Digestion: Study with Video Lessons, Practice Problems & Examples

The digestive system processes food through ingestion, digestion, absorption, and elimination. Essential nutrients, including amino acids, vitamins, and minerals, are vital for bodily functions. The small intestine, with its extensive surface area from villi and microvilli, is crucial for nutrient absorption. Bile and lipases break down fats into micelles for absorption. Water follows solutes via osmosis, aided by aquaporins, particularly in the large intestine, which reclaims water and compacts feces. The liver plays a key role in detoxifying and storing nutrients absorbed from the digestive tract.

Food and Feeding

Hi. In this video, we'll be talking about digestion and the digestive system, the organ system responsible for bringing in food, extracting nutrients from it, and getting rid of the waste. Now food is any substance that has nutrients needed by an organism to live. We consider food to be things like pasta, which is chock full of carbohydrates, meat, which has proteins, or fats, which can be found in things like dairy products. Now, those are all well and good. However, we also need what are known as essential nutrients. These are nutrients that we can't synthesize, and they have to be obtained in the diet. So while we can, for example, make a bunch of amino acids, we can't make all 20 that are used to build proteins. In fact, there are actually 8 amino acids that we can't synthesize, and we call these essential amino acids. And in case you're curious, and don't worry about memorizing this, these include isoleucine, leucine, lysine, methionine, phenylalanine, threonine, valine, and tryptophan. Methionine is actually significant in there, since that is going to be the amino acid coded for by the start codon. Now, it should also be noted, you know, case you're curious about these things, that infants also can't produce histidine. This can lead to certain infantile diseases that, you know, maybe you'll learn about if you go to medical school or something. Now vitamins are organic compounds that are required in small amounts, and are used for a variety of different things, including as coenzymes, which are going to be important parts of enzymes. In fact, usually the essential part to carry out whatever reaction it's responsible for. We also need minerals, which are inorganic substances, required again in just small amounts. However, they're going to be incorporated into proteins and, you know, the active sites of enzymes for example, where the reactions take place, and they also can be important components in hormones. Now minerals also include what we refer to as electrolytes, that are going to be, mineral ions that are super important for maintaining osmotic balance in the body, because remember, water follows solutes. And these will also be super important for nerve signals, and, we'll learn more about that in the section on the nervous system. However, just know that the electrical signals sent through nerves are actually being conducted by the movement of these electrolytes or ions. Lastly, there's also essential fatty acids, and it should be noted that, most animals can produce, you know, all the fatty acids that they need. However, there are certain double bonds that we, as humans can't produce, and those are commonly referred to as omega 3 and omega 6 fatty acids due to the double bond at the omega 3 position and omega 6 position. This just has to do with the chemical naming, so you know don't worry about trying to memorize any of this. It's just a convention in terms of counting the carbon tail from the end, as opposed to the beginning. Anyways, omega threes are found in, you know, things like tree nuts for example. These are often thought of as plant fats. Omega sixes are commonly, referred to as animal fats, and interestingly, it's been theorized that issues with obesity can actually be in part due to the ratio of omega 3 and omega 6s you have in your diet, and Americans tend to have too many omega 6s, not enough omega 3s. So our balance is a little out of whack, and you know, as you know America suffers from obesity quite a bit. Now if I jump out of the way here, I also want to point out how these vitamins can be used. This is a vitamin called riboflavin, because it's got all the flavor. And riboflavin is an essential component of FAD, which you can hopefully see riboflavin's structure in this portion of the molecule. FAD is, of course, the essential electron carrier that it plays a critical role in the electron transport gene, and of course, cellular respiration, production of ATP. So these essential nutrients are truly essential. We need them to build components that aren't just there for, you know, a little added advantage. These build components that are essential to living organisms. Now there are different techniques when it comes to obtaining food. You know, I've always admired, whales that have this stuff called baleen, which you can see here. It almost looks like a comb. It's these bristled, bristled structures that surround their jaws. And you can actually see the baleen in a whale's mouth right there, that's a sperm whale. And what it's doing is it's basically, you know, taking a big bite of ocean water, rich in these tiny little organisms called krill, and then it's going to filter those krill through its baleen, and basically strain its food out of the water. This tactic is known as suspension feeding. Technically it's a type of filter feeding, which is like a, filter feeding is like a type of suspension feeding. And, you know, this is just one of the many ways that organisms can obtain their food. There's also, deposit feeding where, for example, a sea cucumber will kind of eat the I mean, it's kind of yucky, but the sediment deposited material around it. There is substrate feeding, which is basically when an organism lives on its food source, kind of like a caterpillar on a leaf. There's also fluid feeding, like insects sucking your blood, or in a way less creepy and gross example, a nice little hummingbird that eats nectar. Right? Now, we have the dubious title of being mass feeders. Sometimes this is called bulk feeding. Basically, we eat large chunks of food, sometimes whole organisms. Basically, we eat big pieces of other things' bodies, and that is considered like a mass and we feed on it, and it's kind of why I like to think of animals as a very crude form of life. You know, we have to just take in this matter from the outside, and like stick it in our bodies, and then process it, and waste all this stuff that just comes out of us anyways. And, you know, when you compare that to, for example, a plant which you know absorbs sunlight, produces its own sugars, much more elegant way of living. Anyways, that's just my little 2¢ on that. And let's flip the page.

