on the other side of the membrane. Now this all makes sense when we're looking at it in a U tube here where the water is moving from one side back to the other side in order to create an equal solution on both sides of the membrane. So, then let's take it to a cellular environment. Bring some reality to this. When we consider how water moves, we are considering a concept of osmotic pressure, right. Osmotic pressure is driving the water to the area of higher concentration in order to dilute the solution on the other side of the membrane.

    02:40 Tonicity is the concept, and red blood cells give us a great example for exploring that.

    02:47 Let's say we have the environment just as it should be, right. Our blood cells, these are red blood cells. They are floating around in our blood. The solution inside the red blood cell should have the same tonicity or osmolarity as the solution that the red blood cells are floating around in. That again is one of those matters of homeostasis. So we say these solutions are isotonic or isosmotic. If that's the terminology you prefer. So normal cells are floating around in normal extracellular fluid which should have the same tonicity. They are isotonic solutions.

    03:28 What happens though if somehow we end up putting our red blood cells in a hypertonic solution? You have to keep in mind here what's really important is which is hypertonic.

    03:44 Is it hypertonic inside the cell or is it hypertonic outside the cell. For reference here, we are putting the cells which are normal tonicity, normal amount of solute that we would have in our human body. And we're putting them in a glass full of water that is hypertonic.

    04:05 We're putting them in the ocean. There is lots and lots of solute in there.

    04:09 What is going to happen to the nett movement of water? Osmosis is going to go in which direction in this case? So because the solution outside the cell is hypertonic relative to the solution inside the cell the water in the cell is going to move out to try and dilute the external environment.

    04:33 Well, that's not going to be a good situation for the red blood cell because it's going to shrivel up and lose all of its water because it can never dilute the rest of the world.

    04:42 So, with reference, we're looking at the external environment here. So the external environment is hypertonic to the internal environment of the red blood cell which would be hypotonic relative to the external environment. I know this can get a little bit confusing but let's try again by looking at the opposite situation. Now, we've put our red blood cells that belong in a normal solution that we find in our body. Normal tonicity. It should be isotonic but somehow we are running out of solutes for the blood, right. We take our red blood cells and we put them in a hypotonic solution. So the hypotonic solution has less solutes than the solution inside the red blood cells. What's going to happen to water? Water is it going to move into the cell or is water going to move out of the cell? Well because the red blood cell has more solutes in it than the surrounding environment, water is going to rush into that cell in order to try and dilute the environment inside the cell and make it equal with that outside of the cell. We're headed towards isotonic.

    06:06 But the red blood cell is sort of a finite sized thing and so what we have is a lot of movement of water in creates extra pressure, pressure, pressure. Finally the red blood cells would burst. So it's really critical when you're thinking about osmolarity or tonicity that you consider which place is hypertonic, hypotonic. Are we talking about the surrrounding environment or are we talking about the internal environment? Generally in this conversation we will be talking about putting cells into a hypotonic or hypertonic solution. Here is the review of that.

    06:45 Again we have isotonic. That's ideal. That's what it should be like inside the human body.

    06:51 If homeostasis gets thrown off, we may see that there is a hypertonic environment. Too much salt in the environment. We've seen that. Too much salt will cause those cells to shrivel up because water from the cells is coming out of the cells to try and dilute the environment.

    07:11 And then when we have our blood cells put into a hypotonic solution, the water from the outside is trying to dilute the inside of the cell. And because so much water is rushing in, that will cause the cell to burst or lyse. To lyse is to break apart.


    About the Lecture

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


    Included Quiz Questions

    1. Osmotic pressure would pull water out of the cells and they may shrivel
    2. Osmotic pressure would pull water into the cells and they may lyse
    3. No changes
    4. The red blood cells would expend energy to maintain proper shape and function
    5. Cell life span would be shortened by half
    1. Water molecules across the semipermeable membrane
    2. Hydrophilic molecules across the semipermeable membrane
    3. Hydrophobic molecules across the semipermeable membrane
    4. Water molecules across the permeable membrane
    5. Hydrophilic molecules across the permeable membrane
    1. Osmotic pressure
    2. Atmospheric pressure
    3. Absolute pressure
    4. Vacuum pressure
    5. Gauge pressure

    Author of lecture Osmosis – Transport Across Cell Membranes

     Georgina Cornwall, PhD

    Georgina Cornwall, PhD


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