Transcription
So in the previous video, we were talking about the selective reabsorption that happens in the nephron. And I did mention that, um, different parts of the nephron will reabsorb different substances. For example, in the PCT, it is supposed to reabsorb all glucose and amino acids. It does reabsorb most of the water, and it also reabsorbs some salts and urea. The Loop of Henle is supposed to reabsorb water and salts, some of them. And the collecting duct is responsible for reabsorbing just water. You do need to memorize what they reabsorb for the exam, by the way.
So for this particular video, we're going to be focusing on the selective reabsorption in the PCT. So I've cut out the nephron, I've removed the Loop of Henle, DCT, and the collecting duct, and we're just going to focus on what happens in the PCT. And it's quite a heavy topic, and it's quite a popular question in Paper Four as well, because if selective reabsorption for PCT comes out in Paper Four, it's usually about six to eight marks. So a lot of understanding and memorization.
Now, some students will ask the question, why is urea being reabsorbed? Because I thought urea was useless. Uh, the reason why urea is reabsorbed is not because it is useful, but it's just a kind of accidental reabsorption that takes place. So I will explain that part as well.
So the first thing that we have to see here is we're going to look at a section of the PCT where I'm just drawing out one small tubule. Where the, the PCT is like a tube, okay? Where it has its own lumen and it has a wall. And near the PCT is supposed to be blood capillaries, so that reabsorption can happen easily. And the PCT wall is also one cell thick, just like the capillary. Except the capillary is supposed to carry blood, but the PCT is transporting filtrate that was filtered between the glomerulus and Bowman's capsule.
So what I'm doing here is I'm going to magnify that highlighted region. And you can see that the red colored things that I'm drawing out right now is the capillary endothelium. Remember, capillaries are one cell thick. And what are those semicircles? Those semicircles just represent part of the red blood cells. I'm not going to draw the full thing. And I'm just labeling that, I'm labeling that as the capillaries.
Next to the capillaries will be the wall of the PCT. So the PCT wall, as I've mentioned earlier, it's one cell thick. And the wall of the PCT is actually made out of these weird-looking cells, okay? Yeah, the cells, the cells kind of look like they have those, well, I want to say tentacles, but we know that they're not tentacles, are facing the PCT lumen. And, uh, towards the facing the capillary side, however, it has those kind of mouth-like structures as well. So what exactly are these things known as? And of course, the PCT lumen is where the filtrate is located.
Now, between one PCT wall and another PCT wall, there are these kinds of substances where I'm coloring it as green. They form a tight junction. Now, the tight junction is to prevent fluid from the lumen from leaking outwards, because we want selective reabsorption to happen, not total reabsorption. Without the tight junction, the fluid might easily leak out into the capillaries. And with the help of the tight junction, it provides a kind of blockage so that if any reabsorption were to happen, they must go through the cell surface membrane of the PCT wall. Because, because remember, the characteristic of cell surface membrane is it is partially permeable. So with the help of the cell surface membrane, the PCT cells can control what it wants to reabsorb.
And I'm just highlighting certain parts of the PCT cells that is referred to as the microvilli and the basal membrane. Not basement membrane, by the way. This is basal membrane. You must remember that the micro, micro, micro faces the PCT lumen, and the basal membrane faces the capillaries. That's quite an important part. But in both cases, they are just to increase the surface area of the cell surface membrane. When things are folded in biology, it's almost always to increase surface area.
All right, so let's just focus on one of that PCT cells. Remember, the microvilli faces the PCT lumen, and the basal membrane faces the capillaries here, all right? And on the microvilli, because it's folded, it has a lot of these co-transport proteins. Now, co-transporters are basically transport molecules that are able to transport two things at the same time. We studied this in Chapter 7 before, plant transport, where we studied the hydrogen ion, sucrose co-transporters. But in the PCT, however, the two co-transporters are as follows: the sodium ion, glucose co-transporter, and the sodium ion, amino acid co-transporter. But for the purpose of this video, we are just going to focus on the sodium ion, glucose co-transporters. However, the sodium ion, amino acid co-transporters work the same way, so don't worry about that.
Now, on the basal membrane side, however, on the cell surface membrane, they have something called a sodium ion, potassium ion pump. And whenever you see the word pump or pump proteins, you know for a fact that they're supposed to carry out active transport. And also on the basal membrane, we have transport proteins for glucose and amino acids to carry out facilitated diffusion. Don't worry, I know it's a lot, but we're going to put it all together. And within the PCT itself, it has many mitochondria. Chapter 12 and also Chapter 1 as revision. What is the function of mitochondria? Mitochondria are supposed to help carry out aerobic respiration and synthesize ATP, which is required for cells to function, especially in the case of active transport. So you kind of see the link here. Oh, there is a transport protein that needs to carry out active transport, so ATP is required, and the ATP comes from the mitochondria. Simple as that.