Digestion and Digestive Tracts

Animals ingest their food through their mouths. Now, hopefully, it will come as no surprise that something as essential as the ingestion of food will be affected in a very significant way by natural selection. So, that is why there is a wide variety of forms and functions in terms of mouth parts. You can see here, this is the skull of a deer and they have these nice grinding teeth, which are useful when you're an herbivore and you need to grind up plant matter. Whereas this saber-toothed cat here, not going to be doing a lot of eating of plants, and hopefully, you know, you can tell what these bad boys are for. And of course, Darwin's moth, which has a very specialized feeding tube for a very special flower that has a very deep nectary that only it can reach. Now because natural selection has such a powerful effect on mouth parts, you'll often see some interesting examples of adaptive radiation with mouth parts. Adaptive radiation, remember, is when an organism diversifies and gives rise to a bunch of different variations, due to trying to fill different niches. A great example of this is the finches that Darwin saw on the Galapagos, which all had these different types of beaks depending on their food source. They all came from this one common finch ancestor, but on the islands due to all the niches to fill their mouth parts evolved to have the form fit the function.

Now, nutrients are going to be absorbed in a 4-step process. The first step is ingestion, bringing food into the body, specifically into the digestive tract. Then you have digestion, which is the breakdown of the food through both chemical and mechanical, or like physical means, like chewing. Right? Grinding your Mechanically digesting the material. And it should be noted that humans perform what's called extracellular digestion, as do lots of other animals. However, some organisms will perform intracellular digestion, where they actually bring the material into their cells to be digested. Whereas we do it in the lumen or the hollow space inside of our digestive organs, which of course is outside of the cells. Now, you also need to do something with the nutrients you extract from digestion, and that is absorption, where you absorb them into the body to make use of them. Lastly, you're not going to use everything you take in, there's going to be some waste. So you have to eliminate it, eliminate that undigested material in some manner that we need not discuss here.

Now, there are going to basically be digestive tracts that fall into 2 categories. Incomplete digestive tracts, where there's a single opening that food enters, and waste exits. A nice example of this is the gastrovascular cavity of Cnidaria and Platymenesthes, which, you can see 2 cnidaria here. We have a hydra and some type of medusa, you know, jellyfish-like creature, and they just have this, one, I'm just going to write, you know, GVC for gastrovascular cavity. So, basically food is going to go in there, it's going to get digested, that's what's happening, this food's getting digested. And then it's going to exit through the same opening. We, on the other hand, have what we commonly call an alimentary canal, but is technically referred to as a complete digestive tract, and that is to say that it has two openings. Our food enters through the mouth, works its way through this digestive system, which we're going to go through momentarily. So, it's going to make its way through the esophagus here, into the stomach right here. From the stomach, it enters the small intestine, wiggles all its way through the long small intestine until it hits this area, the large intestine, where it finally is formed into feces and will exit through the anus. So, that is the whole of the process of digestion through a complete digestive tract. Let's actually flip the page and take a closer look at this whole organ system.

