Biology: Passive Transport: Osmosis

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10/29/2009

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I want you to start thinking about how materials get through cell membranes other than through the aid of proteins, or, if they are being aided by proteins, things like water. How is water going to get through cell membranes? And then I want you to think about the effect of this. Now, I'm going to start out this particular discussion with a demonstration, and you're going to make a prediction.
I have right here one of those dialysis tubes. You know about dialysis tubes; it's a clear plastic, plastic-like substance that's semi-permeable to small molecules, but not big ones. In the dialysis tube I have table sugar - sucrose. Table sugar is a disaccharide, and therefore pretty big. I'm going to tell you in advance, it can't get out of here. It's a good 10 percent at least sugar solution, which means only 90 percent of it is water. I'll write all this down for you in a second, but I want to get this going. I'm going to put this on the scale, and we're going to mass this, we're getting the mask of this particular tube. Now, let's take a look here. Its mass is going to roughly come in at about 66.7. So we have this thing that ways 66.7 grams, and now I'm going to put it in our Thinkwell pure water container. We're going to leave this for the next 7 or 8 minutes, and we'll come back to it.
Now, let's think about what it is I just did, and what I want you to think about. What I gave you there was, I gave you this pure water. So here's what I have: a Thinkwell pure water container holder, and that was 100 percent water; it's pure water. Within that, I put something that I'm estimating is about 90 percent water. Why 90 percent water? Because it's 10 percent sugar. Well, if it's percent sugar, it must be 90 percent water. I have a question for you. Here's what I want you to predict: What's going to happen? Now, you can imagine it must have something to do with its mass. What do you think is going to happen? Remember the membrane is impermeable to sugar. We'll come back to that in just a few minutes.
This whole concept of movement of water through a membrane has a name, and it's called osmosis. And you're probably saying, "Well, isn't that the same as diffusion? Isn't diffusion the passage of molecules from high to low, along a concentration gradient? Is not osmosis the same thing?" And the answer is: Yes, kind of. Osmosis is kind of the same thing, except now we're not talking about any molecule, we're talking about water. We're talking about passing water across a membrane, again, from a region of high concentration to a region of low. And I want to give you some - we've got to throw some words at you here - we have to think a little bit about this, and we have to definitely make sure that you understand the concept before we see the result. While I'm thinking about it, and before I forget, I'm going to write down that 66.8, because if I forget that, our whole experiment is down the tubes, if you will. So let's - 66.8, we're not going to forget that.
Now, let's see, osmosis. I'm going to show a diagram for you, and you tell me what's going to happen. Prediction number two: I'm going to put a situation just like the one we had. I'm going to put a glass tube in here, and I'm going to make that glass tube tall, perhaps 1 foot, 2 feet, 3 feet. Then I'm going to put at the bottom of this glass tube a swelling, like so. Then I'm going to put a membrane across there, pretty much like the dialysis membrane we've seen. So there's my dialysis membrane. I am going to put in here a 20 percent sucrose solution. Now, think about it. What does that mean? Twenty percent sucrose solution means 80 percent water. But we don't refer to things - you've got to get used to this, guys - you don't refer to things as, "Oh, well, that's a 19 percent water solution." You would say, "Oh, no, it's an 81 percent something else solution." So we talk in terms of what is dissolved in there - the solute - instead of the water as the solvent.
So that's 20 percent sucrose, and I'm going to put that in distilled water. Distilled water, as you know, is 100 percent water, which we can refer to as 100 percent water. Here we go, and this is something that always confuses people, so we've just to get this out of the way, and that's going to be called the terms hypotonic and hypertonic. When you're hyperactive, what do you have? You have too much activity relative to everybody else in the room. So we say, "Man, that guy's hyperactive." See, "hyper" means "a lot." So, if you look at this, we are going to define this system - and it's a system in terms of its tonicity - what's in there, as in solution - its tonicity. So we are going - and watch where these words come from - this is going to be what we call hypotonic, and this is going to be referred to as hypertonic. Why? Well, it has a lot of material solute; it's defined in terms of the solute. So we are defining this as hypertonic, because 20 percent of it is solute; this stuff dissolved in the water. So it is hypertonic, but hypertonic relative to what? Relative to the water outside. On the other hand, we could say that the water is hypotonic, relative to what's inside the tube. In other words, not much solution or solute dissolved in there. So it's hypotonic. And this can change around. For example, if this were 20 percent sucrose, and this were 30 percent sucrose, or if this were 70 percent water, now we have more things dissolved out here, and this is hypertonic, and this is hypotonic. Why? It's all relative.