Okay, now this is going to be very confusing. So what I'm going to do is, instead of drawing that very complicated cell, the, the, the structure of the PCT cell, I'm going to represent the PCT cell as individual squares. All right, it makes it easier for us to understand this. But remember, facing the lumen side is the microvilli, and facing the capillary side is the basal membrane's folded areas. And I've labeled the PCT cells as 1, 2, 3, 4, 5. But what I mean by this is I mean this as a time lapse. It doesn't mean that each PCT cell does a different function. They are all doing the same function, but just read it as how you would read a comic book. So process number one happens in cell number one, uh, process number two happens in cell number two, but it's all just happening in the same cell. So I hope you understand that part.
Now, now again, as always, we represent the water as blue colored dots, um, and the glucose and amino acids as orange colored dots, salts as green colored dots, and the urea as magenta, maroon, or purple, whatever color that is, as those colored dots as well. Now, remember, ultrafiltration happened between the glomerulus and also the Bowman's capsule, where it filtered out a lot of water, glucose, amino acids, salts, and urea. But in the PCT, we want to reabsorb a lot of water back into the blood. We want to reabsorb all the glucose and amino acids back into the blood because we don't want to waste the glucose and amino acids. Uh, we also want to reabsorb some salts. In this case, salts, when I say salts, I mean sodium ions. Of course, there are other things like potassium ions, chloride ions, and such, but we're just going to focus on sodium ions. And inadvertently, we will also reabsorb some urea as well, okay? But don't worry about that.
So look at the PCT lumen. In the PCT lumen, coming from the Bowman's capsule, uh, we can see the water, salts, urea, glucose, and amino acids. Fine. And as a reminder, also facing the capillary at the basal membrane of the PCT wall, there is something called the sodium ion, potassium ion pump. All right. What is the function of that pump? Let's look at it.
Now, in the very beginning, even within the PCT cells, they will have their own sodium ions or salts, represented in those green colored dots, right? The function of the sodium potassium pump is very simple. It receives ATP from the mitochondria of the cell, and it actively pumps the sodium ions into the capillaries. Now, those sodium ions were inside the cell, and they just actively transported out into the capillaries. Don't worry about potassium ions, just focus on the sodium ion part. As you can see the arrow here, the sodium ions from the cell are transported into the capillaries. That's the first thing. Why is this an important thing to happen? Because now, compare the sodium ion concentration in the lumen and the sodium ion concentration in the cell. What do you notice in this case over here? We have something called as a sodium ion concentration gradient, where in the lumen, sodium ion concentration is higher, and in the PCT cells, sodium ion concentration is lower.
So why is this good? Because if you go back to the PCT cell, as a reminder, I told you that they have something known as co-transporters, sodium ion, glucose co-transporters, and sodium ion, amino acid co-transporters. So the co-transporters, I've represented in those pink colored circles on the cell surface membrane over there. So what will happen in this case? Sodium ions start to diffuse into the cell down the concentration gradient, from high to low concentration. But as they diffuse into the cell, automatically, because it's a co-transporter, it will also pull in one glucose molecule, all right? So the glucose molecule, represented by the orange dot, is being pulled inwards. There you go. But also, let's say another sodium ion enters the cell through another co-transporter, and it also pulls in amino acids passively in this case. So in this situation over here, sodium ion and glucose are transported into the cell, or sodium ion and amino acid are transported into the cell. And this process keeps repeating until all the glucose and amino acids are reabsorbed from the PCT lumen right here. So at the beginning, we had some glucose and amino acids in the PCT lumen, but at the bottom portion over here, the glucose and amino acids have all been reabsorbed into the cell now.
So, but they are still just inside the cell. We want them to go into the blood. So look at the third cell over here. Let's just focus on the third cell. The same glucose and amino acids, which were in the cell, now I told you that at the basal membrane, it doesn't just have sodium ion, potassium ion pumps, it also has transport proteins to carry out facilitated diffusion, represented by those green colored things. So what happens over here? Very simple. The glucose and amino acids will move through the transport proteins at the basal membrane, and they are transported into the capillaries by facilitated diffusion. Simple. So reabsorption of glucose and amino acids are now done. There you go.
So remember, the PCT was supposed to reabsorb all the glucose and amino acids, and it has done so. Because do you see any more glucose and amino acids in the PCT lumen? No, you don't. Now, the interesting thing that also happens over here is because it reabsorbed a lot of solutes like salts and glucose, the solute concentration in the PCT lumen decreases. Because compare the solute concentration at the top there, and now at the bottom, there are less solutes because there's less salts, and there are no more glucose and amino acids. And remember, when solute concentration decreases, the water potential of the PCT lumen increases. It's the opposite or inverse relationship that they have with each other. So in this case, when the water potential increases, it creates a water potential gradient. Guess what happens? Where the water potential gradient is higher, uh, the water potential is higher in the PCT lumen, it's lower in the capillaries, and you've guessed it, osmosis will occur, where the water from the PCT lumen moves through the cell into the capillaries by a process known as osmosis. So some, some water have been reabsorbed. There you go.
