Digestion – Digestion and Absorption

by Thad Wilson, PhD

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    00:01 The classic example for any starch such as this is using an enzyme such as amylase to start breaking down the bonds of starch.

    00:11 So let’s talk about starch in more detail.

    00:15 Starch has two different bonds to it.

    00:17 One is 1,4 bonds.

    00:21 And the other is 1,6 bonds.

    00:23 Amylase, which you can see here as the protein with the meat cleavers, is only able to break down 1,4 bonds.

    00:31 And where does amylase come from? You produce a little bit of it in the salivary glands, but the majority of it and the necessary components of amylase are produced in the pancreas.

    00:45 However, even with all the different enzymes that are involved with digestion of carbohydrates, some carbohydrates cannot be digested in humans.

    00:56 Thos involve cellulose, hemicellulose, lignin, gums, and pectin.

    01:01 These are dietary fibers.

    01:05 Some of them are soluble and some of them are insoluble, but you can’t break them down into smaller substrates to absorb.

    01:11 So then, they would just be passed through the GI system.

    01:14 Now, there are some animals that undergo different types of fermentation in the gut that are able to utilize dietary fibers, but we as humans are not.

    01:25 Carbohydrates need to be broken down though even if they are taken in as small sugars such as lactose, sucrose, isomaltose, so forth, need to be broken down into monosaccharides for absorption.

    01:41 Lactose is a great example for this, that’s milk sugar, broken down by a lactase into a glucose and a galactose.

    01:50 And it’s only that glucose or galactose that will be absorbed.

    01:53 However, to undergo that absorption, we also need to have sodium gradient present and that allows us to move through the glucose molecule into the enterocyte from the intestinal lumen There are other brush border enzymes that a person can talk through and we’ll get to those in the next slide.

    02:14 Those are glucoamylase, isomaltase, lactase, and sucrase.

    02:28 Protein digestion is a bit more complex because you utilize more than just amylase and the disaccharidases.

    02:37 You have a cohort of protein enzymes.

    02:41 Those protein enzymes will be broken down depending upon if that protein is a neutral or basic protein.

    02:50 Chymotrypsin, elastase, trypsin, help to form both neutral oligopeptides and basic oligopeptides.

    03:02 The chymotrypsis and elastase allow for the neutral peptide breakdown from proteins and trypsin from proteins to basic oligopeptides.

    03:12 Now, we have two forms of carboxypeptidase that help us then further break down these oligopeptides.

    03:19 And that is carboxypeptidase A and carboxypeptidase B.

    03:26 A allows us to break down the neutral oligopeptides into a neutral amino acids and very short peptide fragments.

    03:33 Basic oligopeptides are broken down by carboxypeptidase B into both basic amino acids as well as short peptides.

    03:43 You need to be either an amino acid or short peptide for you to be able to reuptake into the enterocyte and then eventually into the blood.

    03:54 If we talk through these various peptides, we can classify them in a little bit better way.

    04:01 And that is chymotrypsin is initially secreted as chymotrypsinogen.

    04:09 And that is from the pancreas.

    04:10 However, to activate it, it needs to have an endopeptidase to break down a component of it to make it into chymotrypsin.

    04:20 Pepsinogen is broken down and activated into pepsin, also by endopeptidases.

    04:29 Carboxypeptidases A and B are both broken down into their active forms by exopeptidase.

    04:39 Proelastase and trypsinogen are also broken down by endopeptidases and those are then in terms of their active form, elastase and finally, trypsin.

    04:53 These are the series of enzymes, which are necessary in protein digestion.

    05:02 Now, once you have a substance such as pepsinogen, how in the world do you activate it? Well, you can activate it in a number of different ways.

    05:14 Pepsinogen is activated by a low pH.

    05:16 And pepsinogen again is secreted by chief cells in the stomach, And then luckily, that’s also where acid is produced and therefore, they are coupled together to be activated.

    05:31 Trypsinogen is activated by these endopepsidases as well as other trypsin molecules.

    05:37 All the other brush border pepsidases are activated by trypsin.

    05:43 So, now we can move on to fat digestion.

    05:47 Fat digestion involves some new enzymes.

    05:50 One is lipase.

    05:52 Lipase can be produced either lingually in the salivary glands or from the pancreas.

    05:58 Lipases are necessary to break down triglycerides into both the glycerol backbone as well as fatty acids.

    06:08 Cholesterol requires a cholesterol esterase And what this does is break down cholesterol esters into cholesterol and fatty acids.

    06:18 And finally phospholipase A2 will allow us to break down phospholipids into things like fatty acids.

    06:26 So here, we have the primary enzymes of lipase, cholesterol esterase and phospholipase A2.

    06:37 But there are a couple of other items that are necessary to help out this lipid digestion.

    06:43 Those are bile salts, because bile salts help emulsify the fat.

    06:49 Colipase is necessary to bind with lipase to undergo triglyceride digestion.

    06:58 Cholesterol esterases and phospholipase A2 round out fat digestion for cholesterol and phospholipids.

    About the Lecture

    The lecture Digestion – Digestion and Absorption by Thad Wilson, PhD is from the course Gastrointestinal Physiology.

    Included Quiz Questions

    1. Amylopectin
    2. Cellulose
    3. Hemicellulose
    4. Pectin
    5. Gums
    1. Amylose
    2. Sucrose
    3. Lactose
    4. Isomaltose
    5. Glucoamylase
    1. Alpha 1,4
    2. Alpha 1,6
    3. Beta 1,4
    4. Beta 1,6
    5. Alpha 1, beta 6
    1. Carboxypeptidase A
    2. Chymotrypsin
    3. Pepsin
    4. Elastase
    5. Trypsin

    Author of lecture Digestion – Digestion and Absorption

     Thad Wilson, PhD

    Thad Wilson, PhD

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