Nutrient absorption in the brush border -
But what exactly is the role of the brush border enzyme in the digestion? what exactly does is help to digest? for e. fats, proteins ,carbs,etc. Are there any practice questions or quizzes that can be taken for the digestive system?
Video transcript Voiceover: Did you know that the stomach flu can make you temporarily lactose intolerant? It's true. So, in this video, we will talk about how that happens.
And in detail, how the small intestine works. So, as a review, once you put food into your oral cavity or your mouth, it's chewed up and then sent down the esophagus where it ends up in the stomach where it's churned and then introduced to acid where it gets broken down into chyme and then delivered into the first part of your small intestine.
Now, the small intestine has three different parts to it. So, let's take a better look. So, the first part of the small intestine is called the duodenum, the duo-denum.
This receives the chyme that just got processed in the stomach and it's the part of the entire GI tract where the most digestion occurs. The most breakdown of food products will happen in the duodenum. All right, so the next part of the small intestine is called the jejunum.
I'll just write that right here, the jejunum. And this is the part of the entire GI tract where the most absorption occurs, anywhere. So, the most absorption of nutrients is going to happen in your jejunum, the jejunum.
Then, finally after your food passes to the jejunum, it gets to the last part of the small intestine and that's called the ileum, the ileum I-L-E-U-M.
And the ileum, now this doesn't have a superlative like the most digestion or the most absorption, but there are some pretty important things that are absorbed here. Things like vitamin B12, vitamin A, D, E, K. So, there are some important things that are absorbed here.
So, I'm just going to write important absorption. There are some important things that are absorbed in your ileum. Now, the busiest part of your small intestine is the duodenum, because there are a bunch of things that are involved in this digestion process.
So, there are four key things to keep in mind. First of all, your stomach is going to be delivering a bunch of chyme or processed food into the duodenum. So, you're going to be working with all this chyme here.
In addition, you're going to have some stomach acid that process food into chyme. That's going to be present in the duodenum. In addition to the stomach, the liver and the gallbladder are also going to be important to deliver bile to your duodenum.
So, they give bile. And as I'll talk about in a subsequent video, bile is composed of two different things. Bile salts and bile pigments. And beyond the liver and the gallbladder, the pancreas also delivers a couple of very important enzymes for digestion here.
So, I'm just going to write enzymes for now. And in a minute, I'm going to go through and talk about which enzymes are delivered by the pancreas. And then finally the duodenum itself has what are called brush border enzymes, brush border enzymes.
That are very important for activation of certain enzymes and also for digestion of several nutrients that we're going to discuss.
So, let's talk a little more about this brush border. Now, if I would have make a little drawing of the duodenum right here. Remember that first part of our small intestine. I would draw just a little tube, connected right there. And then blow up the wall if I want to take a better look at what's going on right there.
We would find then that there is a whole bunch of things than just meets the eye. First of all, the wall isn't just a straight line. There is actually a bunch of infoldings that are present on the wall to help increase surface area. Think about it.
If we're trying to digest as much as we can here, we need to make sure that there are a lot of projections or a lot of space where we can make contact with the food that's passing by. So, if this is the inside of our duodenum, and this is the outside, just like how we drew up here. That's in and that's out.
You can notice that this wall here has a whole bunch of projections on it. These projections are called villi V-I-L-L-I , villi. And a single one of them is just called a villus, just a single villus. And these are just a couple of folds or these outpouchings that help increase the surface area of our duodenum.
Now, that's not where the story ends. If we take a closer look at one of these villi, then we'd find that there are even more projections sitting on that, even smaller microscopic projections.
So, for this single villus that had a horizontal line right here for its shape. Fair to draw it out here. You'd notice that it's not a straight line, but instead these also have a bunch of projections that are present on them.
And if you were to guess why all these projections are in there? I'm sure you'd say to further increase our surface area. That's sort of the name of the game when we're in the small intestine.
To increase our surface area. And so these little guys cutely enough are called microvilli, microvilli. And a single one is just called a microvillus. So, we've got these villi right here or the single villus that you can see if you just blow up the wall of the duodenum.