Mouth and Esophagus

In humans and other mammals, digestion isn't sequestered to one single organ. It actually occurs in multiple organs as the food moves through the body. And as we'll see, certain organs are specialized for certain types of digestion. Now, the first place where digestion begins is actually in the mouth itself. The food there is going to be mechanically digested and subdivided, and this serves the benefit of actually increasing the surface area of food particles, which is going to be really useful later on when we get to chemical digestion because this increased surface area is going to help increase the efficiency of the chemical breakdown. Now, there actually is some chemical digestion going on here. You see, salivary glands release a substance, of course, saliva. And saliva is a mixture of water, mucus, and enzymes. It actually contains something called salivary amylase, and amylase breaks down carbohydrates. And this salivary amylase is going to break down carbohydrates into maltose and what are called dextrins. Now maltose is basically 2 glucose subunits. You can see a maltose right here, behind my head. A single one of these is glucose. Now, behind me, let me jump out of the way here, this is what you could think of as a dextrin. Now it wouldn't be quite as long, this is basically saying, you know, that you just are repeating this unit some number of times. Now, you know, it's not going to be the, you know, 100 long it's it shows in this figure. A dextrin is, you know, going to be like a couple units of glucose linked together. So the idea is that salivary amylase breaks down carbohydrates into small units, but not into glucose itself. That's the point to take away. Now, mucus, speaking of glucose and all that good stuff, mucus is actually made from glycoproteins that form a slimy substance when they mix with water. And of course, there's water in saliva, so you get that nice slimy mucus, and this is going to be good because it lubricates the food bolus. And the bolus is basically what, you know, that ball of chewed up food that you swallow is referred to in a technical sense. Now salivary amylase isn't the only enzyme there. Actually, also going to have lingual lipase, which is basically an enzyme that's going to break down fats. Right? Lipase, lip-like lipid, ace. So this is going to break down fats, and it's released along with the saliva, but it's actually coming from glands in the tongue, as opposed to these salivary glands that you see here. So these are salivary glands, and the lingual lipase is going to be coming from a gland in the tongue. You don't need to worry about the details of that. Here you can see a nice overview of the mouth, and once that food is chewed up, or you know that bolus as we should probably call it, be technical about it, once that bolus is ready to be swallowed, it's going to move through the pharynx to the esophagus. The pharynx is back here, it's kind of like the throat region, or you know, the back of the mouth region, and it's going to connect to the esophagus, which is an organ that connects the mouth to the stomach. As you can see here that's going to lead to the stomach, And you should note that there's actually another opening in here, the, opening, the larynx, which will, it's commonly, you know, called like the windpipe, that's going to lead to the lungs, that's where air goes. So these guys actually share an area together, which is, you know, why if you've ever eaten or drank anything for long enough as a human, I'm sure you've had some water or food very unpleasantly go down your windpipe. That's why, because they are right there next to each other. Now once the bolus is in the esophagus it needs to be moved to the stomach. In part, gravity is going to help with this, but the esophagus is lined with smooth muscle that's going to go through these rhythmic motions known as peristalsis. This is basically a wave-like contraction in the smooth muscle that has the effect of pushing the food bolus down through the esophagus. So, it's it's, you know, a little hard to visualize in a static image, but essentially, this right here, these are these, smooth muscle contractions that are in a wave-like motion going to travel down the esophagus and cause that food bolus to move towards the stomach. Now, birds actually, or some birds I should say, have an interesting modification to the esophagus known as a crop. And this is, you can see this right here, it's this kinda like big chunky ball that's, attached to the esophagus, which is this tube here leading from the mouth. In case you're curious, this other tube you're seeing from the mouth is actually the windpipe right there. So anyways, their esophagus has this big chunky area on it, and it is known as a crop and it is modified for food storage. This is, you know, how all those birds will go eat a bunch of food, and then they regurgitate it to their young, you know, they're actually storing it in their crop. And, that is again, just part of the esophagus that's been modified by evolution. With that, let's turn the page.

Stomach

The stomach is a muscular organ where digestion really starts to get serious. And that's because the stomach actually creates this acidic environment that is ripe for protein digestion. Now, before entering the stomach from the esophagus, the bolus has to move through a sphincter, which is a circular muscle with a hilarious name that maintains the constriction of an orifice. Basically, it's like the gatekeeper for a particular orifice. Now there are 2 sphincters in the stomach worth noting, and you don't need to worry about the cardiac sphincter from the esophagus into the stomach, and then the pyloric sphincter. So this is cardiac, this one is pyloric, and the pyloric sphincter leads to the intestine. Now, the stomach is full of gastric juice. This is digestive fluids that are secreted by the stomach. And when the food mixes with the gastric juice it is no longer considered the bolus; it is now considered chyme. Which is just a lovely sounding name for something that I'm sure looks and smells fabulous.

Now, there are some interesting specialized cells in the stomach that perform the important functions needed for digestion. The first cell I want to talk about is the parietal cell. This is going to be the type of cell that not only secretes but makes hydrochloric acid. Acid. And this hydrochloric acid is going to help break down foods, in part, it's going to help denature proteins. If you recall, proteins are very sensitive to pH changes, and a lot of them are sensitive to acidic environments. So by creating an acidic environment, it's going to help denature proteins, making them easier to, you know, chemically degrade, which is going to be the job of this enzyme that we'll talk about momentarily called pepsin, and the hydrochloric acid produced by these parietal cells actually activates pepsin. It's stored in an inactive form called pepsinogen. Again, we'll get to that in just a second. And this hydrochloric acid is also going to help kill pathogens. Yeah, I mean, you know, you often will be eating bacteria that's on the food you're eating, or, you know, like other harmful things. This acidic environment can help kill some of those before they can really cause problems for your body. Now it should be noted there's a hormone called gastrin that's actually secreted in response to food entering the stomach, and this has the effect of causing the parietal cells to increase hydrochloric acid production. And it is production because, as I'm sure you can imagine, hydrochloric acid is not something that you can just store. Right? It's a very powerful acid. You don't want that just sitting around. So these parietal cells don't actually store it, they produce it. And you don't need to worry about the specifics of how they produce it. As you can see it, it involves a variety of pumps and channels and exchangers. You know, just very quickly we have this, pump here. It's an ATPase, meaning it, burns ATP to pump, to, pump particles and it's going to bring in potassium and pump out a proton. And you have this exchanger here that's going to, get rid of a bicarbonate and bring in a chloride ion, and then the chloride ion is going to be, also moved into the stomach lumen along with your proton, and guess what? Proton plus chloride ion, that's hydrochloric acid right there. So don't worry about the specifics there, just know that these parietal cells, you know, use a chemical process to make this substance on the fly. Because again, can't really be storing acid in your body. A little dangerous. Now there's also chief cells. Didn't mean to give parietal cells all the attention, chief cells are, you know, equally, if not more important because they secrete pepsinogen, which is again an inactive form of an enzyme called pepsin. This inactive form is actually a special type of enzyme called a zymogen, which, again, it's a type of enzyme that's going to be stored in an inactive form, and usually it's because it's safer to store when it's inactive. See, the thing is pepsin is a protease, so it's going to degrade proteins. You can't really be storing proteases around your cells because they're going to degrade all the protein products you make. I mean, cells make proteins for everything. These proteases are just one of the things they make, and a lot of the proteins they make are essential to their life. So yeah, proteases floating around, they're just going to be gumming up your day. They're going to be totally ruining everything you make, ruining all your hard work. So they have to be stored as zymogens. We call the zymogen pepsinogen. And at low pH, pepsinogen actually converts itself into pepsins, called an autocatalytic process.

