Hungry Microbiome: Starch Digestion

March 7, 2020

(Narrator) This is Armando. This video was created as part of the Hungry Microbiome Project, which I made at CSIRO. In this video we’ll focus on starch digestion. We will first review the structure of starch, and finally we will look at the steps involved in starch digestion. So we begin here with a human eating a sandwich. The bread contains starch. Starch is made up of two forms of glucose polymers. These are the linear amylose, and the branched amylopectin. Amylose is a linear chain of glucose, linked together by alpha 1-4 glycosidic bonds. Amylopectin are linear chains of glucose with branch points, creating a tree like figure. In amylopectin the glucose molecules are also linked by alpha 1-4 glycosidic bonds, but the branch points are linked by alpha 1-6 glycosidic bonds. So now let’s see what happens when starch is ingested. What happens first is that the mouth will break down the starch, both physically and chemically, physically by the jaws, teeth and tongue, chemically by the salivary glands. The salivary glands secrete saliva, but also the enzyme within it called salivary alpha amylase. What alpha amylase essentially does is that it will break these bonds here, the alpha 1-4 glycosidic bonds, so alpha amylase hydrolysis alpha 1-4 glycosidic bonds. Hydrolysing in this respect means breaking down. Amylase will only break down the starch partially, and then from the mouth the starch will travel towards the stomach here. Starch is only partially hydrolysed into oligosaccharides and shorter polysaccharides once it reaches the stomach. Starch is only hydrolysed partially because once the starch comes down to the oesophagus, into the stomach, the amylase becomes inactivated. This is because the acidic environment of the stomach actually inactivates the salivary amylase, and so starch digestion does not occur within the stomach. The stomach will only mix the content around, and then allow the starch to reach the small intestine. And it is within the small intestine where most of the digestion and absorption of starch takes place. Now let’s zoom into the small intestine and see what happens with the starch. So we’re zooming into the lumen of the small intestine. Within the lumen of the small intestine we can find the cells, the cells of the intestine known as enterocytes. The enterocytes are also called the absorptive cells, because they absorb the nutrients. But the enterocytes also contain these enzymes called brush border enzymes that play a role in the digestion of starch. Below the enterocytes we have the blood stream, so when starch reaches the small intestine it is already in a partially hydrolysed form, remember. Now when starch reaches the small intestine another organ known as the pancreas, which is situated here, will begin to secrete alpha amylase. So there was alpha amylase secreted from the salivary glands and the pancreas. The pancreatic alpha amylase will be secreted into the small intestine where it will break down the alpha 1-4 glycosidic bonds, just as what the salivary amylase did. So pancreatic alpha amylase hydrolysis the alpha 1-4 glycosidic bonds, breaking down the starch further. The enterocytes, as I mentioned, also have enzymes called brush border enzymes that participate in the digestion of starch. These enzymes include maltase, which will hydrolyse maltose. Now maltose is essentially two glucose molecules linked together, so maltase will hydrolyse these. Then you have another brush border enzyme called sucrase/isomaltase. Isomaltase will hydrolyse both the alpha 1-4 glycosidic bonds and alpha 1-6 glycosidic bonds. So what is important to know is that isomaltase will hydrolyse the branch points of starch here. Therefore starch will encounter all these enzymes in the small intestine and will be digested, will be broken down into their building blocks, which is glucose. So through starch digestion we will end up with many glucose molecules. Now within the lumen of the small intestine we also have many sodium ions that actually play a critical role in the absorption of glucose into the body. What happens is that on enterocytes we find these transporters called sodium glucose linked transporters, or SGLTs. The sodium glucose linked transporters are found on the apical surface of the enterocytes, so on the top, and what they do is that they function as a co- transporter for both sodium and glucose. Two sodium ions will enter for one glucose molecule. Once glucose is within the cell it can be reabsorbed by the blood stream through a glut two transporter. The glut two transporter is found on the basal surface of the enterocyte. When glucose is in the blood stream it will increase blood glucose levels. The glucose can be used as energy by tissues, or it can be stored away in the liver as glycogen. So that was the fate of starch, once it is broken down in the small intestine, it is absorbed by the body and can be used as energy. However, not all portions of starch is digested in the small intestine. The portion of starch that resist digestion in the small intestine are known as resistant starch. And this fraction of starch will essentially reach the colon. So what happens to this resistant starch when it reaches the colon? So here we’re just zooming into the colon. The colon is also known as a large intestine. The resistant starch will reach the colon after escaping digestion in the small intestine. Now within the colon the resistant starch will actually undergo fermentation by the gut microbiota, so zooming into the colon, here we have the colon cells and the mucus. The resistant starch here will encounter all these bacteria within the colon, and through bacterial fermentation the bacteria will produce a by-product such as short chain fatty-acids, which will be subsequently used by the human body. And of course starch that is not fermented, or absorbed, or digested will be waste, and it will be excreted by the human body. And that is where I end this video. I hope you enjoyed it. Thank you for watching. [Music playing]

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