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Lecture 30

PHYSIOL 201 Lecture 30: Lecture 30

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Elizabeth Rust

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Lecture 30: Small Intestine Slide 2 When we look at the wall, there are folds called villi and epithelial cells whose membranes are also folded -Lots of folding -The folding of the cells are called the brush border Blood vessels in the submucosa will go up the villi As things are being absorbed across the epithelial cells, they can enter the capillaries or enter lymph duct called lacteral Slide 5 Length varies depending on how tall you are You can live without some parts of the small intestine but one major part is the ileum that you can’t live without Slide 6 The ileum has transporters that are only present in the ileum for the absorption of vitamin B12 and recycling of the bile salts that come from the liver So you end up with some nutritional deficiency when someone doesn’t have an ileum -B12 is important for red blood cells production -So these individuals can’t make read blood cells -Bile salts help increase the breakdown of fats so without the bile salts going back into the liver, we decrease efficiency of fat digestion Slide 8 In order to absorb carbohydrates, we have to break them down to their monosaccharide form We have an enzyme that comes from the pancreas called pancreatic amylase -Amylase breaks carbs down into a disaccharide called maltose and then into chains called dextrins The brush border expresses these enzymes that will do the rest of the breaking down of carbs -Dextrinase and glucoamylase breakdown the dextrins into maltose, sucrose and lactose (these are three disaccharides) -Then three other enzymes break down the disaccharides: maltase, sucrase, and lactase -These products can be absorbed; the other things before cannot be absorbed There is a Na cotransporter for glucose and galactose On the basolaterial side we have facilitated diffusion -Fructose crosses the apical membrane using facilitated diffusion Slide 10 GLUT is what transports glucose into our cells and also fructose into the cell through facilitated diffusion GLUT is a facilitated transporter SGLT (sodium, glucose cotransporters) use secondary active transport Na/K pump generates a Na gradient which can be used as an energy source for the transporters Slide 11 Pancreatic enzymes come from the pancreas -As CCK levels go up, CCK stimulates the acinar cells to release all these enzymes -Amylase was one -Trypsinogen, trypsin, and procarboxypeptidase are all coming from the pancreas -They are not active when they are released because when released, if active, they would be digesting the proteins inside the cell -Released in the inactive form and are activated in the intestinal lumen What they do: Trypsin and chymotrypsin split proteins into smaller peptides (not all the way down to the single amino acid) Carboxypeptidase splits peptides into amino acids from the carboxy end Brush border enzymes express aminopeptidases (so they don’t come from the pancreas) and they split peptides in to amino acids from the amino end Slide 12 From the pancreas into the lumen, we have trypsinogen, chymotrypsinogen and procarboypeptidase Then we have these two brush border enzymes -Enterokinase and it’s job is to take trypsinogen and break it down to trypsin -Trypsin then activates chymotrypsinogen and procarboxypeptidase Slide 13 With amino acids, we can absorb individual amino acids or small peptides (two or three amino acids together) We have at least 7 different amino acid transporters to transport the 20 amino acids -These are all Na coupled Short chains are absorbed by active transport coupled to H+ gradient Only the basolateral membrane can move amino acids using facilitated diffusion Slide 14 We have the Na coupled amino transport and the brush border enzymes and the pancreatic enzymes Across the basolateral membrane we have facilitated diffusion transporters We also have H+ being secreted and it moves back into the cell via this H/Peptide transporter -So we’re not adding hydrogen, keeps coming back in -This gradient is used to move the peptides -So this is a hydrogen active coupled transporter that moves the di and tri peptides into the cell Within the cell, they will be taken up into vesicles that have enzymes that will break them down into the individual amino acids -Because we can’t transport the little peptides across the basolateral membrane like we can across the apical membrane Na/K pump creates the gradient Slide 17 Fat is more complicated because fats are not water soluble -How we get through this is by emulsifying the fat, trying to get as small as possible fat droplets so that we can digest those triglycerides easily The interior of the lumen is filled with water -So fats form these droplets to avoid contact with the water What we have is bile -In terms of fat digestion, bile salts are amphilitic molecules that have a hydrophilic and hydrophobic side -Can wrap around the fats and make it similar to water -This allows us to have smaller fat droplets -Then the enzyme which is hydrophilic can get past it and be more efficient There is actually a coenzyme -The enzyme ligase breaks down fat -Colipase is necessary so it can dock and enter and get to the bonds in the triglycerides -So we need both of them which come from the pancreas Slide 18 Liver makes bile and stores it in the gall bladder So the first step is to release the bile -Secret
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