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  • >>Dr. Ketchum: In this lecture video we are going to focus on digestion and absorption

  • of carbohydrates. So were working with the gastrointestinal system, but focusing

  • on carbohydrates. And so, so far weve provided you an overview of the gastrointestinal system

  • and it’s processes, the functional anatomy, which weve talked about the major organs

  • and the accessory organs, and now what were going to do is discuss, well once that food

  • enters into your digestive system or your GI system, how is it broken down? And then how

  • are those nutrients absorbed into the blood? Okay, so the focus once again is going to

  • be on carbohydrates.

  • So in a typical diet, we actually consume about 250 to 800 grams of carbohydrates. Most

  • of these are consumed as disaccharides or polysaccharides. But, here’s the concern

  • or here’s the, the, the important part: Only monosaccharides can be absorbed. So be

  • sure you understand what we mean bycan be absorbed.” So that means that were going

  • tocan take them from the lumen of the gut, and we can transport those monosaccharides,

  • we can absorb them into the blood, from lumen to the blood.

  • So the disaccharides and polysaccharides that we typically consume are things like

  • sucrose, which would be table sugar, lactose, which would be milk sugar, maltose, starch,

  • glycogen and cellulose. So pause the video for a moment and determine which of these

  • are disaccharides and which of these are polysaccharides. So sucrose, lactose and maltose are all

  • considered disaccharides, whereas starch, glycogen, and cellulose are all considered polysaccharides.

  • So cellulose cannot be digested. So if you just ate a salad for example, the cellulose

  • in the lettuce cannot be digested. So why is it still important? Well, that’s providing

  • the dietary fiber that’s important for intestinal mobility. In other words, you have to

  • have contents passing through your intestines, otherwise youll get very ill,

  • and you can actually die from that if your contents of your gut don’t

  • pass through. So how then are we going to go about digesting these

  • disaccharides and these polysaccharides so that we can absorb the resulting monosaccharide?

  • And so let’s look at digestion of starch. So if you recall, starch is a polysaccharide that consists of glucose

  • monomers. So each one of these green structures is a glucose subunit or a glucose monomer.

  • And so since starch is a polysaccharide, we cannot digest it, or we cannot absorb it into the blood.

  • So we have to be able to digest it. So here weve ingested starch. Maybe, for example, you had some

  • pasta for dinner. Or you had crackers for dinner that have a lot of starch in them.

  • This carbohydrate, weve got to start breaking it down. So we have two enzymes that can assist with the break

  • down of starch: salivary amylase and pancreatic amylase. Salivary amylase is produced by your salivary

  • glands and pancreatic amylase is produced by the pancreas. And so what this enzyme

  • can do is break down starch into two different products. You can wind up with limit dextrins

  • when you break down starch. Which are basically short-branched polysaccharides, or you can

  • wind up with maltose, which is a disaccharide, which consists of two glucose monomers. Okay,

  • so that doesn’t do us much good either, because we cannot absorb disaccharides and

  • we certainly cannot absorb short polysaccharides either. So we have to break these down even

  • further. So how do we do that? Well that’s going to require additional enzymes.

  • So dextrinase will break down those limit dextrins. So dextrinase catalyzes the reaction

  • where the limit dextrins are further broken down into glucose subunits. If it’s maltose

  • that were concerned with, the enzyme maltase will break down our maltose into two glucose

  • subunits. Okay, because once you have the glucose, and glucose is a monosaccharide,

  • then the body can absorb that monosaccharide. Now as far as some of these other disaccharides

  • that we see like sucrose, sucrose is broken down by sucrase, the enzyme sucrase, into

  • its subunits which are fructose and glucose. And then lactose, that sugar that’s found

  • in milk, is broken down into its subunits galactose and glucose via the enzyme lactase.

  • And so lastly we have glucoamylase, which will take any polysaccharide and break that

  • polysaccharide down into glucose. So what we wind up with because of all of these enzymes are

  • all of these monosaccharides. So be sure you know which of these are monosaccharides, which

  • are di and which are polysaccharides.

