in this video we're going to discuss the biochemical processes in digestion So when we're talking about digestion we're talking about the catabolic or the breakdown of large molecules macroolelecules down into their base units or monomers And it is these monomers that are then small enough for absorption through the epithelia of the GI tract Now the enzymes that are involved are going to be intrinsic enzymes So these are enzymes secreted by the GI tract themselves or via the accessory glands that we've talked about Hydraysis or the use of water is the chemical process that allows us to break down larger molecules into smaller ones because water is going to be added to the chemical bond in order to break it So let's discuss the digestion of carbohydrates So carbohydrate digestion begins in the mouth So keep in mind that only monossaccharides are absorbed Now the three main monossaccharides that we usually ingest are going to be glucose fructose and galactose So only those three predominantly are going to be absorbed Now in the mouth the first enzyme that these carbohydrates encounter and in most cases we're consuming polysaccharides So these are longer chains of monossaccharides is salivary amalayise So salivary amaase begins the breakdown of polysaccharides like starch and glycogen Uh we also have pancreatic amalayise Uh so keep in mind that there is no digestive enzyme in the stomach for carbohydrates Digestion in the stomach Uh assuming there was some digestion in the saliva in the mouth at that point you have oligosaccharides So that particular enzyme salivary amala works best around slightly acidic to close to neutral pH So when the material when the bolus gets into the stomach if it falls into the gastric juices that sort of deactivates salivary amalayise However in somebody who might eat a larger meal not all of the food items get into the gastric juice Some might hang out sort of in the fundus and so the action of salary amalase might be a little bit more prolonged but within the stomach proper there is no specific enzyme to break down carbohydrates So it's not until we get to the deadnum via secretions from the pancreas So pancreatic amaase is released into the dadum and that further facilitates the breakdown of uh polysaccharides into olosaccharides Um so it's it functionally it's the same as salivary amaase It's not breaking things down into monossaccharides So in the microvilli uh you have brush border enzyme So specifically you have u brush border enzymes dextrinase gluco amalayise and this will convert those oligosaccharides into disaccharides and then you have other enzymes like lactase malt sucrace where they will take those disaccharides respectively lactose moltos and sucrose and break it down into its constituent monossaccharides And often times that breakdown So sorry ju just to go back here when we break down these uh or use these enzymes these brush border enzymes lactase maltase and sucra these are actually next to protein complexes that will readily then absorb those simple sugars So what about the digestion of proteins so when we're talking about the digestion of proteins we're talking not only about proteins that were consumed in the diet but also some of your own digestive enzymes are going to be broken down ultimately into amino acid monomers And so the beginning of protein digestion begins in the stomach So that low pH environment denatures the protein So it disrupts the three-dimensional structure and it makes it a little bit more accessible to proteasis and the proteiase that is present in the stomach is pepsinogen So it's in an inactive state that is then activated under low pH environments And so pepsin breaks down the polyeptide chain again into sort of igopeptides right it's not breaking it down into single amino acids And when the kim goes into the small intestine there are a number of other proteases that have a role in digestion So we have pancreatic proteasis trison and kimotriin So they cut at specific amino acids to get even smaller iligopeptides And then you have an enzyme called caroxyeptidase So caroxyeptidase is an enzyme that specifically cuts at the caroxal terminus of an illegal peptide So remember a traditional amino acid has a central carbon It has a variable group that is different from each of the 20 amino acids you have an amino group and you have a caroxile group So this is a traditional amino acid So when I build an amino acid chain so an igopeptide you're going to have an amino group the central carbon the caroxile group connected to the next amino group connected to that central carbon connected to the caroxile group So we refer to this end of the peptide as the amino end and this end as the caroxile end So what caroxyeptidase does is it cuts basically at the caroxile end So one amino acid at a time So it chews away the caroxile amino acid at a time So it helps to break down that oligopeptide Now the brush border enzymes you have an amopeptidase and an amopeptidase sort of complements the caroxyeptidase because it's going to break down one amino acid at a time on the amino end So you're basically chewing that polyeptide or oligopeptide at both ends to make it one amino acid shorter each time Now add that you have trison and kimotriin which are cutting oligopeptides at specific amino acids It's giving you shorter oligopeptides to digest And then lastly you have depidacases where if you have a igopeptide that only consists of two amino acids a deptide this enzyme facilitates the bond being broken between those two amino acids So here we have an example of those amino acids uh being released from the oligopeptides So you have your pancreatic proteasis and then finally those brush border enzymes which allow you to generate from your oligopeptide your amino acid The absorption let's write that here The absorption of those amino acids is usually through a mechanism known as secondary active transport So it's not using ATP directly It's using the energy in the sodium ion gradient because sodium is higher outside of the cell compared to inside So sodium is going down its gradient and it's pulling along the amino acid up a concentration gradient So this is on the apical side and on the basil lateral side you have an amino acid carrier that then just allows through passive transport to move that amino acid from the absorptive epithelial cell ultimately into the capillaries So next we have the digestion of lipids Now there is a lingu lipase uh and it does a little bit of digestion about 10% of the digestion but it doesn't do it in the mouth in the saliva It's not until that linguual lipase gets into the stomach where that lower pH environment really activates that linguual lipase Now it's not breaking the lipids down a whole lot um but you know it is sort of starting to uh facilitate um the breakdown of triglycerides Okay So fatty acids and monoglycerides uh can be recombined to form triglycerides Um but what happens in the stomach well you have gastric lipaces Well again not a whole lot of digestion is going to occur in the stomach either It's not until you get to the small intestine where you start to pre-treat the fats And this treat pre-treatment is involving the bile salts and the emulsification And we talked about this earlier how emulsification doesn't break down chemical bonds but it sort of opens up the lipids from being large aggregates into smaller aggregates So it increases their surface area so you're more efficiently breaking down those lipids And so pancreas releases the bulk of the enzymes that break down lipids These are pancreatic lipaces And the pancreatic lipaces will take a triglyceride and remove two out of the three fatty acids And what you have left so you have your two fatty acids and what you have left is a monoglyceride So this is a threecarbon molecule called glycerol that is attached to a single fatty acid chain That's a monoglyceride This is then absorbed via the lymphatic system and we'll talk about absorption uh in the other video uh with a little bit more detail but just to give you an overview you have a larger aggregate fat globule The bile salts break it down into smaller pieces which increases surface area The pancreatic lipaces break it down into fatty acids and monoglycerides So they will form these structures called myils Um and the myils will be absorbed via the epithelial cells into the lactal and then eventually it carries it away from the intestine in the lymphatic system So the lymphatic system is going to be very important in the absorption of lipids and lipid soluble vitamins So when we get to the digestion of nucleic acids uh the enzymes here are going to be pancreatic ribboucleases deoxy ribboucleases and they're going to basically break down the chains of nucleotides into nucleotide monomers And it's the brush border enzymes called nucleioide nucleioidases and phosphotases that break the monomer down into their composite three components The fivecarbon pento or pento sugar either ribos or deoxyibbos phosphate ions and then the free base the adanine the guanine cytosine and thymine and uricil