in this video we will 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 amalo and the branched amalo pectin amose is a linear chain of glucose linked together by Alpha 124 glycosidic bonds Amal optin are linear chains of glucose with Branch points creating a tree likee figure in Amal optin the glucose molecules are also linked by alpha1 124 glycosidic bonds but the branch points are linked by alpha1 126 glycosidic bonds so now 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 gland secret secret saliva but also the enzyme within it called salivary Alpha amas what Alpha amalay essentially does is that it will break these bonds here the alpha 124 glycosidic bonds so alpha amas hydes alpha 124 glycosidic Bonds hydrolyzing in this respect means breaking down Amala will only break down the starch partially and then from the mouth the starch will travel towards the stomach here starch is only partially hydrolized into oligosaccharides and shorter polysaccharides once it reaches the stomach starch is only hydrolyzed partially because once the starch comes down to the esophagus into the stomach the amas becomes inactivated this is because the acidic environment of the stomach actually inactivates the salivary amalay 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 Lum of the small intestine we can find the cells the cells of the intestine known as inyes the interos sites are also called the absorptive cells because they absorb the nutrients but the ocytes also contain these enzymes called brush B enzymes that play a role in the digestion of starch below the interos sites we have the bloodstream so when starch reaches the small intestine it is already in a partially hydrolized form remember now when starch reaches the small intestine another organ known as the pancreas which is situated here will begin to secrete Alpha amas so there was Alpha amase secreted from the salivary glands and the pancreas the pancreatic Alpha amas will be secreted into the small intestine where it will break down the alpha 124 glycosidic bonds just as what the salivary amalay did so pancreatic Alpha amas hydrolized the alpha 124 glycosidic bonds breaking down the starch further the interos sites as I mentioned also have enzymes called brush water enzymes that participate in the digestion of starch these enzymes include molas which which will hydr moltos now molos is essentially two glucose molecules linked together so molese will hydrolize these then you have another brush butter enzyme called sucra Isom molas isomaltase will hydroly both the alpha1 124 glycosidic bonds and alpha1 126 glycosidic bonds so what is important to know is that Isom molas will hydroly the branch points of starch here therefore starch will encounter all these enzymes in the small intestine and will be digested it 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 interos sites we find these Transporters uh called sodium glucose linked Transporters or sglts these sodium glucose link Transporters are found on the appical surface of the anas sites 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 bloodstream through a glute 2 transporter the glute 2 transporter is found on the basal surface of the interos side when glucose is in the bloodstream 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 one it once it is broken down in the small intestine it is absorbed by the body and can be used as energy however not all 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 resistance stch 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 resistance starch will actually undergo fermentation by the gut microbiota so zooming into the colon here we have the colon cells and the mucus the resistance starch here will encounter all these bacteria within the colon and through bacterial fermentation the bacteria will produce a byproduct 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 will be excreted by the human body and that is where I end this video hope you enjoyed it thank you for watching [Music]