9. Transport Across Membranes
Passive Transport: Diffusion and Osmosis
1
concept
Diffusion
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Hi in this video, we're gonna be talking about passive transport, focusing specifically on diffusion and osmosis. So, first, let's talk about diffusion diffusion is the movement of molecules towards equilibrium and it's unassisted considered passive transport, passive movement and generally it's limited to small, uncharged, non polar molecules. And so this is because these types of molecules require no energy input across the membrane and as long as they're moving the molecules from an area of high concentration to an area of low concentration. Now with simple diffusion, what we're going to see is this term called a partition coefficient. And what this measures is actually measures the ability of non polar soliah bility of something in a non polar solvent. And so why am I talking about this? And why am I defining this here? Well, because generally if the molecule that's trying to pass the membrane has greater lipid soluble itty, meaning that it dissolves really well and non polar solvents or things like oil, then it's generally going to diffuse faster across the membrane. Um And so that's kind of a measurement that's used to describe things that will move across the membrane through diffusion. So, simple diffusion, you can see here you have molecules moving both ways across the membrane, there's no different um things assisting them and they just are able to move. Now, facilitated diffusion is different because that's the assisted Movement of molecules. However, these molecules are still moving from an area of high concentration to an area of low concentration, they just can't get through the membrane without some kind of assistance. And so um one of the interesting things is if you remember the Michaela's minton equation that we talked about way back in Chapter three for enzymes. Um what you will notice is that Michaela's minton equation can actually be used to look at the kinetics of diffusion. Especially facilitated diffusion. So you may see these equations or things in your book and if you do and decide that you want to know how to calculate them, feel free to go back to those equations and review them. Um And so what this is assisted movement. So what is the assistance provided by provided by proteins? Two classes of them called channel proteins and carrier proteins channel proteins move molecules by providing a channel through which they pass, which makes sense. Carrier proteins move molecules by binding to the molecule and undergoing some type of confirmation all change which propels the molecule to the other side. So we're looking at the difference here we have a protein channel which you can see just allows for movement into the south. Whereas carrier proteins are different because they actually undergo a conformational change. So we have a molecule come in. There's a confirmation all change of the protein which propels the protein to inside the cell. So these are a little different. Now, transport proteins are further classified by how many molecules they can transport at once. So, you know report transports one molecule at a time sim poor transports to but it does so in the same direction, and an anti report does too, but in opposite directions. So the unit report is going to be the fastest method of facilitated diffusion, because it's really only doing one and it's much easier than binding to separate molecules. Um And so if we look here, we have our, you know, porters importer anti porter. So in the, you know, porter transport one molecule here molecule A. Across the membrane, the sim porter transports A and B and does so in the same direction, and the anti porter does A and B, but does so in opposite directions. So those are the three classes of transporters in facilitated diffusion. So now let's move on.
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concept
GLUT1 Transporter
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Hi in this video, I'm going to be presenting two different examples of transporters that you're going to read about a lot in your textbook that are examples of facilitated diffusion. So the first is the glucose transporter. Now there are many types of glucose transporters but the one that you're probably gonna read about most is the glue. One unit port. So first do you remember what a uniform does, Right. So it transfers a single molecule across the membrane and so for the glut one or the glucose transporter that's moving glucose uh from an area of high concentration to an area of low concentration. So what you see here is glucose, you have all these glucose molecules and the glut transporter moves them to an area of low concentration. Now, if we were to just kind of guess what cells in the body do you think have a lot of glucose transporters? So think about where in your body there's gonna be a lot of glucose that needs to get into the cell, Right? So that's going to be in your gut cells and your stomach cells and your intestines. These cells are going to need a lot of glucose transporters because you've just eaten a meal with a lot of glucose in it and it needs those cells need that glucose. So it has to be able to transport them into cells. So glucose transporters are a really important example of this facilitated diffusion. Now, another one is the sodium calcium anti porter. And unfortunately I don't have a great image of this. But first do you remember what an anti porter does? Right, So that does two molecules transferred to molecules and does them in different directions. So here in this example we have sodium and that's going this way and we have our calcium and that's going this way and this anti porter is extremely important in regulating muscle contraction. And we're going to talk about and a lot of different topics later that calcium is really important for cell biology and especially um you know regulating different signals. But in order for it to do that it has to be able to get into the south. So there are actually a lot of different ways calcium gets into the cell for a lot of different functions. And so for this one, calcium is really important in regulating muscle contraction through being transported through the sodium calcium and. Aiport. So those are two examples of facilitated diffusion. But now let's move on.
3
concept
Osmosis
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in this video, we're gonna be focusing on focusing on osmosis. So osmosis is the diffusion of water across a semi permeable membranes and water movement depends on the solute concentration. So, if you remember back to intro chemistry, what a salute is. It's the thing that dissolves in water. And so water moves from a low solute concentration, which would mean that it has a high water concentration to a higher solute concentration or low water. We use salute to describe water movement just because it makes more sense than using water. Um and so when in a hyper tonic solution, a cell swell and in a hyper hyper tonic solution, they shrink. Now there's another word here, isotonic and that is solutions that have similar concentrations and cells. And you're probably familiar with these terms from your intro classes Now, Osmotic pressure is another term that you're going to read about and that pressure is or that term is the describes the pressure needed to stop water flow across the membrane. So whether that's concentration or actual physical pressure, it's just, you know, the conditions that allow for the water flow to stop moving across the membrane now in osmosis, water has to get across the membrane. So how does it do that? Well, it does it through these proteins called aqua por ins and their channel proteins that allow for water to cross the membrane. So how do how does water actually flow through Aqua porn's well, the surface of the inside of the channel, through which water flows actually has the ability to form hydrogen bonds with water. And so as a water molecule enters the aqua por in, it forms hydrogen bonds and therefore displaces the water molecule that was sitting there before which then goes on to create another hydrogen bond. So these water molecules just create all these different hydrogen bonds as they move up the aqua por in and across the membrane. And so the important aspect of this is that confirmation, all changes are not needed. And so there's really fast transport because the water molecules are just breaking and reforming these hydrogen bonds as they move through the aqua foreign. Now you can imagine that different cell types and different organisms need different amounts of aqua porn's because they react differently with water. So for instance, a frog has to have a completely entirely different, you know, number of aqua porn's than humans do because we live in very different, you know, water environments. So, Agua por ins are really important. So when we're looking at hyper tonic hype, a tonic and isotonic solutions, here's examples of what they look like. So, in our hyper tonic the water is flowing out, the cells are shrinking in the isotonic. There there's an equal amount of water flow into and out of the cell and so it remains a normal size and in a hip a tonic solution, we have water flowing into this cell, which is really causing this swelling of the cell. So that is osmosis. So now let's move on
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Problem
ProblemWhich of the following describes the diffusion of water across a membrane?
A
Simple Diffusion
B
Facilitated Diffusion
C
Osmosis
D
Uniporters
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Problem
ProblemThe plasma membrane by itself is impermeable to all charged molecules.
A
True
B
False
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Problem
ProblemWhich of the following transport proteins transports two molecules across the membrane in the same direction?
A
Uniport
B
Symport
C
Antiport
D
Biport
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Problem
ProblemCarrier proteins that transport molecules via facilitated diffusion require energy from ATP.
A
True
B
False
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Problem
ProblemIsotonic is a term that describes what?
A
The cytoplasmic side of a membrane has a higher solute concentration
B
The extracellular side of a membrane has a higher solute concentration
C
Both sides of a membrane have equal solute concentrations