Biology: Simple and Facilitated Diffusion

Not Bad

10/29/2009

~ 10243406

I think its a great video but he could of added examples and differentiation of carrier proteins and channel proteins in passive transport. Also the notion of aquaporins wouldve been great but this was a good video all in all.

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When we start talking about the way materials pass through membranes and around cells, there are two processes that are unbelievably important. One of them I want to talk to you about right now, and that's going to be called passive transport. Well, passive transport is exactly what it sounds like. You've heard, I'm sure, of a substance called ATP, and ATP provides energy. Well, not here. Passive transport is about passiveness. In other words, transport that occurs by virtue of the molecular function of the material that is being transported.
A good example of this, one you probably have heard of, is something called diffusion. Diffusion is a key ingredient into the processes of life. Diffusion works something like this: Imagine if you will - here's diffusion in its simplest. No, it's not a cell; I just want to show you just a little bit of simple diffusion happening here. Do you ever notice someone perhaps walks into the room and they have a particular odor about them, perhaps they have some perfume on, and you're on the other side of the room? And within a few seconds or minutes, depending on the strength of the perfume, you pick up the smell that they're emanating. Why? Diffusion. You see, molecules move randomly.
Well, let's take a look at something like that. I've just put some water in here, and I put it in quite a while ago so there wouldn't be all sorts of currents in there, so you didn't think I was cheating. And I'm going to throw some food coloring in there, and I want you to watch the diffusion that's occurring. As that thing drops to the bottom, it's not just plummeting, it's spreading out. Well, if you've had any chemistry at all, you know about Brownian movement of molecules. You know that molecules are never completely at rest and they're constantly moving. And now we see a phenomenon of diffusion. The molecules of the food coloring are moving from an area where they were in a high concentration to an area of low concentration. To put it in vernacular, they're spreading out; the molecules are spreading out from high to low.
Now, here's a question for you: Do you think that diffusion would change based on anything besides what it's in? And I hope you know the answer to that one. Here's the scoop: I talked about a concentration gradient. Well, can you imagine where a gradient may not be as steep as others? That whole analogy of gradient works here. There are some determinants in - okay, so here are some determinants on a diffusion situation. What's going to determine the speed, the rate of a diffusion in, say, a watery solution, or aqueous solution? Well, the first thing we mentioned is the concentration gradient. How steep - now, this was a very steep concentration gradient. This had concentration food dye molecules and no food molecules, but can you imagine a situation where it would be different? Of course you can, because you can imagine where there might be 100 percent diffusing to 90 percent, and maybe that won't be quite as fast. That's going to determine its speed.
Think of something else from your chemistry - how about temperature? Temperature will certainly make a difference. Why? Molecules move faster when they're warmer. How about charge? You guys are water experts, and you and I both know that the solubility of something will determine how quickly it spreads out and passes through the water. And last but not least, the size, or I'm going to say the diameter, of the molecules that are diffusing. A heavier, if you will, molecule may diffuse through water at a different rate than a lighter one. A heavier perfume may take longer to get across the room, or shorter, than a lighter one. So it depends. Now you think about that, because all of these things are attributes of materials passing in and out of cells.
Let's prove it. I'm going to ask you to do a little bit of a prediction with me. But before I do that, I have to show you a very simple demonstration. In this flask bottle, I have starch. In this, I have something called an indicator for starch; it's a substance called iodine. Let's see what iodine does when mixed with starch. Now, iodine is brown. This may not look brown to you, but it is, it's just very concentrated brown. And as we put it into this pipette, I hope you can see that it's brown. But guess what? That won't matter, because watch what happens when iodine is mixed with starch. That is clearly not brown. We have a darkening, a blue-black color. Now, we use indicators a lot in biology and in science of all kinds. I want to do an experience with you now, and I'm going to ask you to make a prediction. Here's what I'm going to do: I'm going to take this flask, and in here is water. And in that flask, I'm going to put quite a bit of iodine; we're going to brown this up pretty well. I want it fairly concentrated, because we only have about 3 or 4 minutes here, and we need some results, because time marches on. So that's fairly good concentration of iodine in there; we could have just poured it.