  • Okay, now the locations for these enzymes are on the brush border of the small intestine.

  • And so if you recall if you looked at the lining of the small intestine, you would see villi.

  • And at the tip of the villi are the microvilli.

  • And well put the microvilli in green.

  • And so located on the brush border are enzymes. So this enzyme here is called the brush border enzyme. And

  • many of the enzymes that we talk about, excluding salivary and pancreatic amylase are brush

  • border enzymes. So now once we have our carbohydrate broken down into a monosaccharide,

  • now we can absorb the monosaccharide. Here what we will do is focus on the absorption of

  • glucose and galactose. So here we have the lumen, we have the epithelial cells,

  • and here we have the blood. So the goal is to absorb glucose...and galactose

  • and even fructose, okay? All monosaccharides. So were going to start on the basolateral

  • membrane. And were going to start with a type of transport that I’m going to label

  • a one. So now inside of an epithelial cell you know the concentration for sodium and

  • potassium. So pause the video, write in your concentration for sodium and potassium inside

  • of your epithelial cell and then see if you can’t determine what type of transport is

  • occurring in number two as well as number three, and number four. So if youve

  • already paused the video and determined what type of transport’s occurring at 1, 2, 3,

  • and 4, let’s see if you were correct. So were going to start with the concentrations

  • for sodium and potassium inside the epithelial cell. So we noticed that sodium is being pumped from

  • a low concentration toward a high concentration. Potassium is being pumped from a low concentration

  • toward a high concentration, and this requires ATP. So number one must be primary active

  • transport. So let’s look at number two now up on the apical membrane. So on the apical membrane

  • we have this purple thing, right? That’s representing a carrier protein. Sodium is going from a high

  • concentration in the lumen toward a low concentration within the cytoplasm of the epithelial cell. So

  • high to low using a carrier. Then we notice glucose is going from a low concentration

  • in the lumen to a high concentration in the epithelial cell. So if one ion is going along

  • a gradient, that’s releasing the indirect energy to drive the second solute against

  • its gradient. So number two must be sodium glucose co-transport, which is a type of secondary

  • active transport. Okay, so now once we have glucose at a high concentration inside the

  • epithelial cell, here it’s going from a high concentration toward a lower concentration,

  • but it’s using a carrier protein. So number three must be facilitated diffusion. And then

  • finally we have number four. So in number four, you can see how potassium is moving from

  • a high concentration in the epithelial cell through a channel toward a low concentration.

  • So number four must also be facilitated diffusion. So that explains how glucose is absorbed,

  • the mechanism for glucose absorption in the small intestine. Now there’s

  • also galactose absorption, which is the exact same as glucose. So instead of writing glucose

  • on the diagram, scratch that out and you could put galactose and it would be exactly the

  • same as what we just discussed for glucose. What’s different is fructose.

  • So fructose is actually absorbed by facilitated diffusion across both of the membranes. So

  • by both of the membranes, we mean apical membrane as well as the basolateral membrane.

  • So now, how do the brush border enzymes play a role? So here we have a brush border enzyme

  • called maltase. So this would be the lumen of the small intestine. Remember that most

  • of your digestion and absorption takes place in the small intestine, specifically the duodenum.

  • So maltose is the brush border enzyme that’s going to take maltose and catalyze a reaction

  • to break that bond between those two glucose units into your monosaccharide glucose.

  • And then we can absorb glucose just like I described in the previous diagram. The only extra step

  • here is that you have a brush border enzyme called maltase to help break down that disaccharide.

>>Dr. Ketchum: In this lecture video we are going to focus on digestion and absorption

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人間の生理 - 炭水化物の消化と吸収 (Human Physiology - Carbohydrates Digestion and Absorption)

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    王維希 に公開 2021 年 01 月 14 日
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