Here's a question for you: What do you think the word isotonic means? Well, if you remember some of your chemistry, you know what isomers were. "Iso" means "the same." So when two things have the same concentration, relative to each other, they're isotonic.
Now, before we get back to our experiment, I want to give you some examples of this, and let's use a plant, because plants are extremely susceptible to osmotic change. Plants are extremely susceptible to what their solution is around their cells, because - think about it - plants live by pulling water in out of the ground. Let's take a plant that is in - the plant is a happy plant, nice, standing up, it just rained an hour ago - let's think of what's going on around it. If the ground is wet, and there's a lot of water around the plant, we now have a hypotonic solution relative to the plant. In other words, outside the plant cells, there's a lot of water relative to the water inside of the plant.
So what's going to happen? The water is going to pass from the outside to the inside through the membrane. You're going to get osmosis inward, in the inward direction. That's a good thing, because what will happen then is the plants will have - they'll be full of water, they're water vacuoles will be full. We call that turgidity. In other words, the plant's cells will be turgid, and the plant will be happily standing.
We then go to a situation where it's isotonic. In other words, things are drying up. You're not going to have any net flow of water into the cell. You see why? It's going to be a little of this, a little of that. It's isotonic; it's not going to be in balance toward the cell, so the plant is going to start getting a little limp. We're going to call that a flaccid plant.
Now let's get to the real bad news. Let's dry out the environment of the cell and make the environment of the cell hypertonic. Now, the opposite is going to happen; the plant is going to start losing its water to the environment. We call that drying out, and that causes death, and that is a process called plasmolysis - the loss of the cytoplasm. That's the same thing that happens when you - did you ever hear of "Water, water everywhere, and not a drop to drink"? That's why you die if you drink salt water, because your blood salt level gets high, high, high, high, high - you're drinking salt water, you're drinking salt water, you're drinking salt water - your blood salt level gets high, your water level goes low, and no matter how much water you drink, you're getting saltier and saltier and saltier, so you're brain is saying, thirstier and thirstier and thirstier, because it's a water balance thing. You end up eventually sucking water out of your cells - that's called death.
All of that being said, let's take a look at our demo one more time, and take a look at what's going to happen here. The first thing we want to do is have this thing at zero. Now we've had this thing in there for about 6 minutes, and what we want to do is we want to have this thing at about - we gave it a little bit of a balance there - I think it was 66.8 or 66.7 - but you know what? What's a 10th of a gram amongst friends? I am so confident. Now, see what I'm doing, I'm taking this out of water, because I don't want you guys to say, "You cheated, it was all wet." Here we go, let's see what happens. Now, I don't know if you can see this or not, so if you can't, I'm going to lie to you. So what's going on is this thing, as the thing is coming in, it's getting - okay, so we are at about, oh, 68 at this point.
Well, let's think about what happened there. What happened there is this: Well, holy mackerel, this is very cool stuff. If you think about what happened there, what did we have in here? We had sugar water. What happened was the pure water went into the bag. We had an osmotic movement from the 100 percent water into the bag. Now, let's just think of one ramification of this. Ready? Suppose I had left it there forever. What would have happened? Would they ever have equaled? Would this ever have been 100 percent? No. We've stumbled on a concept called osmotic pressure, and we have a pressure of osmosis into that bag which can actually do work. If I left it there long enough and this membrane were weak enough, it would pop. Osmosis is a very powerful force in cells. Passive transport - I'll bet you it doesn't sound so passive anymore.
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