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OK. There you go. Um Can you see the Tylo um slide? Yeah, I can. Perfect. Perfect. So yeah, as I was saying, the only one we're gonna focus on is the first one but these three were mentioned um on your lecture slides. So the session plan for today is to cover these four areas. But first we're gonna cover some brief concepts that they mentioned in your lecture and these are things that I'm sure you're familiar with. So I won't spend too long on them. The first thing to be aware of is units. So they will try to catch you out with them. So be careful of your units. Um It can come up in PM and BRS and it's really important for this lecture where we're looking at um concentrations of irons. Um And there are some tips you can use to help you remember, for example, I like to remember um femtomolar starts with af and so does 15 and nanomolar starts with A nine sorry starts with A N and so does nine and those two come up quite a bit. So diffusion and osmosis, I'm sure you've heard of um both these processes before. So I won't spend too long going through it. But the key points that they wanted you to be aware of specifically with diffusion is that it's rapid over a microscopic distance and it's slow over a macroscopic distance. And it makes sense, right? Because it's just a passive diffusion, sorry, passive movement of um a solute down its concentration gradient. And so because it's slow over a macroscopic distance, we've developed um circulatory systems which helps bring individual cells within diffusion range. And we've got, you know, a cell membrane is a diffusion barrier and it allows us to have different concentrations of certain substances. Either side of the membrane, lipid soluble molecules can cross more easily than water soluble molecules. And we look more about um the transport of solutes across the membrane in a moment, but just briefly with osmosis, um just remember that water follows, follows irons. So it um moves towards a hypertonic environment. So yeah, now we're gonna look at how molecules cross the epithelium. So there's two main um processes. The first one is paracellular transport. So literally between the gaps in cells between two cells. And we've also got um the movement of things directly across one cell and this is transcellular transport. So we're gonna look at both of them. But first, we're gonna focus on transcellular transport. So, so have you test you of that? So, so can across the cell membrane by simple diffusion facilitated transport and active transport. And we have different proteins that help us um with each of them different, sorry, we've got two transport proteins, carrier proteins and channel proteins. And here is the diagram of each of them. You need to be aware that channel proteins are allow faster transport than carrier proteins. And this makes sense, right? If we think about what a channel protein is, it's just a pore in the same membrane that allows um solutes to move through it. Whereas with carrier proteins, there needs to be a conformational change in the protein to allow the transport. Um So some examples of channel proteins that you may have come across ion channels and they can be open at certain times closed at certain times, you would have also come across voltage gated channels, ligand gated channels and mechanically gated channels and just be careful with the ligand gated channels because the ligand can bind outside the cell and inside the cell to open the channel. So what about carrier proteins? Um The main way that they're differentiated is based on how many things are moving and in what direction. So if it's one thing moving in one direction, it's a uniportal. And when we have two things moving together, we call that coupled transport and it can be in the same direction through a symporter or opposite directions through a antiporter. So um we haven't talked about um we transport that requires energy and transport that is passive. So we've got facilitated transport, facilitated diffusion and active transport and facilitated diffusion is passive. It doesn't require any energy. It's just based on a concentration gradient. So something moving down that concentration gradient, active transport requires energy and it's broken down into two types. So primary and secondary primary is when we directly use at P to power the transport. Whereas with secondary um the pa the transport of one thing is dependent on the concentration gradient that is created by actively transporting something else. And here you've got nice illustrations showing them all side by side. We're gonna look at some examples of actual transport. So these are some that you may have come across already. Um The sodium potassium A TPA S pump comes up almost everywhere and we're gonna look at the SGL T one and the glute five and glute two transporter. Now, so you might be familiar with this um glucose and lactose move through the SGL T one transporter on the apical membrane by secondary active transport. And it's co transport with sodium ions. So again, we've got a sodium potassium ATPase pump and that creates a concentration gradient of sodium ions um in the cell. So it moves three sodium ions out of the cell, two potassium ions into the cell. And so now we've got a concentration um gradient for sodium and glu glucose and lactose can move with sodium into the cell. Um And yeah, it's effective when we've got a low concentration of sugars in the gut lumen. So how does the sugars move out of the cell on the basolateral membrane? We've got this carry protein glute two. And that moves glucose and lactose by facilitated diffusion and it's high capacity. So it works. Um so it moves a lot of things at once but it is low affinity. So um it means it's not as effective at low concentrations. Um And so, yeah, I talked a lot about apical side, basolateral side. Just remember the basolateral side is the side um next to the blood stream and the apical is facing the gut lumen. So fructose is absorbed a bit differently. It's by the glute five coprotein on the apical membrane and it's by facilitated diffusion. So this diagram wasn't in your lectures, but I thought it's quite nice showing them both side by side. And it's also effective at low concentrations of um fructose in the gut lumen. OK. So this, this is the bulk of um the teaching session today, water and electrolytes and the other vitamins and irons. Um Most of the water in the gi tract is absorbed. So 99% and it's powered by the absorption of irons. And that's really important. What for what we're gonna talk about in a moment, most of the water is absorbed in the small intestine, specifically the Jejunum. So just remember that and um the movement of irons is mostly by passive diffusion calcium and iron. We'll talk about in a moment, but we, um, they're not completely absorbed and we'll talk about why it's regulated later on. So, I'm sure you've seen this diagram before. Um, the key takeaway is just that most of the water that absorb, that is absorbed is in the small intestines of 8 L. Whereas only 1.4 L are absorbed from the colon. So, where does this water come from? Because we only ingest about 2 L as you can see here. Well, it's all listed. So um saliva and gastric secretions as well as other glands secreting fluid. Um all this water needs to be reabsorbed. So we said that the absorption of water is driven by the absorption of ions. So mostly by sodium and as we go down the gi tract, the absorption of sodium becomes more efficient. So we've got multiple mechanisms, multiple transporters moving sodium into the cell. A lot of them are cotransport. You'll see that sodium is um I guess a popular thing to be cot transported with. Um So essentially, we're moving a lot of sodium into these enterocytes that line the gut lumen. And so what happens to that sodium inside the cells, they actually move into the gaps between the enterocytes. So the lateral intracellular spaces and this is by again, sodium potassium H PA S. So we also have other ions that move. So chloride again with sodium potassium and bicarbonate. The key thing is potassium is slightly different. So it's by that paracellular pathway that we talked about earlier. So it directly goes into the gaps between cells. Um and it's a passive um er passive transport. Um So essentially these intracellular spaces are becoming very hypertonic, we're having a very high concentration of ions there. And so what is going to happen? Well, water is gonna follow. Um so water moves by osmosis um into these spaces. So again, this diagram wasn't in your lectures, but I thought it's quite nice. It was quite nice at illustrating this. Um Yeah. So you can see it's moving into these uh intracellular spaces. You've got tight junctions between the cells and some water does move into the cells because we've got a lot of solute there but only a little bit. Um And so yeah, the water that moves in distends the intracellular channels and causes increased hydrostatic pressure. And so iron and water move across the basement membrane or the epithelium and are carried away by the capillaries.