Again, don't worry about memorizing all these details, just know that pepsin is kept in an inactive form, which we call pepsinogen, and at low pH, which is provided by this nice hydrochloric acid, it will turn into pepsin and start breaking down proteins into smaller polypeptides. And you can see a nice little model of that there, parietal cell and chief cell working together. It's a match made in heaven. Lastly, I want to mention the mucus cells, which secrete mucus to lubricate and protect the stomach from all that nasty acid. I mean, the tissues of the stomach need protection from the harsh stuff that's in the stomach, and the mucus cells provide that. So don't forget the stomach is a very muscular organ in addition to, you know, secreting this hydrochloric acid, and the protease pepsin, it's muscular. It's going to, you know, physically move and churn the chyme in there to help with digestion. Now just like birds had a crop that, or some birds have a crop that modified esophagus, some birds have a modified stomach called a gizzard. And basically, if you think about it, birds have beaks, so they're not going to be very good at chewing, and that can be a problem when you want to break down your food so that it's easier to chemically digest. Right? We have, like, teeth and mouths to do that. Birds, don't have that option, which is why they have a gizzard. And a gizzard is basically a special stomach that is going to contain stones, and sand, and grit, like, you know, harsh abrasive materials in there, and it's going to help grind up their food. Right? The stomach, again, it's a muscular organ. If you have all this grit and stones and sand in there, physically grind the food that as a bird, you know, you swallowed in a big chunk because you couldn't chew it. And it should be noted just because I think it's kind of funny that the way the stones and sand and stuff gets in there is the birds actually swallow them. So just, if you're ever in a bad mood, picture birds swal ... With that, let's flip the page.

Small Intestine and Pancreas

After the stomach, the food, or as we should call it now, the chyme, will enter the small intestine through the pyloric sphincter. The small intestine is a long tube in which digestion is going to occur, and also, this is very important, absorption. This is where you get to finally get the payoff for this and absorb those nutrients. And, the small intestine is actually going to be assisted by the pancreas and the liver, who are going to provide some nice secretions that will help with the digestive process. Now, small intestines often lead to some misconceptions because it's actually longer than the large intestine; it just has a smaller diameter. And in fact, it's really long, it's like 6 meters long, which, you know, if you think that a meter is, like, approximately, like, a little more than 3 feet, you're talking about roughly 20 feet here. Obviously, humans aren't 20 feet long, and that's because our small intestine is all wrapped up to minimize the amount of space they take up, but because they're super long, they have a lot of surface area. And this surface area is actually even longer, due to some cool modifications that the small intestine has. Now first, if we look at the small intestine here, you can see that the tissue on the inside of the intestine has these folds in it. Right? That's what these little lines are. Basically, the tissue itself is made to fold to increase the amount of surface area on the inside of the small intestine. Now this fold, if you zoom in, is actually covered in what are called villi. So basically, this fold has all these little projections on it, these villi, and these villi are in turn covered in what are called microvilli. So the villi, the cells of the villi rather, the enterocytes have these little hair-like structures on their tips called microvilli. This is going to result in a crazy amount of surface area. Right? We have the tissue itself folded, those folds are covered in villi, and those villi are covered in microvilli. So we are maximizing surface area here. Now villi actually surround blood vessels, as you can see in this image here, and they also have what's called a lacteal in them. And, this structure here, it's a lacteal, and basically, it's a lymphatic vessel. The lymphatic system is going to be a system involved in both circulation and immune functions, and we'll talk about it at a different time. Just know that it has projections into these villi, as do blood vessels. Now the duodenum is what we call the start of the small intestine. That's where the chyme is going to enter. And the duodenum secretes hormones in response to the chyme. Two of these hormones are secretin, which is a hormone that's going to stimulate bicarbonate release from the pancreas, and that's super important because remember, the chyme is filled with acids. And you don't want all that acid sitting in your poor small intestines, so they're going to ask for some bicarbonate from the pancreas to help neutralize the acid. The other hormone is cholecystokinin (CCK). That's what they call it in med school and stuff; that's what everyone calls it; it's a lot easier. So CCK is going to be the hormone that stimulates the pancreas to secrete digestive enzymes. See, not only does the pancreas provide a nice bicarbonate solution for the chyme to cool off, so to speak, it's also going to secrete a bunch of enzymes that digest the food coming from the stomach. Now these include things like nucleases, which break down nucleic acids, pancreatic amylase, which, similar to salivary amylase, is going to break down carbohydrates into maltose and dextrins, as well as pancreatic lipases that break triacylglycerides into two fatty acids and a monoacylglyceride. Now, the pancreas will also secrete proteases. And two of the important ones to take note of are trypsin and chymotrypsin. And these are proteases that break down proteins into smaller polypeptides. It should be noted that they actually will only break specific polypeptide bonds. That is peptide bonds between specific types of amino acids. So just like other enzymes, they are very specific as to what they will act on. What's cool about these is they're actually also zymogens, like pepsinogens, so they're released as trypsinogen and chymotrypsinogen. And this enzyme called enterokinase, which is produced by the small intestine, is what will activate trypsinogen, and that leads to the activation of other proteases, which will help break down those polypeptides. Now, with that, let's flip the page.

