Hi. It's Mr. Andersen. This is on transport across a cell membrane. If you haven't watched the video on cell membranes, so the parts of the cell membrane like the phospholipids, the glycolipids, the proteins, cholesterol, if you haven't watched that, make sure you do that first because I'm assuming that you know all the parts of the cell membrane as we talk about transport.

So imagine right here, we've got on this side, we've got some gas, it's in a container, but it's locked within that container. So it sealed within the container. So if I were to open up that container on one side, what's going to happen? Well, these molecules are moving around, they're constantly bumping off of each other. And so when you open up a new space on this side, they're simply going to move into that. Now how much energy does that require? It requires no energy at all: they're just randomly moving around, they're going to randomly move into that space. Likewise, if I remove the container on the outside, what's going to happen? They're going to randomly keep moving. And so that process that random movement is something called diffusion. Now, if we want to move those molecules in the other direction, we can do that in a cell. And lots of times we have to do that in the cell. But it's usually going to require energy to do that. And so when we do that, we're going to cash in some ATP. And that's called active transport.

And so to summarize what I'm going to talk about in this podcast, there are two forms of transport we have passive and then active. And so the greatest form of passive transport that I'll talk about, or the most common is going to be called diffusion. Diffusion is just that random movement of particles. It's super important, because that's how you get oxygen into your body. And that's how you get rid of things that we don't need like carbon dioxide. A specific type of diffusion is osmosis. And Osmosis is simply diffusion of water across a semi permeable membrane. That has a huge impact on cells. Because if they are in a hypertonic, hypo or isotonic environment, they're either going to lose gain, or nothing's going to happen to the water inside them, according to osmosis. And so that's something that we have to battle.But we can also use to our advantage, a specific type of passive transport is called facilitated diffusion.

It's just like diffusion. But we need to use proteins to actually move the material across these things. Passive transport requires no energy. Active transport is where we need to cash in remember a little bit of ATP to move things across their gradient or against their gradient, what that means to move against your gradient is to move where you don't want to go. And so proteins in ATP are required to do active transport as the most famous type of active transport as the sodium-potassium pump. I'll talk about that. And it's important in maintaining a gradient on nerve cells. And then on the large scale form of active transport are both endocytosis and then exocytosis. So that's just not moving a few molecules, it's moving big particles, even organisms across a membrane.

And so let's get started. First type is going to be called diffusion. Remember, diffusion requires no energy, it's just molecules moving around randomly, and then filling in a space. And so in this diagram, right up here, I've got two gases, we'll call this gas A, and then gas B, which is going to be a little bit darker, they're separated by a wall. And so these particles are randomly moving around. If I remove that barrier, and then check back on it a little bit later, we're going to find that each of those molecules have spread up according to their gradient. In other words, the gray is going to move in this direction, and the black is going to move in that direction. And so that would be called moving with their gradient or along their gradient. Now, it's not just a linear path, you can see right here that it's going to be a random bounce that whole time. Where's this play out inside our body? Well, these are the alveoli. Alveoli are going to be in the lungs. And so our lungs are one way. In other words, you breathe in air, and eventually it goes all the way down to the level of the alveoli, which are the small sacs of really thin cells. And then we're just simply going to have diffusion across that gradient. You have a lot of oxygen when you breathe in, in the alveoli. And so that's going to flow right into the capillary beds. And likewise, we have a lot of carbon dioxide in our capillary beds, and that's going to flow back into the alveoli. And so that requires no energy. And so when I go like that, and take a big breath, that oxygen is going through into my alveoli, it's going into my blood supply. And in fact, it's moving into the cells in my body according to diffusion requires no energy. Likewise, when I breathe out, carbon dioxide is coming out through a process of diffusion as well. How much energy does that require? None.

Let's go to the next one, then. A specific type of diffusion is called osmosis. So osmosis, if we were to define it is the diffusion of water across a semi permeable membrane. And so this is the YouTube experiment. In the YouTube experiment, what we have are two different concentrations of water, let's think of this as sugar water. And this as less sugary water. Now, the sugar can't move across the membrane, but the water can. And so where's the water going to move here? The water, over time, is going to move from an area of high water concentration to low water concentration. And so if you were to watch this YouTube experiment, you'd see that on this side, the water is mysteriously rising up because you can't see the sugar that's dissolved inside the water. But it's going to do that until the concentration on either side of that semi permeable membrane is going to be the same. In other words, the ratio of water to the sugar molecules is exactly the same. And that's why when you throw salt on a slug, the slug is going to shrivel up. And the reason why we get some water is that the water is going to move from an area of high water concentration inside the slug to low water concentration on that salty area on its surface.

Where does that play out as far as humans go? Well, this is a red blood cell. And so red blood cell is surrounded by plasma, and the concentration of the plasma is the same as the red blood cell. And the reason why is that we're going to have water flowing in and water flowing out. In other words, it's an equilibrium. But we're not going to have it go radically in one direction or the other. If you were to inject saltwater into our blood, what would happen to it? Well, if you think about that, there's going to be saltwater out here. So there's going to be a lower concentration of water outside of the blood cells, and so the water is going to flow out. And that's going to cause the blood cells to shrivel up. Likewise, if you were to inject distilled water into blood, what's going to happen? Now we actually have more water outside the blood cells, so the concentration of water is greater out here, it's going to flow into the blood and it's actually going to lyse the cell: it's going to explode the cell. And so if you're surrounded by a liquid that has a higher solute concentration, we call that hypertonic, if it's low, or we call that hypotonic. And then eventually, when we reach equilibrium, we call that isotonic. But it's the movement of water across a semi permeable membrane. What the semi permeable membrane is, in this case, is the cell membranes, that membrane that surrounds all living things. An example of diffusion where we still don't require energy. 

