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How do things move across a cell membrane?

Created by Raja Narayan.

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Video transcript

- [Voiceover] In our bodies, the cell is the smallest unit of life, and just like larger units of life, like the entire human body, the cell needs nutrients that are, at times, available outside of their cell membrane, and they also make waste products that they need to get outside in order to survive, and so an important function of living is the ability to transport things, to transport things across a cell membrane, to transport things across a cell membrane. In the next couple of videos, we're going to talk about these different types of transport mechanisms. In this video, in particular, I'll give you an overview of the different types before we go into detail. A good rule of thumb to have in the back of your head as we talk about this is that the bigger the passenger or the bigger the waste product or the nutrient that we're going to be talking about that we need to transport across our cell membrane, that means the bigger the car that we're going to need to get them across. You'll see that as we go in order right here from the smallest solutes or nutrients or waste products to the largest things that we transport across our cell membrane. We've got specialized mechanisms to make these things move. The first example of a transport mechanism I want to talk about is just a simple potassium leak channel. I'll call this a potassium leak channel, and you might be wondering, "Well, why do we call it a leak channel? "Why don't we just call it just a regular channel?" Well, the reason is that potassium has a large concentration inside of our cells. In fact, there are more potassium ions that are found inside of our cells than there are outside of our cells, and this is the exact opposite of what we see compared to sodium. For sodium, there's a ton of sodium ions that hang out outside of the cell, so they have a large concentration here, and there's a smaller portion of them that are located inside of the cell. When there's an open channel or an open gate present, potassium naturally wants to flow down its concentration gradient to go outside of the cell. This type of transport mechanism is called passive transport, passive transport, and we'll talk about another mechanism in a moment that's called active transport. Why do we call this passive? Well, the reason we make this distinction is because no energy was used in order to allow our potassium ion to transport across the membrane. Now, at this point, I also want to mention that there are a couple of different flavors of passive transport we'll talk more detail about. There's something that's called diffusion, which is the movement of solutes in space, and we'll talk about that, what that means. There's osmosis that relates mainly to the movement of water, and then there's also filtration. We'll talk about filtration as it occurs in the kidney, and then finally, there's also what's called facilitated diffusion, facilitated diffusion, which, unlike diffusion as I've listed above, facilitated diffusion utilizes the help of a protein channel, which is exactly what we talked about right here. This is facilitated diffusion that I gave you the example of for the potassium leak channel. Now, the other thing you should be asking yourself is, "How is it that we had a large concentration "of potassium in the cell in the first place "or a large concentration of sodium outside of the cell?" Well, the honest answer is that there was energy that was used to set up that ion gradient, and so here we go. We're going to move towards a bigger concept here where instead of just moving one ion across, we're going to move two at the same time, and truth be told, it's a little more complicated than that, but just to simplify it for now, we're going to talk about a process that allows for sodium to be pushed against its concentration gradient to be kicked out of a cell to a place where there's already a bunch of them hanging out, as well as potassium, moving from a place where it's very comfortable and happy into this area where there is already a high concentration of potassium ions. This is not the direction these ions wants to flow. In fact, we're going to need a little bit of energy to make that happen, and so that energy is in the form of ATP, so that's adenosine triphosphate, which is broken into adenosine di--, as in two, phosphate, and there's a little phosphate group that broke off here as well. This protein is affectionately called a sodium potassium pump, a sodium potassium pump, and in some books, you'll see it called a sodium potassium ATPase because you break them all, you'll have ATP to use it. This is an excellent example of something that's called active transport, active transport, and as the name suggests, relative to when we talked earlier about passive transport, active transport uses energy, and it uses energy in this scenario by breaking a molecule of ATP. Now, as sort of a side note here, this is specifically known as primary active transport because we've directly used ATP here. A direct use of ATP is primary active transport, which may lead you to ask, "Well, are there other types "of active transport as well?" The answer to that question is yes, you