Hey folks, Dr. Nair here, and in this video we're going to study carbohydrate digestion and absorption. And so, in the previous videos, we kind of went through the whole digestive tract and basically described everything that was happening in each of those sections. So the oral cavity, we had these processes happening in the stomach, we had these processes, so on and so forth. In the rest of the videos, we are going to look at digestion through a different lens. So, in these coming videos, this video and the next ones, we're going to take each macromolecule, carbohydrates, proteins, and lipids, and see and kind of watch their journey along the digestive tract.
See how they get digestion and then absorb. So the questions we're trying to address here is what's being broken where, how, and at what level? A lot of questions in one there. And so just as a brief overview, because we're going to see this over and over again here, right? We start with the oral cavity where we have a lot of mechanical digestion.
If you're carbohydrate, there's some chemical digestion happening there. Then you get to your stomach where we get mechanical and chemical digestion happening here. We have hydrochloric acid. We have some enzymes like pepsin, some light pieces not mentioned in the slide here. I hope doing some chemical digestion as well.
Then we have further. digestion in the intestines. Yes, we all, not mentioned here, but we also have mechanical digestion, especially we're talking about lipids, bile, we're talking about, we also have a lot of chemical digestion as we have the whole suite of enzymes coming from the pancreas as well. And then it eventually leads to, that's a typo here, or not a typo, actually it's not, I tricked myself. We have even further chemical digestion as these molecules are getting closer and closer to the epithelial lining of the intestines mucosa because it's going to even break down right before we have absorption here.
And so as you hopefully see from left to right, we're going from large pieces to overly smaller pieces to even smaller pieces to even smaller pieces. And with that we're kind of phasing out the mechanical digestion which is going to be really heavy in the beginning. and really favoring the chemical digestion towards then right before we see absorption here. All right, so let's see what this looks like, this whole kind of pattern that I've just described here with just carbohydrates. Oh, my slide is telling me something else.
Okay, here let's talk about hydrolysis here. So we're going to see hydrolysis a lot, a lot, a lot in digestion here. Why?
Because this is the main way enzymes, whether we're talking about lipases, amylases, proteases, you name it. This is the main way we're going to take larger molecules and break them down into smaller molecules. Basically, we're going to be doing these decomposition reactions where we take big molecules, break them out into smaller molecules. And if you remember the dehydration synthesis reactions that we talked about way back in the chemistry of biomolecules, lecture series. Hydrolysis is basically the opposite, right?
In dehydration synthesis, if you remember, we combine two molecules together. Let's say we combine two amino acids together, create a peptide bond here, and that resulted in a water kind of being pulled out of the molecule. So we ended up with, you know, two amino acids joined together and some water.
Hydrolysis is the opposite. To take a molecule and break it up into two small molecules, it's going to use up. water molecules. So that's basically what we're showing here. In this picture here, we have one water molecule being used up, an enzyme is being involved, it will break a bond here, and now we make two smaller molecules.
So, this is what we call hydrolysis. Hydro for water, and this is for basically breaking up, breaking apart here. And so, that's what's happening in hydrolysis. So anytime you think about an enzyme working to break molecules down, it's using this reaction which we call hydrolysis.
And this is actually another reason why you need water when you have your meals too. Because, well, if you want to break down the protein, carbs, and lipids, right, you need water for hydrolysis. Alright, to the carbohydrates. So, this is what carbohydrate digestion and absorption looks like from top to bottom here. So again, we have carbohydrates enter our mouths.
Let's pretend these are like rice or bread or baked potatoes. You choose your carb, that's what's in this imaginary mouth here. And we have chemical digestion happening from the get-go.
Of course, we have mechanical digestion. So we're chewing up that bread or rice. We're breaking it up into smaller physical pieces here. Of course, that's happening.
But specifically for carbohydrates, we have chemical digestion happening from the get-go. With our amylase that's in our saliva here, we're going to break down the polycarbohydrates into basically smaller polycarbohydrates. So breaking up starch into smaller pieces of starch, essentially, here. But that's already happening from... the get-go when food is entering our oral cavity.
From there, the next time we really pick up on anything digestion related with carbohydrates is actually at the intestines. So food does get going to the stomach and of course there's a lot of digestion there. There's mechanical digestion happening there as we have the stomach churning, but we don't actually have any chemical digestion happening there.