Nutrient Absorption

Hello, everyone. In this lesson, we are going to be talking about the functions of the small intestine, and how nutrients like fats and sugars are going to be absorbed through the small intestine. Okay. So, in the digestive system, the absorption of the nutrients that comes from your food is going to happen in the small intestine. The small intestine's main job is to absorb all of the energy and nutrients from the food that you eat. And absorption is going to happen through the epithelium cells that line the small intestine. Now, these are very selective cells. They're only going to let through certain substances. And each substance like sugar, or fats, or proteins are going to have their own unique method of getting into these cells. They may have a specific cell receptor. They may have a way that a vesicle enters the cell. They're going to have unique forms of transport into these epithelial cells. But just know that these epithelial cells in the small intestine absorb very specific nutrients molecules. And they're going to require specific transport proteins to do these particular processes. Now, whenever we talk about the movement of nutrients into these cells from the small intestine, it's actually going to require energy. Now, remember there are 2 different types of active transport or transport that requires energy. One is going to be primary active transport where you utilize ATP. And the second one is going to be secondary active transport where you're going to utilize the potential energy of a molecule. Now, both of these types of active transport are going to be used in these cells, and I'll show you a more specific example of both of these types of transport. But just know that transport is active, and it does require an energy source of some type. Because you're generally going to be moving these substances against their concentration gradient. Okay?

Now, once all of these nutrients have been absorbed into these cells, these cells are generally going to give these nutrients to the bloodstream. And the blood vessels from the microvilli, or the villi that line the inside of the intestine cells are going to converge at the hepatic portal vein or hepatic portal system. So, a portal system is basically just a system of blood vessels. And the hepatic portal vein is going to be a portal system or a blood vessel system that is going to specifically take things to the liver. And the way that I knew this is because the prefix Hepat, h e p a t is actually Greek for liver. So, if you ever see that prefix, you know we're talking about the liver. And you guys can see that right here. The Hepatic Portal System is going to be the system that leads these blood vessels to the liver. So this is going to transport nutrients directly liver. Now, you got to think about why would we want nutrients to go to the liver. Well, generally, remember the functions of the liver. It is going to be able to detoxify any toxic substances that we may consume into our bodies, and it also stores a lot of things. It can store glucose. Remember, it stores glucose in the form of glycogen. It can also store iron. It can store copper. And it also stores a lot of vitamins. So the liver is going to be the first stop that these nutrients get to because it stores a lot of this stuff. It's going to store glucose, our main energy molecule, in the form of glycogen, and it also destroys toxins. Remember that our liver is our main detoxifier. So, if we eat anything that might be toxic, it's going to go straight to the liver, and the liver's going to do its best to destroy those toxins. So, generally, the things that we absorb in our small intestine are going to enter the blood vessels. They are going to travel through the hepatic portal vein right to the liver, so the liver can either detoxify whatever we've eaten, store some of it, or send it on its way. Okay?

