Absorption and Digestion of Fats Flashcards
what are the two most important organic components of bile?
bile salts and lecithin
describe the structure of bile acids
24 carbons with a pKa of 6 (non ionized) and contains alpha oriented hydroxyl groups and beta oriented methyl groups, creating distinct polar and nonpolar sides allowing them to act as emulsifying agents
describe synthesis of bile acids
cholesterol is a precursor, a multistep process that occurs in the liver, the rate limiting step is done via cholesterol-7-alpha-hydroxylase which is downregulated by cholic acid and upregulated by cholesterol
2 main bile acids are cholic acid and chenodeoxycholic acid
describe bile salts
before leaving the liver, bile acids are conjugated via an amide bond between the carboxyl group of the bile acid and the amine group of either glycine or taurine, both glycine and taurine have pKas that make the ioninzed in body pH, so they amplify the amphipathic effects and thus are what is actually in the bile, glycine to taurine ratio is 3:1
how does bacterial flora affect bile?
this can change bile salts into bile acids or convert the bile acids into different bile acids
what is enterohepatic circulation?
most of the bile that is secreted is reabsorbed by the ileum and recycled to the liver, the bile acids that were converted via bacteria are then reconverted to bile salts, thus only a small proportion of bile is lost and this is synthesized newly each da
cholestyramin and dietary fiber both cause bile secretion
what is cholelithasis?
when cholesterol is secreted into the bile, it must be accompanied by enough bile acids to make it soluble, otherwise it precipitates out in the gallbaldder, creating gallstones, this has 3 causes: 1) malabsorption of bile acids from the gut 2) obstruction of the biliary tract, interrupting enterohepatic circulation 3) heptaic dysfunction leading to decreased synthesis of bile acids
describe lipid breakdown in the stomach
this occurs due to acid-stable lipases 1) lingual lipase is secreted at the back of the tongue 2) gastric lipase both of these are good at breaking down TGs with short FAs, such as those found in milk are important in pancreatic insufficiency
what is emulsification?
emulsification is increasing the surface area for lipid to be digested and occurs via two mechanisms 1) bile salts 2) peristalsis occurs in the small intestine
describe TG breakdown
an esterase from the pancreas breaks down the glycerol bindings to FAs 11 and 3
colipase binds FAs at 1:1 ratio and keeps them anchored at the lipoaqueous interface
describe choelsteryl ester breakdown
most cholesterol is in the free form but 15% is esterified, this is broken down by cholesterol ester hydrolase
describe phospholipid breakdown
phospholipase A removes 1 FA and lysophospholipase removes the other
describe absorption of FAs, cholesterol, and PLs
they are transported to enterocytes where they are absorbed in micelles formed by bile salts, with the hydrophobic residues internal and the hydrophlic ones external, the short and medium length FAs do not require micelles
what happens to lipids upon entering the enterocytes
they are taken to the ER, large FAs are acted upon by tiolase to become FA-COAs and then reattached to 2-monoacycglyerol to form TG via monoacylglycerotransferase and diacylglyerotransferase short and medium FAs don’t need this and go with albumin
lyshophospholipids are re-made into phospholipids
cholesterol is esterified to FAs by acyl CoA: cholesterolacyltransferase