And look at the end over there, some urea can also be reabsorbed passively. Now, the reason for this is as follows: I told you that urea is less soluble and less toxic because they're not so polar. They can also diffuse through the phospholipid bilayer, okay? Even though we don't want it to happen, it just does happen, okay? So simple diffusion of urea will happen, where some of the urea will just get accidentally reabsorbed into the capillary. For the most part, it is an unavoidable process.
So this is how selective reabsorption in the PCT actually takes place. So look at the top, you had a lot of salts, a lot of water, a lot of glucose, and a lot of urea. But after selective reabsorption in the PCT occurs, there are less salts, no more glucose and amino acids, less water, and also lesser urea, because that's what the PCT was supposed to do. It was supposed to reabsorb some salts, all the glucose and amino acids, some urea, and some water. That's it.
So let's just go through it again. So number one, the sodium ion and potassium ion pump actively pumps sodium ion into the capillaries. This creates a sodium ion concentration gradient between the PCT lumen and the cells. Co-transport of sodium ion and glucose or sodium ion and amino acids occur from the lumen into the cells. And then the glucose and amino acids will move into the blood by facilitated diffusion. The increase in water potential in the lumen causes water to be reabsorbed by osmosis. And urea is also passively reabsorbed. For the purpose of the exam, number one until number five is important. Okay?
So if you're still kind of confused, let's look at it again. But now let's just focus on one PCT cell. All right? So remember, the PCT cell microvilli faces the lumen, basal membrane faces the blood capillaries. On the microvilli, they have co-transporters such as the sodium ion, glucose co-transporter and the sodium ion, amino acid co-transporters. And the basal membrane have sodium ion, potassium ion pump and glucose carrier proteins. And of course, I'm just drawing out a mitochondrion inside the cell. And of course, uh, inside the cell also, they contain some sodium ions that were just floating around in the cytoplasm.
So first thing first, what's supposed to happen? The mitochondrion produces ATP, and the ATP is used to power the sodium ion, potassium ion pump. The pump carries out active transport, where it actively transports the sodium ions within the cell into the blood capillaries. Why is this important? This is important because we want to create something called as a sodium ion concentration gradient. Where is the gradient, by the way? The gradient is between the lumen and also the cell, where in the lumen, sodium ion concentration is higher, in the cell, sodium ion concentration is lower.
So what happens in this case? As things usually do, the particles will try to move from a higher to lower concentration by diffusion. But in this case, as the sodium ion moves into the cell, it also pulls in one glucose molecule. This is co-transport, all right? It does it again, and it does it again, okay? It just takes it another glucose, okay? It starts diffusing, and every time it diffuses into the cell, it will pull in the glucose. And guess what? All the glucose or amino acids will be reabsorbed from the lumen into the cell. So that's it.
So essentially, what happens then is the glucose and amino acids, represented by those orange dots, will move through the carrier proteins, and they will diffuse into the blood. As you can see here, I'm just focusing on glucose because, you know, it just focuses on one molecule. So the glucose moves into the blood capillaries through the glucose carrier protein, and the process is known as facilitated diffusion.
Now, by doing so, as the reabsorption happens, it causes the solute concentration in the PCT lumen to decrease, and it causes the water potential in the PCT lumen to increase. All right? And as a result of that, water will also be reabsorbed by osmosis. It moves from the PCT lumen into the cell and into the blood capillaries, right? So water has been reabsorbed. And of course, some urea will also be reabsorbed by simple diffusion, okay? Because they can move through the phospholipid bilayer, not so easily, but they still do move through the phospholipid bilayer a little bit, all right?
So that is the selective reabsorption that happens in the PCT cells. What you must also know is you must also talk about the adaptation of the PCT to carry out selective reabsorption. The first one is the tight junction to prevent the leakage of filtrate, so that the filtrate can only be reabsorbed through the cell surface membranes. Because if it goes through the gaps, the cell, the kidney might reabsorb too many things, and we don't want that to happen. So reabsorption must take place through the cell surface membrane, because the cell surface membrane can actually control what is passing through the cell. All right.
Microvilli, the adaptation, the purpose of the microvilli is to increase surface area to contain co-transporter proteins. The basal membrane also is to increase the surface area facing the capillaries, by the way, so it can contain more sodium ion, potassium ion pumps and transport proteins, the glucose and amino acid transport proteins for facilitated diffusion. And of course, within the PCT cells, they also have many mitochondria. And what's the purpose of having many mitochondria? So that they have a higher ATP production to power the sodium ion, potassium ion pump. So I hope you understand the selective reabsorption that is carried out by the PCT.