Alright. So, now, let's look at how these epithelial cells actually work. So, we're going to go down a little bit, and we are going to look at these cells. We're going to talk about how these epithelial cells in the small intestine actually do absorb glucose, because glucose is what we're really interested in because this is going to be the molecule that we use for cellular respiration, the main molecule. So glucose utilizes something interesting. It utilizes secondary active transport to cross those epithelial cells. And it's going to do this in a very unique way. The way it's able to do this is because of the functions of the Sodium Potassium ATPase, or as more commonly referred to as the Sodium Potassium Pump. Now, generally, when you think of the Sodium Potassium Pump, you're thinking of neurons because it's utilized to create that membrane potential in neurons. But the sodium potassium pump has another function as well. Now, the sodium potassium pump is used in these epithelial cells to create these concentration gradients. So, let's have a look at these cells right here. So, these cells are going to be our epithelial cells. And that's going to be kind of like a chunk here. Let's say that we took this chunk of these epithelial cells inside of the small intestine and we expanded it so we could see these cells. That's basically what we're looking at. Now, remember, whenever we are talking about these epithelial cells in the small intestine, they're going to be very specialized cells, and they're going to have 2 different sides. And, these 2 different sides are going to be called the apical side, which is this side with the little tiny microvilli right here. This is the apical side, and this is the side that is going to face the lumen of the small intestine. That means the inside of the small intestine. So, basically, the apical side is looking into your small intestine. And then, we're going to have the basolateral side. So, the basolateral side of these cells is this side. And this side is going to face the extracellular matrix and it's going to face your blood vessels. So, this is where this is the side that the nutrients are going to exit whenever it's trying to go to those blood vessels. So, we're going to have these two sides of these epithelial cells. And they're going to have tight junctions right here that actually make it so none of the juices inside of your small intestine actually leak into your body. Thank goodness. And they have these 2 different sides of these cells cells because these 2 different sides are specialized in the proteins that they have in their cell membrane. So on the apical side, we're going to have the glucose transporter, and on the basolateral side, we're going to have the sodium potassium pump. So, in fact, whenever we're looking at this blue structure right here, this is the sodium potassium pump. These blue proteins that you guys see right here are the sodium potassium pumps. So, what do the sodium potassium pumps do? Remember, they're going to be pumping sodium out of the cell. They're going to be pushing sodium out of the cell, and they are going to be pushing potassium into the cell. So, what that means for our cells is we're going to have high potassium concentrations, and low sodium concentrations inside of this cell. Now, whenever we're talking about the apical side, we're going to have this protein which is going to be a sodium glucose cotransporter. So these red proteins are the sodium glucose cotransporter. The reason they're a cotransporter is because both the sodium and the glucose are going in the same direction. They're moving together. The sodium potassium pump, on the other hand, is an antiporter. They're going in opposite directions. Now, the sodium glucose cotransporter is going to move sodium and glucose into the cell at the same time. Now, how does this work? Well, because there's such a low concentration of sodium inside of these epithelial cells, the sodium is moving with its concentration gradient. But the glucose is going to be moving against its concentration gradient. So, basically, this is called secondary active transport because this type of active transport, it is utilizing the potential energy of the sodium to move the glucose. The sodium wants to move into the cell because it's moving down its concentration gradient, and when it does, it drags the glucose with it. So, glucose uses sodium's energy via its concentration gradient to pull itself into the cell, and then you are going to have glucose and sodium inside of the cell now. And then the sodium potassium pump is going to actively pump that sodium back out of the cell. So, now, you have actively absorbed glucose into these epithelial cells. Now, something else to note that is not drawn here is we're going to have these other proteins, which I'm going to draw in green right here. This protein is going to actively transport glucose out of this cell. So, we're going to have glucose exit the cell, and it's going to exit the cell via a glucose transporter. Sometimes these are just abbreviated GLUT, g l u t. But these are glucose transporters which are going to transport glucose into the bloodstream, and out of these epithelial cells. So, then glucose will enter the bloodstream right here. Now, this would be an example of a glucose transporter. Now, the glucose transporter isn't really going to require any energy because there's a very high concentration of glucose inside of these epithelial cells, the glucose is going to use the transporter to just go down its concentration gradient and exit these epithelial cells into the blood, but it can't simply diffuse through the membrane, so it's going to have to use these glucose transporters that you guys can see over here. So it's going to need its own protein on the basolateral side of these particular cells to actually exit the epithelial cells. So that's the basis of how absorption of nutrients works. This was specifically for glucose, but just so you guys know, most of the nutrients that you absorb is kind of going to happen this way. We'll talk about fats in just a second because they're going to have a special form of absorption, but most substances are going to be absorbed the way that glucose is going to be absorbed. So just to recap, glucose uses utilizes secondary active transport because it utilizes the potential energy of the sodium ions to enter the cell, and the potential energy of the sodium ions is built via the sodium potassium pump concentration gradient that it makes, because it's transporting sodium out of the cell, establishing a sodium gradient. And then, the sodium glucose cotransporter is going to utilize that potential energy. And then, glucose is carried over the basolateral membrane through a glucose transporter via facilitated diffusion, which doesn't require any energy. Okay, guys?