And then if you blow up a single villus, you'll find that they have a whole bunch of microvilli that are found on them too, to increase our surface area because that allows for better digestion. And when we say brush border enzymes, these are a whole bunch of enzymes that are present on this brush border.
You guys think about it. These villi and these microvilli, they're no different from bristles on a comb. They act to increase surface area or places where you're going to have interaction with food that you want to digest.
And so there are enzymes that are present on this brush border. So, just to make the point, all of these microvilli and villi together, that's what makes up the brush border of our duodenum.
Pancreatic lipase breaks down each triglyceride into two free fatty acids and a monoglyceride. The fatty acids include both short-chain less than 10 to 12 carbons and long-chain fatty acids. The nucleic acids DNA and RNA are found in most of the foods you eat.
Two types of pancreatic nuclease are responsible for their digestion: deoxyribonuclease , which digests DNA, and ribonuclease , which digests RNA. The nucleotides produced by this digestion are further broken down by two intestinal brush border enzymes nucleosidase and phosphatase into pentoses, phosphates, and nitrogenous bases, which can be absorbed through the alimentary canal wall.
The large food molecules that must be broken down into subunits are summarized in Table 2. The mechanical and digestive processes have one goal: to convert food into molecules small enough to be absorbed by the epithelial cells of the intestinal villi.
The absorptive capacity of the alimentary canal is almost endless. Each day, the alimentary canal processes up to 10 liters of food, liquids, and GI secretions, yet less than one liter enters the large intestine.
Almost all ingested food, 80 percent of electrolytes, and 90 percent of water are absorbed in the small intestine. Although the entire small intestine is involved in the absorption of water and lipids, most absorption of carbohydrates and proteins occurs in the jejunum. Notably, bile salts and vitamin B 12 are absorbed in the terminal ileum.
By the time chyme passes from the ileum into the large intestine, it is essentially indigestible food residue mainly plant fibers like cellulose , some water, and millions of bacteria.
Figure 5. Absorption is a complex process, in which nutrients from digested food are harvested. Absorption can occur through five mechanisms: 1 active transport, 2 passive diffusion, 3 facilitated diffusion, 4 co-transport or secondary active transport , and 5 endocytosis. As you will recall from Chapter 3, active transport refers to the movement of a substance across a cell membrane going from an area of lower concentration to an area of higher concentration up the concentration gradient.
Passive diffusion refers to the movement of substances from an area of higher concentration to an area of lower concentration, while facilitated diffusion refers to the movement of substances from an area of higher to an area of lower concentration using a carrier protein in the cell membrane.
Co-transport uses the movement of one molecule through the membrane from higher to lower concentration to power the movement of another from lower to higher.
Finally, endocytosis is a transportation process in which the cell membrane engulfs material. It requires energy, generally in the form of ATP. Moreover, substances cannot pass between the epithelial cells of the intestinal mucosa because these cells are bound together by tight junctions.
Thus, substances can only enter blood capillaries by passing through the apical surfaces of epithelial cells and into the interstitial fluid. Water-soluble nutrients enter the capillary blood in the villi and travel to the liver via the hepatic portal vein.
In contrast to the water-soluble nutrients, lipid-soluble nutrients can diffuse through the plasma membrane. Once inside the cell, they are packaged for transport via the base of the cell and then enter the lacteals of the villi to be transported by lymphatic vessels to the systemic circulation via the thoracic duct.
The absorption of most nutrients through the mucosa of the intestinal villi requires active transport fueled by ATP. The routes of absorption for each food category are summarized in Table 3.
All carbohydrates are absorbed in the form of monosaccharides. The small intestine is highly efficient at this, absorbing monosaccharides at an estimated rate of grams per hour. All normally digested dietary carbohydrates are absorbed; indigestible fibers are eliminated in the feces.
The monosaccharides glucose and galactose are transported into the epithelial cells by common protein carriers via secondary active transport that is, co-transport with sodium ions.
The monosaccharides leave these cells via facilitated diffusion and enter the capillaries through intercellular clefts.
The monosaccharide fructose which is in fruit is absorbed and transported by facilitated diffusion alone. The monosaccharides combine with the transport proteins immediately after the disaccharides are broken down. Active transport mechanisms, primarily in the duodenum and jejunum, absorb most proteins as their breakdown products, amino acids.