are absolutely correct, and I'll give you an example from the gut. In the gut, when we eat things like salty food might have some sodium in it, or it might be sweet food and it's got some glucose in it as well, and as you already know, there's a lot of sodium that's outside of our cells, in the extracellular space. Well, our intestines, our guts, are lined with things that are called enterocytes, and they're just basically cells of the G.I. tract, and they're just like normal cells where they want sodium to enter. This protein that's on our enterocyte here will let sodium enter, but because we're in the gut, another magical thing happens. This glucose molecule will enter as well. This is an example of something that's called symport, symport, where both molecules are moving in the same direction, one where we're using sodium to flow down its ion gradient and then glucose, going from outside of the cell into the cell. It's basically coming along for the ride, which is why this is a great example of what's called secondary active transport, secondary, meaning that we used energy. Certainly we use energy here, so I'll say it uses energy here but not directly. This, in fact, uses a gradient that was already set up, so a gradient set up using energy. In that sense, it's indirectly using our ATP molecule, or indirectly using our energy that generated this sodium gradient. That's what happens when we have our sodium and our glucose absorbed into our intestine. After it's absorbed into our enterocytes, they actually need to go to our bloodstream, so I'll draw a little blood vessel right here. That's where our nutrients need to go to to flow elsewhere in our body. In order to get out of our enterocyte and end up in our bloodstream, we actually use the same type of channel to help our glucose molecule that just entered our enterocyte to leave the cell and travel to this blood vessel or this capillary. What's unusual is that, again, sodium is in play here, but this time, it's actually going in the opposite direction, but this completely makes sense. Remember, there's a higher concentration of sodium outside of our cells, and so it naturally wants to flow down its ion gradient into the cell. Still, this is an example of secondary active transport, but instead of symport, now we have what's called antiport, antiport because our two molecules are moving in the opposite direction of each other. We'll talk again about secondary active transport in a part of the kidney call the nephron. For now, let's talk about bigger molecules trying to cross the membrane. As you see, we've been getting progressively larger from one ion to two ions to now we're talking about a bigger molecule like glucose. What about if we have a giant protein that wants to enter the cell? What I've drawn here is not the protein. Instead, I'll draw the protein inside of here. I'll just write a.a. because you know proteins are made of amino acids. There's a couple of amino acids in here that want to enter our cell. This guy is a vesicle. A vesicle is just a small pocket of cell membrane that's surrounding some type of cargo or some type of thing we're trying to transport. In this case, it's this protein. Remember, it's the exact same membrane we have around our cell, and once this vesicle gets close enough to our membrane right here, it'll actually fuse, and in doing so, will then create an opening from the inside of the vesicle, connecting it to the inside of our cell, which means that our amino acid can actually enter inside. These steps that we saw here as we went from here to allow our amino acid to enter the cell is a process that's called endocytosis, endocytosis, which as you can see here, means for something to go into a site or a cell. Because we're moving something larger than what we talked about before, this uses a lot of energy, uses a lot of energy directly here to make this process happen. You can actually have the opposite case as well, where a vesicle that's made inside the cell, as we'll talk about sometimes will be made off of what's called the Golgi apparatus, and that's an organelle within your cell. It'll create a vesicle that'll have some protein that we want to get rid of. I want to make this different, so instead of amino acids in here, I'll write ACh or acetylcholine, which is a type of neurotransmitter we have in our body. Once this vesicle with acetylcholine is made from the Golgi apparatus, it floats towards our cell membrane, similar to how we saw earlier, and then these membranes will fuse, the membrane of our vesicle, as well as the membrane of our cell, causing the acetylcholine to be kicked out. This process where this vesicle came here and then caused the acetylcholine to get out of our cell is called exo, exocytosis, which is why I called it the opposite of endocytosis because something is exiting our site or a cell, and this is similar to endocytosis in that it uses a lot of energy to happen. These are all the main processes or mechanisms we use to move things across our cell membrane, and they're very important to understand. Just to give you kind of an idea why, when things are wrong with our proteins or when viruses hijack some of the mechanisms that we use to move things across our membrane, bad things can happen. The more we know here, the better we can fight a lot of our different infectious diseases.