But we do require protein to something called facilitated diffusion. So an example of this could be if we're moving, it looks like sugar molecules right here. So sugar molecules like this, but we're moving it through a protein, or we're moving it through a protein that has a different conformation. Confirmation is the shape of the protein, it's still moving along its gradient. In other words, if you look up here, we have a greater concentration of sugar greater concentration of this molecule appear, it's still moving along its gradient, in other words, from a high concentration to low concentration. But since it's requiring a protein to do that, we call that facilitated diffusion. An example of that I made a little animation here, we can use something called the glucose transport. That's a lot I love that word. So the glucose transport protein is going to sit right within that phospholipid bilayer. So it's a protein inside here. Now we've got our glucose here out here. And if you think about it, its gradient is in the top to the bottom. In other words, we have more glucose on the top than we do on the bottom. But it can't move through, glucose is too large to move through this, this membrane. And so as it randomly moves along, there will eventually be a connection. So we make a connection right here, there's a chemical connection or bond right here, that causes a conformational change in the glut and conformational change in this glucose transport protein. And so what that means it's, it's simply going to change its shape, as it changes its shape, and it's going to force that glucose in this direction. And so the glucose is still moving around randomly. But since we're using this protein to do that, then we call that facilitated diffusion, it's still moving along its gradient. And it's going to keep moving along its gradient until it hits another one, and it's going to move in that direction. Now, if you think about it, what if we want to move the glucose in the opposite direction? What if we want to make the glucose instead of moving from a high concentration to a low from a low concentration to a high, where we might we see that might be during in the for example, the lining of your small intestine. We've got a lot of glucose in the cells inside there, but maybe not a lot of glucose inside the inside our stomach inside the small intestine.

Let's say we want to move in the other direction. Well, then we could tap something called co-transport. So we could use for example sodium out here and as sodium flows in this direction we can carry glucose in the other direction. But right now, I'm hinting at the next form of transport. And that's called active transport. Active Transport requires energy. And so the most famous of all active transport proteins is probably called the sodium-potassium pump. And sodium-potassium pump looks just like this, it's a protein. But what it's going to do is it's going to move sodium outside of the cell, and it's going to move potassium inside the cell. And if you think about it, we're moving sodium out here. And if you look at it, there's actually more sodium already out here. And we're moving potassium in the other direction, there's more potassium here on the inside. And so to do that, we have to use ATP. And so you can see right here that we have adenosine triphosphate that's attaching a phosphate to the sodium-potassium pump. And as it does that, it causes a change, it's causing a change in the shape, which is moving the sodium to the outside, it's moving the potassium to the inside. And then we have to use more ATP to do that. And so it's a constant supply of energy required to sodium to maintain that sodium-potassium pump. But all the nerves inside our body may use a sodium-potassium pump, and lots of cells inside our body use the sodium-potassium pump to keep that correct balance of sodium on the outside and then potassium on the inside. But that's called active transport and requires one ATP for to move every three sodium ions and every two potassium ions to the inside.

Big scale movement across a membrane is called endocytosis, and exocytosis. And so endocytosis means moving cells inside. So when would you do that? Well, this right here is a phagocyte, a phagocyte, is going to be a white blood cell, this is going to move around, and it's going to eat invading cells. And so if you think of all of these green bubbles out here as bacteria, we want to destroy the bacteria. And so we have these phagocytes. And what they'll do is they'll actually fold their membrane in, and as they do that, they create a sphere, it's called the phagosome. And that phagosome is going to contain all of these invaders these pathogens inside it. And so it's not just a few molecules, we're talking about a lot of material, there's even liquid in here as well. So that phagosome will move to the inside of the cell, it will then attach to a lysosome. And it makes something called the phagolysosome. And what happens there, these digestive enzymes are going to pour into this phagosome, it's going to digest the material on the inside, we can then make antibodies based on the shape of that after it reaches the nucleus. But since we're taking a large amount of material, that's endocytosis. This takes energy, of course, we're going to move it against its gradient. So we're also going to move this membrane so that requires ATP to do that. So it's a form of active transport. And then finally, Exocytosis is simply moving in the opposite direction. And so a great example of that you're probably familiar with this is a nerve signal moving in this direction. In other words, we have an action potential moving in this direction. And so we have to send that signal across the synapse, which is going to be this gap between two different neurons. To do that, we use what are called neurotransmitters. Neurotransmitters are these molecules that are moving across that synapse to the other side. And then they're going to start an action potential on the other side. And so that nerve signal can keep moving in this direction. But to do that, we have to release a lot of neurotransmitters. And so that process is called exocytosis, or the release of large amounts of material; those will go across and they'll open up these gated channels on the other side. And since we're moving a lot of material, that's called exocytosis. And so again, in summary, if you're not adding energy, it's called passive transport, if you are it's called active transport. But both of these are ways to move materials across the cell membrane. And I hope that's helpful.


Last modified: Tuesday, October 18, 2022, 10:47 AM