Alright. So, now, let's talk about how fats are going to be absorbed and broken down inside of our bodies because that is a little bit different. Because fats are going to be nutrients that we need, but they're very special nutrients because they're going to be nutrients that are hydrophobic, they're going to be broken down and absorbed a bit differently. Okay. So fats are going to be broken down by this very important substance called bile. It sounds really gross, but it's very, very important. Bile is going to be a substance that is basically going to break down these fats, these hydrophobic molecules by increasing their surface area and pulling them apart. So, bile and lipases, which are going to be the special proteins, are going to break down these fat molecules. And, then these fat molecules can be absorbed by the enterocytes. Now, enterocytes is just another way to say epithelial cells in the small intestine. They're the same thing. So fats are going to be broken down into the smaller components by bile and lipases, and they're going to be broken down into these smaller components or smaller pieces called micelles. So this is a micelle right here. And basically, a micelle is how these hydrophobic fats, hydrophobic lipid molecules are going to form this structure to decrease the amount of surface area that they have interacting with hydrophilic, the hydrophilic interior of our cells. So we're going to break these fats up into all these tiny pieces called micelles, and then our epithelial cells can actually absorb those micelles. Now, what's the point of bile? What does it actually do? Well, the cool thing to know about bile is it's going to increase the surface area of fats. And this is also called emulsification. So it's going to increase the surface area of these fats, these lipids. And then it just makes it easier for lipases or these, proteins that break down lipids to break them apart. It just makes it easier. So, bile, kind of, takes these fats and stretches them apart, and then the lipases come in and they break the fats apart. Now, whenever a fatty food enters your small intestine, your body needs to understand that it's going to have to start breaking down this food a little bit differently than it does with glucose. So we're going to have these special hormones. So you're going to have your small intestine say, hey. There are fats in here. We need our bile and our lipases, so we need to make this signal. And, it's going to be the CCK signal. And, this is going to be a hormone that stimulates bile production in the liver. And a good example of that is actually down here. So, let me go out of the picture so you guys can actually see it. So, this is going to be the diagram of your digestive anatomy, and right here in orange we're going to have the small intestine. And once the fats enter the small intestine, the CCK hormone is going to be released from the small intestine, and it's going to travel to the liver, which actually generates and creates bile. And it's going to travel to the pancreas, which can actually start breaking down sugars because it creates insulin, and it's going to travel to the gallbladder, which you guys can see is this little structure right here. And the gallbladder actually stores bile for the release into the small intestine. So, the CCK hormone is going to stimulate the release and the production of bile. Now, remember, like I said, bile is going to be produced in the liver, and it's stored in the gallbladder. So, you guys can actually see these bile ducts right here in green through the liver. You guys can see it's being made in the liver, and it's actually going to be stored right here in the gallbladder. Now, some people will have issues with their gallbladder, and your gallbladder can be removed. It is not necessary for you to actually create bile. It's just important to know that with people who have their gallbladder removed, they can't store bile. So they have to not eat as many fatty foods at one time because they won't be able to digest it. So you just have to change your diet a little bit, but people without a gallbladder still function just fine. They just can't eat as many fatty foods because they don't have any bile stored up to actually break down those fats. Now, bile salts are going to be an important component of bile. I just want you guys to know that these are going to be molecules that are amphipathic. That's always hard for me to say, meaning they're both hydrophobic and hydrophilic, basically just know that they help bile do its job by emulsifying those fats. So then, once the fats are emulsified, broken down, how are they going to enter those cells, those epithelial cells? Well, they're going to enter in this form called these chylomicrons. These are going to be a special way that we package fats for transport through the body. And these chylomicrons are going to be these lipoprotein-associated complexes. So this is a chylomicron, right here. Which you guys can see right here. And it is going to have lipids right here, and it is going to have proteins. And this is, basically, just a shipping container for these digested fats. So, the digested fats are going to be in here. So, those are the digestive fats. And they're going to be stored in these chylomicron, kind of, packages. And this allows them to be transported around the body. So, they're going to be transported into the lacteal. Just so you guys know, a lacteal is going to be a vessel, a lymphatic vessel, and the lymphatic vessel just simply leads to the blood. So to transport fats, which are very hydrophobic molecules in the hydrophilic blood, we're going to need to actually package them in these chylomicrons or they won't be able to transport in the hydrophilic blood. So we package them this way, and then it is able to go to the lacteal, which will place those chylomicrons in the blood, and then we can transport fats anywhere in the body that we need those fats. So that is going to be how fats are broken down and packaged and transported. So fats are going to be broken down by bile and lipase proteins, and they are going to be packaged and transported in these chylomicrons. So this lesson just went over how the small intestine does many of its important functions. Just remember, the small intestine is utilized for absorption of nutrients, including glucose, vitamins, and fats. Now let's go on, and let's talk about the functions of the large intestines and its importance in absorbing water.

