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 nefron 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 UA Loop of Henley 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 nefron I've removed the loop of Henley 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 4 it's usually about 6 to 8 marks so a lot of understanding and memorizationscripture now some students will ask the question why is Ura being reabsorbed because I thought Ura was useless uh the reason why Ura 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 tubu 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 filate that was filtered between the glomus 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 color things that I'm drawing out right now is the capillary endothelium remember capillar is one Celtic 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 now next to the capillaries will be the wall of the PCT so the PCT wall as I've mentioned earlier it's one Celtic 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 T 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 likee structures as well so what exactly are these things known as and of course the PCT Lumen is where the filate is located now between one PCT wall and another PCT wall there are this kind 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 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 basil membrane not basement membrane by the way this is basil membrane you must remember that the micro micro micro faces the PCT Lumen and the base 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 micro faces the PCT Lumen and the basil membrane faces the capillaries here all right and on the microv because it's folded it has a lot of these Co transport proteins now Coe 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 cor Transporters however the sodium ion amino acid cor Transporters work the same way so don't worry about that now on the basil 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 basil membrane we have transport proteins for glucose and amino acids to care 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 car 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 the 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 basil membranes 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 color dots um and the glucose and amino acids as orange color dots salts as green color dots and the Ura as magenta maroon or purple whatever color that is uh as those color dots as well now remember ultra filtration happened between the glomerulus and also the Bowman's capsule where it filtered out a lot of water glucose amino acids salts and UA 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 ion and inadvertently we will also reabsorb some Ura 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 Uria glucose and amino acids fine and as a reminder also facing the capillary at the basil 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 color dots right the function of the sodium potassium pump is very simple it receives ATP from the mitochondrian of the cell and it actively pumps the sodium ions into the capillaris now those sodium ions were inside the cell and they just actively transported out into the capillaries don't worry about potassium ions just for 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 cor Transporters and sodium ion amino acids cor transporter so the cor Transporters I've represented in those pink color 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 c 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 am 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 basil 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 color things so what happens over here very simple the glucose and amino acids will move through the transport proteins at the basil membranes and they are transported into the capillaries by facilitated diffusion simple so rear absorption of glucose and amino acids are now done so 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 PC 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 assets and remember when solute concentration decreases the water potential of the PCT Lo 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 Ura can also be reabsorbed passively now the reason for this is as follows I told you that UA is less soluble and less toxic because they're not so polar they can also diffuse through the phospholipid by layer okay even though we don't want it to happen it just does happen Okay so simple diffusion of UA will happen where some of the Ura 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 UA but after selective reabsorption in the PCT occurs there are less salts no more glucose and amino acids less water and also lesser Ura cuz that what the PCT was supposed to do it was supposed to reabsorb some salts all the glucose and amino acids some Ura 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 qu transport of sodium ion and glucose or sodium ion and amino acids occur from the Lumin 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 Ura is also passively reabsorbed for the purpose of the exemp 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 microv faces the lumen basil membrane faces the blood capillaries on the microv they have Co Transporters such as the sodium ion glucose Co transporter and the sodium ion amino acid Co Transporters and the basil membrane have sodium ion pottassium ion pump and glucose carrier proteins and of course I'm just drawing out a mitochondrian 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 mitochondrian produc uses 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 con 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 the sodium 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 uh you know it just focus 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 okay moves from the PCT Lumen into the cell and into the blood capillaries right so water has been reabsorbed and of course some Ura will also be reabsorbed by simple diffusion okay because they can move through the phospholipid Bayer not so easily but they still do move through the phospholipid B layer 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 to prevent the leakage of filtrate so that the filtrates can only be reabsorbed through the cell surface membranes because if it 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 micro the adaptation the purpose of the micro is to increase surface area to contain Co transporter proteins the basil 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