Now, I've prepared a tubing. Let me show you what this tubing is. This tubing is a tubing called dialysis tubing. Now, dialysis tubing is a plastic tubing that is permeable. But it's only semi-permeable. Elephants can't fit through its doors. Pigs and horses can. What does that mean? Well, its walls are made so that only small molecules can pass through. So what I'm going to do is I'm going to tie one end of this, I'm going to put a starch solution in here, and then I'm going to tie the other end of it. We're going to have a bag full of starch, which I've prepared in advance. So here I have a bag full of starch molecules, and I'm going to ask you to make a prediction. I want you to base it on your knowledge of chemistry, and on what you just saw right here. I'm going to submerge this in here, like so, and I'm going to leave it there for about two minutes. And we'll come back to it, and we'll see what happens. No, I'm not just going to stand here for two minutes staring at you, because you know there's more I have to tell you about.
I want to tell you about a process of diffusion that is helped along by proteins, and that process of diffusion is called facilitated diffusion. And, once again, here come those proteins in the plasma membrane that are going to help with this process called facilitated diffusion. Sometimes, you need to get stuff across the cell membrane faster than a material will let you. Sometimes, on the other hand, materials won't move across membranes. Perhaps they're hydrophilic. And you know that cell membranes are hydrophobic in the center; and, therefore, the stuff can't get through.
Well, for this situation, we have a model with membrane proteins, and these membrane proteins are just - I mean, there it is. Look at this diagram and how beautifully that is illustrated. What's going on here? You see, here's the scoop: The protein is facilitating the diffusion of the material through. It's helping it, see? That's what facilitation is; it's helping. So what's happening is, this protein, without the involvement of ATP - remember we talked about that before, ATP is not going to be involved in this spontaneous situation - the material to be moved in moves into the protein, causing a conformational shape. You're saying, "Okay, what's that mean?" Remember conformational shape means - or conformational change in that shape - a conformational change means a shape change.
So the protein goes in, the fact that the - or the material goes into the protein - the fact that the material goes into the protein, causes the protein to change its shape, and now the material can pass through. No energy used, just the physics of molecules. But nevertheless, we're helping diffusion out. Facilitated diffusion - that's an important one. Can you think of some other ways that we might facilitate diffusion besides this kind of, sometimes we call this the ping-pong model? Well, let's think of some other ones. Certainly, that one would be a channel. So we can form channels. How about this one, how about a gated channel? Imagine, if you will, that that channel - now, we've just ended up putting a little bit more selectivity on that protein - imagine instead of something as blatantly open as this, there was a gate across here, and that gate only opened at certain times to allow diffusion. Now we have what is called a gated channel. The best thing about these membrane proteins is, you can think of reasons they might work and how they might work, and that's the coolest part of all.
Now, one last thing: I'm not going to forget our demonstration - what did you predict? What did we have? We had the flask of iodine, and in that flask was a test tube full - not a test tube, but a dialysis tube - full of starch. Remember starches are polysaccharides. Remember that makes them big. Iodine is merely iodine, I[2] in the gaseous form; iodine's just iodine. Now, to be quite honest, this is a compound of iodine, which we'll call Lugol's iodine, but it's still a very small molecule. Have you reasoned out that the iodine should go in there? And if it does, what should happen? Let's see if the iodine went in there. Okay, so, so quickly, the iodine has passed in to the membrane, and you can see that the starch has changed color. Diffusion has occurred through this fake plastic membrane, and look at the liquid itself - no change of color there. The starch has not moved out. This is an example of diffusion. I don't think it takes much imagination for you and I to picture this happening in our cells - great stuff.
Cell Biology
Cell Transport
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