0:00 In this video, we will focus on starch digestion. 0:04 We will first review the structure of starch. 0:07 And finally, we will look at the steps involved in starch digestion. 0:13 So we begin here with a human eating a sandwich. 0:18 The bread contains starch. 0:20 Starch is made up of two forms of glucose polymers. 0:25 These are the linear amylose and the branched amylopectin. 0:31 Amylose is a linear chain of glucose, 0:34 linked together by alpha-124 glycosidic bonds. 0:39 Amylopectin are linear chains of glucose with branch points, 0:43 creating a tree-like figure. 0:46 In amylopectin, the glucose molecules are also linked by alpha-124 glycosidic 0:51 bonds. 0:52 But the branch points are linked by alpha-126 glycosidic bonds. 0:59 So now let's see what happens when starch is ingested. 1:04 What happens first is that the mouth will break down the starch, both 1:08 physically and chemically. 1:10 Physically by the jaws, teeth and tongue, chemically by the salivary glands. 1:17 The salivary glands secrete saliva, but also the enzyme within it called saliv 1:23 ary alpha-amylase. 1:25 What alpha-amylase essentially does is that it will break these bonds here, the 1:31 alpha-124 glycosidic bonds. 1:35 So alpha-amylase hydrolyzes alpha-124 glycosidic bonds. 1:40 Hydrolyzing in this respect means breaking down. 1:45 Amylase will only break down the starch partially, and then from the mouth the 1:50 starch will travel towards the stomach here. 1:54 Starch is only partially hydrolyzed into oligosaccharides and shorter polysac 1:59 charides, once it reaches the stomach. 2:02 Starch is only hydrolyzed partially because once the starch comes down to the 2:08 esophagus into the stomach, the amylase becomes inactivated. 2:15 This is because the acidic environment of the stomach actually inactivates the 2:20 salivary amylase. 2:22 And so starch digestion does not occur within the stomach. 2:26 The stomach will only mix the content around and then allow the starch to reach 2:32 the small intestine. 2:35 And it is within the small intestine where most of the digestion and absorption 2:41 of starch takes place. 2:43 Now let's zoom into the small intestine and see what happens with the starch, 2:46 so we're zooming into the lumen of the small intestine. 2:53 Within the lumen of the small intestine we can find the cells, the cells of the 2:57 intestine known as enterocytes. 3:00 The enterocytes are also called the absorptive cells because they absorb the 3:06 nutrients. 3:07 But the enterocytes also contain these enzymes called brush-border enzymes that 3:13 play a role in the digestion of starch. 3:17 Below the enterocytes we have the bloodstream. 3:22 So when starch reaches the small intestine, it is already in a partially hydro 3:29 lyzed form, remember. 3:32 Now when starch reaches the small intestine, another organ, known as the pancre 3:37 as, which is situated here, will begin to secrete alpha amylase. 3:44 So there was alpha amylase secreted from the salivary glands and the pancreas. 3:51 The pancreatic alpha amylase will be secreted into the small intestine where it 3:55 will break down the alpha 1 to 4 glycosidic bonds, 4:00 just as what the salivary amylase did. 4:06 So pancreatic alpha amylase hydrolyzes the alpha 1 to 4 glycosidic bonds, 4:12 breaking down the starch further. 4:16 The enterocytes, as I mentioned, also have enzymes called brush-border enzymes 4:22 that participate in the digestion of starch. 4:26 These enzymes include maltase, which will hydrolyze maltose. 4:33 Now maltose is essentially two glucose molecules linked together, so maltase 4:39 will hydrolyze these. 4:43 Then you have another brush-border enzyme called sucrase isomaltase. 4:52 maltose will hydrolyze both the alpha 1 to 4 glycosidic bonds and alpha 1 to 6 4:58 glycosidic bonds. 5:01 So what is important to know is that isomaltase will hydrolyze the branch 5:08 points of starch here. 5:11 Therefore starch will encounter all these enzymes in the small intestine and 5:16 will be digested. 5:17 It will be broken down into their building blocks, which is glucose. 5:24 So through starch digestion, we will end up with many glucose molecules. 5:34 Now within the lumen of the small intestine, we also have many sodium ions that 5:39 actually play a critical role in the absorption of glucose into the body. 5:46 What happens is that on enterocytes, we find these transporters called sodium 5:53 glucose linked transporters, or SGLTs. 5:59 These sodium glucose linked transporters are found on the apical surface of the 6:03 enterocytes, so on the top. 6:07 And what they do is that they function as a co-transporter for both sodium and 6:12 glucose. 6:13 Two sodium ions will enter for one glucose molecule. 6:18 Once glucose is within the cell, it can be reabsorbed by the blood stream 6:23 through a GLUT2 transporter. 6:26 The GLUT2 transporter is found on the basal surface of the enterocyte. 6:32 When glucose is in the bloodstream, it will increase blood glucose levels. 6:37 The glucose can be used as energy by tissues, or it can be stored away in the 6:43 liver as glycogen. 6:46 So that was the fate of starch. 6:48 Once it is broken down into the small intestine, it is absorbed by the body and 6:54 can be used as energy. 6:57 However, not all portions of starch is digested in the small intestine. 7:04 The portion of starch that resists digestion in the small intestine are known 7:09 as resistant starch. 7:13 And this fraction of starch will essentially reach the colon. 7:21 So what happens to this resistant starch when it reaches the colon? 7:26 So here we're just zooming into the colon. The colon is also known as a large 7:32 intestine. 7:34 The resistant starch will reach the colon after escaping digestion in the small 7:39 intestine. 7:41 Now within the colon, the resistant starch will actually undergo fermentation 7:47 by the GLUT microbiota. 7:49 So zooming into the colon, here we have the colon cells and the mucus. 7:59 The resistant starch here will encounter all these bacteria within the colon. 8:04 And through bacterial fermentation, the bacteria will produce a by-product such 8:12 as short-chain fatty acids, 8:14 which will be subsequently used by the human body. 8:21 And of course starch that is not fermented or absorbed or digested will be 8:27 waste and will be excreted by the human body. 8:31 And that is where I end this video. Hope you enjoyed it. Thank you for watching 8:35 .
