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 C w/ a pKa of 6 (nonionized)
contain a-oriented hydroxyl groups and b-orientated methyl groups, creating a distinct polar and non polar side, allowing them to act as emulsifying agents
describe synthesis of bile acids
cholesterol precursor, multistep process that occurs in the liver.
rate limiting step is done via cholesterol-7-a-hydroxylase, which is down regulated by cholic acid and upregulated cholesterol
two main bile acids are cholic acid and chenodeoxycholic acid
describe bile salts
before living 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 them ionized at 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?
can change bile salts into bile acids or covert bile acids into different bile acids
enterhepatic 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 reconverted to bile salts. thus, only a small proportion of bile is lost, and this is synthesized newly each day
cholestryamin and dietary fiber both cause bile secretion
cholelithaisis
when cholesterol is secreted into the bile, it must be accompanied by enough bile acids to make it soluble. otherwise, it preciptates out in the gallbladder, creating gallstones. this has 3 causes
- malabsorption of bile acids from the gut
- obstruction of the biliary tract, interrupting enteroheptic circulation
- hepatic dysfunction leading to decreased synthesis of bile acids
describe lipid breakdown in the stomach
occurs d/t acid-stable lipases
- lingual lipase- secreted at the back of the tongue
- gastric lipase
both of these are good at breaking down TGs w/ short fatty acids, such as those found in milk
important for patients w/ pancreatic insufficiency
emulsification
emulsification is increasing the surface area for lipid to be digested and occurs via 2 mechanisms
- bile salts
- peristalsis
occurs in the small intestine
describe TG breakdown
an esterase from the pancrease breaks the glyerol bindings to FAs 1 and 3. colipase binds the FAs at a 1-1 ratio and keeps them anchored at the lipoaqueous interface
describe cholesteryl ester breakdown
most cholesterol is in free form, but 15% is esterified. this is broken down by cholesterol ester hydrolase
describe phospholipid breakdown
phospholipase A removes 1 fatty acid, and lysophospholipase removes the other
describe absorption of fatty acids, cholesterol, and phospholipid
the are transported to enterocytes where they are absorbed in micelles formed by bile salts, with the hydrophobic residues internal and the hydrophillic ones external.
short and medium length FAs do not require micelles
what happens to lipids upon entering enterocytes
taken to the ER.
large FAs are acted upon by thiolase to become FA-CoAs, and then reattached to 2-monoacylglycerol to form TG via monoacylglyceroltransferace and diacylglyceroltransferase
short and medium FAs dont need this and just ride with albumin
lysophospholipids are remade into phospholipids
cholesterol is esterified to FAs by acyl CoA:cholesterolacyltransferase
cystic fibrosis has important effects that result in pancreatic deficiencies and poor absorption of certain nutrients
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