So, The size of our polycarbohydrates that we had from back when we were chewing food up in our oral cavity, they're about the same size, actually. More or less, by the time it gets into our small intestines. Because there's not really any chemical digestion.
A whole bunch that happen in the stomach when it comes to carbohydrates. So we have these polysaccharides entering our small intestines, and they're going to be introduced to more amylase. this time the amylase comes from the pancreas. And so again, we're going to take these polysaccharides, break them down with more amylase, chemically break down amylase, and at this point, we're getting our polysaccharides really close to the disaccharide level.
So here we are looking at the lumen in the intestines here. So here's our pancreatic amylase here. It's found some starch here, some polysaccharide here. and it is basically breaking up these bones with hydrolysis, right? And the end result here is going to be disaccharides here.
And so what happens here is these disaccharides are still too big to be absorbed. That's right. In order to absorb nutrients, they have to be small enough to enter the cells of the intestinal mucosa.
and specifically small enough to enter through membrane proteins. So membrane proteins here are going to be the big facilitator of our nutrient absorption here, for the most part. So we need molecules that are small enough to enter through these membrane proteins.
And of course, polysaccharides, too big. Disaccharides are even too big for our human intestinal cells here. So we have to break them down even further to the monosaccharide level.
And so here we have enzymes at what we say the brush border of our, you know, intestinal cells here. So this is, we're looking at the apical side of our cells here. There's a bunch of these enzymes.
So when the disaccharide comes in contact with these small intestinal cell, small intestinal cells here, The enzymes at the apical level are going to break the disaccharides into monosaccharides here. Now our monosaccharides are small enough to enter the cell, but it's not as simple as that. For us humans, we don't really have a membrane protein that just lets these monosaccharides come in by themselves.
We have a cotransport instead. This is where we'll take monosaccharides with a sodium and bring them both inside these intestinal cells here. Now, this is relying on, this is secondary active transport, right? Relying on the concentration gradient of sodium. We assume that sodium concentration is high here, low inside the cell.
So sodium is moving down its concentration gradient. We're taking these monosaccharides along with the ride and bring them inside the cell. Great.
So at this point, We are on our way to absorption. We have monosaccharides inside the intestinal cells here, out of the lumen of the digestive tract. What happens now?
Well, At this point here, these monosaccharides will simply just diffuse down their concentration gradient. As monosaccharides get built up in the small intestinal cells here, they will travel the basolateral side. So this is the basal layer and this is the lateral, you could say, sides of the cell. So if we just want to talk about this whole area here, we just say basolateral. It kind of combines all the terms together except apical, right?
So here's the apical there. Here's the basolateral side, the opposite side. And so here we have a carrier protein here, a membrane protein here, that allows basically facilitated diffusion to occur to let these monosaccharides travel down their concentration gradient from high to low to the other side of the intestinal cells. And then, again with diffusion, and now the situation is simple diffusion, Those monosaccharides can then move from these interstitial spaces here into the plasma.
And now, aha, we finally have carbohydrates in our bloodstream. Absorption is achieved. Now, there's one small thing we have to solve here. This co-transport. Remember, sodium has to come in along with our monosaccharides to make this work.
But you can imagine. as sodium kind of slowly starts piling up here, right? Every time we bring a monosaccharide, we're bringing another sodium here. As we bring in more and more sodium inside the cell, we increase the concentration of sodium inside the cell, which diminishes our concentration gradient, which is bad because that means we'll slow down our absorption of these monosaccharides, right?
Because we're doing secondary active transport here. So how do we fix this? On the... the basolateral side here, we have what we call this sodium potassium ATPase.
Sometimes people call it the sodium potassium pump. And so what happens here is we spend ATP, we're literally spending energy doing this to take that sodium, pump it out to the outside world in exchange for potassium. Why in exchange for potassium?
We'll talk about that in another video way down the line when we talk about the nervous system. But for now, just. just go with it.
But the really important thing here is now we have sodium leaving our cell, so this basolateral ATPase, sorry this basolateral sodium potassium ATPase, and that reduces the concentration of sodium inside the cell, allowing this co-transport to just hum along, take sodium from the outside with the monosaccharide, bringing the cell letting that monosaccharide flow down its concentration gradient into the bloodstream here. Alright folks, that is it for this video here. As I mentioned before, or hopefully I mentioned this before, we're going to see this pattern again and again for proteins and lipids.
So really get this down pat, and I'll see you in the next video. Bye.