Large Intestine

Where solutes go, water follows. And absorption of water in the process of digestion is no different. As nutrients and solutes are absorbed by the small intestine, water will actually be pulled in as well through osmosis. This is useful because it will actually reclaim water that was lost in saliva and mucus, as well as digestive juices. It also absorbs the water that came in with or from the digested materials. And of course, water can't move very easily through membranes, although it can get through membranes because it's small enough. It is assisted by these channels called aquaporins that allow for efficient passage of water. And you can see an example of an aquaporin right here, letting all these little water molecules pass through this membrane. So, we're basically at the end of our journey now. We've gone from the mouth, down the esophagus, into the stomach, through the small intestine, which involves going through the starting point, the duodenum, then through the jejunum and the ileum. Don't need to worry too much about the segments of the intestine. But finally, after the small intestine, we make it to the large intestine, which is this structure that kind of frames the small intestines. Right there, that is the large intestine. Now, it's actually shorter than the small intestine, which is why I think the naming convention is a little weird, but it's wider, which is where it gets its name from. And its main function is to absorb water. When you think large intestine, think water absorption. It also is there to help compact feces, which in part is due to absorbing water from them. So, the beginning of the large intestine is called the caecum. And it's kind of just like a little sac, you can see it right here, it's this little area of the large intestine. And in some animals, like herbivores, this structure is actually going to be specialized for cellulose digestion. So, you can see in the rabbit's digestive system here, right here is the end of the small intestine, beginning of the large intestine, so food, you know, move through the large intestine this way. And here is our cecum. See how big it is compared to this teeny little one in humans? That's because rabbits are herbivores, and the cecum is going to be a specialized structure that allows them to digest plant material better. Now, after the cecum comes the colon, and this is like the main section of the large intestine. This is the real show. And what's super cool about the colon is it's home to a microbiome of bacteria, and these are actually essential to your life. This is cutting edge research right now actually, and people are finding out more and more every day just how important these bacteria that live in your colon are. And of course, the colon is going to be this whole big portion here. And you can see that it has special names, you don't need to worry about knowing all the different parts of the colon, you just need to know that the colon is where, is the main portion of the large intestine, and it's where these, where these bacteria live. Now the last part of the intestine is the rectum. Let me jump out of the image here, so you can see behind me. We have the rectum. That is where feces are stored, for elimination, or as they wait for elimination, I should say. You know, it's not always a great time to have bowel movement, especially if you're an animal in the wild and you need to look over your shoulder and make sure nothing's going to eat you or something. So, you know, this is a useful structure to allow you to wait until the moment's right. Now, it should be noted that some organisms actually have what's called a cloaca. And this is a special orifice that excretes both urine and feces. And this is because urine will flow from their kidneys into their large intestines and then all that nice lovely stuff comes out the same end. So, yeah. Cloacas are pretty gross. That's, you know, going to be something that you find in birds and snakes and stuff, it's just it's icky. The last thing I want to talk about is the appendix. I was going on and on about how cool gut bacteria are. Well, the appendix is this little extension on the cecum, you can see it right here, this just this teeny little nub, and it basically houses useful gut bacteria. It's like a backup. It's a store of gut bacteria. So, for example, if you, you know, due to illness or something, lose a lot of the bacteria in your gut, the appendix can help replace them. It also contains some tissue related to immune function, but you don't really need to worry about knowing the details of that. Basically, just know that it's not the useless structure that people used to think it was, in fact, it's thought to be quite important for housing those gut bacteria, which again, you need to live. They're actually essential to your life. That's all I have for this video. I hope you guys like gut bacteria as much as me. I'll see you next time.

Here’s what students ask on this topic:

What are the main functions of the digestive system?

The digestive system has four main functions: ingestion, digestion, absorption, and elimination. Ingestion is the intake of food through the mouth. Digestion involves breaking down food into smaller molecules through mechanical and chemical processes. Absorption is the process by which nutrients from digested food are taken up into the bloodstream, primarily in the small intestine. Finally, elimination is the removal of indigestible substances and waste products from the body through defecation. Each of these functions is essential for maintaining the body's nutrient balance and overall health.

How does the small intestine maximize nutrient absorption?

The small intestine maximizes nutrient absorption through its extensive surface area, which is increased by folds, villi, and microvilli. The inner lining of the small intestine has folds called plicae circulares, which are covered in tiny finger-like projections called villi. Each villus is further covered in even smaller projections called microvilli. This structure significantly increases the surface area, allowing for more efficient absorption of nutrients. Additionally, the small intestine contains specialized cells and transport proteins that facilitate the uptake of specific nutrients into the bloodstream.

What role does the liver play in digestion?

The liver plays a crucial role in digestion by producing bile, which helps break down fats into smaller molecules that can be absorbed in the small intestine. Bile is stored in the gallbladder and released into the small intestine when needed. The liver also detoxifies harmful substances, stores nutrients like glucose in the form of glycogen, and regulates various metabolic processes. It acts as a central hub for processing and distributing nutrients absorbed from the digestive tract, ensuring that the body maintains a balanced internal environment.

What are the differences between complete and incomplete digestive tracts?

Complete and incomplete digestive tracts differ primarily in their structure and function. A complete digestive tract, like that of humans, has two openings: a mouth for ingestion and an anus for elimination. Food passes through a series of specialized organs, including the esophagus, stomach, small intestine, and large intestine, allowing for efficient digestion and nutrient absorption. In contrast, an incomplete digestive tract has a single opening that serves both as the mouth and the exit for waste. Organisms with incomplete digestive tracts, such as cnidarians, have a gastrovascular cavity where digestion and nutrient distribution occur in a less specialized manner.

How do bile and lipases work together to digest fats?

Bile and lipases work together to digest fats in the small intestine. Bile, produced by the liver and stored in the gallbladder, emulsifies fats, breaking them into smaller droplets and increasing their surface area. This process, known as emulsification, makes it easier for lipases, which are enzymes secreted by the pancreas, to break down the fats into fatty acids and monoglycerides. These smaller molecules can then be absorbed by the enterocytes (epithelial cells) lining the small intestine, where they are packaged into chylomicrons for transport through the lymphatic system and eventually into the bloodstream.