Almost all 95 to 98 percent protein is digested and absorbed in the small intestine. The type of carrier that transports an amino acid varies. Most carriers are linked to the active transport of sodium.
Short chains of two amino acids dipeptides or three amino acids tripeptides are also transported actively. However, after they enter the absorptive epithelial cells, they are broken down into their amino acids before leaving the cell and entering the capillary blood via diffusion.
About 95 percent of lipids are absorbed in the small intestine. Bile salts not only speed up lipid digestion, they are also essential to the absorption of the end products of lipid digestion. Short-chain fatty acids are relatively water soluble and can enter the absorptive cells enterocytes directly.
Despite being hydrophobic, the small size of short-chain fatty acids enables them to be absorbed by enterocytes via simple diffusion, and then take the same path as monosaccharides and amino acids into the blood capillary of a villus.
The large and hydrophobic long-chain fatty acids and monoacylglycerides are not so easily suspended in the watery intestinal chyme. However, bile salts and lecithin resolve this issue by enclosing them in a micelle , which is a tiny sphere with polar hydrophilic ends facing the watery environment and hydrophobic tails turned to the interior, creating a receptive environment for the long-chain fatty acids.
The core also includes cholesterol and fat-soluble vitamins. Without micelles, lipids would sit on the surface of chyme and never come in contact with the absorptive surfaces of the epithelial cells. Micelles can easily squeeze between microvilli and get very near the luminal cell surface.
At this point, lipid substances exit the micelle and are absorbed via simple diffusion. The free fatty acids and monoacylglycerides that enter the epithelial cells are reincorporated into triglycerides.
The triglycerides are mixed with phospholipids and cholesterol, and surrounded with a protein coat. This new complex, called a chylomicron , is a water-soluble lipoprotein. After being processed by the Golgi apparatus, chylomicrons are released from the cell. Too big to pass through the basement membranes of blood capillaries, chylomicrons instead enter the large pores of lacteals.
The lacteals come together to form the lymphatic vessels. The chylomicrons are transported in the lymphatic vessels and empty through the thoracic duct into the subclavian vein of the circulatory system.
Once in the bloodstream, the enzyme lipoprotein lipase breaks down the triglycerides of the chylomicrons into free fatty acids and glycerol.
These breakdown products then pass through capillary walls to be used for energy by cells or stored in adipose tissue as fat. Liver cells combine the remaining chylomicron remnants with proteins, forming lipoproteins that transport cholesterol in the blood.
Figure 6. Unlike amino acids and simple sugars, lipids are transformed as they are absorbed through epithelial cells. The products of nucleic acid digestion—pentose sugars, nitrogenous bases, and phosphate ions—are transported by carriers across the villus epithelium via active transport.
These products then enter the bloodstream. The electrolytes absorbed by the small intestine are from both GI secretions and ingested foods. Since electrolytes dissociate into ions in water, most are absorbed via active transport throughout the entire small intestine.
During absorption, co-transport mechanisms result in the accumulation of sodium ions inside the cells, whereas anti-port mechanisms reduce the potassium ion concentration inside the cells. To restore the sodium-potassium gradient across the cell membrane, a sodium-potassium pump requiring ATP pumps sodium out and potassium in.
In general, all minerals that enter the intestine are absorbed, whether you need them or not. Iron —The ionic iron needed for the production of hemoglobin is absorbed into mucosal cells via active transport. Once inside mucosal cells, ionic iron binds to the protein ferritin, creating iron-ferritin complexes that store iron until needed.
When the body has enough iron, most of the stored iron is lost when worn-out epithelial cells slough off. When the body needs iron because, for example, it is lost during acute or chronic bleeding, there is increased uptake of iron from the intestine and accelerated release of iron into the bloodstream.
Since women experience significant iron loss during menstruation, they have around four times as many iron transport proteins in their intestinal epithelial cells as do men.
Calcium —Blood levels of ionic calcium determine the absorption of dietary calcium. When blood levels of ionic calcium drop, parathyroid hormone PTH secreted by the parathyroid glands stimulates the release of calcium ions from bone matrices and increases the reabsorption of calcium by the kidneys.
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