this chapter traces the triacylglycerol molecule, the most common form of dietary lipid, from its ingestion to its eventual metabolism in skeletal muscle or adipose cells. triacylglycerol molecules are composed of a 3 carbon glycerol backbone and 3 fatty acids of varying chain lengths esterified to each carbon. digestion of lipids begins in the mouth and stomach, with secretions of lingual and gastric lipase, respectively. these enzymes hydrolyze the shorter chain fatty acids off of the triacylglycerol molecule, and are thus more active in children and infants who drink greater amounts of cow's milk, which contains triacylglycerol molecules with a higher proportion of short chain amino acids.
the main digestion of lipids begins in the intestines. when the stomach's acidic contents are dumped into the intestines, the enzyme cholecystokinin is secreted and stimulates the release of bile salts from the gall bladder. bile salts, in cooperation with the peristaltic action of the intestines, emulsify and breakdown the dietary lipids into smaller droplets, increasing the surface area to which pancreatic lipase has access to. pancreatic lipase is the main digestive enzyme for lipids and its net effect on a triacylglycerol molecule is to break it down into two free fatty acid molecules and a 2-monoacylglycerol molecule. these breakdown products are repackaged in bile salt "micelles", which then travel to the intestinal epithelium and are absorbed into the microvilli.
at this point the bile salts remain in the intestinal lumen (eventually to be reabsorbed in the ileum, transported to the liver, and stored back in the gall bladder), but the contents of the micelles are absorbed into the intestinal "enterocyte" cell. the previously disassembled fatty acids and 2-monoacylglycerol molecule are reassembled in the smooth ER of the enterocyte, and packaged into another protein chamber, called a "chylomicron", before leaving the enterocyte cell. the chylomicron, one of several types of "lipoproteins", basically serves as a small, hydrophilic container for the hydrophobic triacylglyceride molecules, and is outfitted with apoproteins which are involved in cell signaling downstream.
when chylomicrons are released from the enterocytes, they receive more apoproteins (E and CII) from yet another lipoprotein, HDL, and become "mature" chylomicrons. these two apoproteins allow the lipids to be metabolized in muscle or adipose cells- apoprotein E allows uptake of the chylomicron into cells, and apoprotein CII stimulates LPL, an enzyme which metabolizes the chylomicron's contents within the cell.
questions
1. what is the structure of a triacylglycerol?
2. why are gastric and lingual lipase more active in children and infants?
3. describe the action of bile salts on lipids.
4. what are bile salts stimulated by?
5. what is the major enzyme that digests triacylglycerols?
6. pancreatic lipase is secreted along with...
7. what is the function of bicarbonate?
8. bicarbonate secretion is stimulated by...
9. what does colipase do?
10. describe the effect of pancreatic lipase on triacylglycerols.
11. what happens to the fatty acids and monoacylglycerol that is produced from pancreatic lipase?
12. describe the life of a micelle.
13. what parts of a micelle get absorbed into the intestinal lining?
14. describe the fate of bile salts after being used in the intestine.
15. how does the absorption of short chain fatty acids differ from that of medium and long chain?
16. what happens to fatty acids in the endoplasmic reticulum of intestinal cells?
17. what are chylomicrons and why are they necessary in lipid digestion?
18. where are fatty acids resynthesized within intestinal epithelium cells? where are apoproteins synthesized?
19. what happens to chylomicrons after they are released from intestinal epithelial cells?
20. what distinguishes a "nascent" chylomicron from a "mature" one?
21. what is LPL?
22. describe the effect of insulin on LPL?
23. how does the Km for LPL compare in muscle and adipose cells? what is the implication?
24. what is the fate of the portions of chylomicrons that remain after digestion by LPL?
answers
1. a glycerol moiety with three fatty acids esterified to each carbon.
2. because they preferentially metabolize lipids with short chain fatty acids, which are consumed in large quantities by children in the form of cow's milk.
3. bile salts act as a "detergent" and bind to fat globules, increasing the surface area and thus the efficiency of metabolism by pancreatic enzymes.
4. cholecystokinin, secreted by intestinal cells when stomach contents enter the intestines.
5. pancreatic lipase.
6. colipase and bicarbonate.
7. bicarbonate neutralizes gastric acidity, raising pH to ~6.
8. secretin, which is secreted when acid enters the duodenum.
9. binds to dietary fats as well as lipase, increasing lipase activity.
10. pancreatic lipase hydrolyzes the fatty acids at positions 1 and 3 (of any chain length), resulting in 2 free fatty acids and 2-monoacylglycerol.
11. they are bound into micelles, tiny microdroplets which are emulsified by bile salts.
12. after formation, micelles travel through the "unstirred water layer" and then are absorbed into the microvilli of the intestinal lining.
13. fatty acids, monoacylglycerol, other dietary lipids are absorbed, but bile salts remain in the lumen.
14. 95% of bile salts are resorbed in the ileum, where they are transported via entero-hepatic circulation back to the liver, which stores the bile in the gall bladder for the next digestive cycle.
15. short chain fatty acids can be absorbed directly onto the intestinal lining without any packaging, and are transported to the liver via portal circulation (rather than lymph) bound to serum albumin.
16. fatty acids and monoacyl glycerides are re-synthesized into triacylglycerides and packaged into chylomicrons.
17. chylomicrons are small protein packages that contain triacylglycerides, proteins, and phospholipids. they are necessary because lipids are not water soluble and would therefore coalesce and block blood flow.
18. smooth ER, rough ER
19. they are transported via the "chyle" in the lymphatic system and enter the blood through the thoracic duct.
20. a nascent chylomicron is one that is recently secreted by the intestinal epithelial cells. a mature chylomicron is a lipoprotein that has had additional apoproteins transferred to it via HDL, such as apoprotein E and CII.
21. an enzyme on capillary endothelial cells that digests the triacylglycerol molecules within the chylomicrons.
22. insulin upregulates production of LPL so that after a mealtime, when triacylglyceride levels are high, fatty acids will be hydrolyzed.
23. the Km is lower in muscle cells, implying that muscle cells can more easily metabolize fatty acids, even when the concentration is low.
24. degradation and recycling by hepatocytes.
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