Terms: 70, 90, amphipathic, bile, bile acids, capillary, cholesterol, chylomicro

Question

Terms: 70, 90, amphipathic, bile, bile acids, capillary, cholesterol, chylomicrons, detergents, emulsification, endoplasmic reticuli, ferry, gastric, hydrophilic, hydrophobic, like, lingual, liver, lymphatic, micelles, surface area, water

In alphabetical order, __________ and ________ lipases are the acidic lipases responsible for the luminal digestion of triglyceride lipids that occurs in the oral cavity and/or the stomach. However, ~____% to ____% of the ingested triglycerides and most of the other ingested lipids (i.e. esterified cholesterol molecules and phospholipids) reach the duodenum without having undergone any enzymatic digestion. Because lipids are more soluble in each other than in the aqueous, water-based fluids in the lumen of the GI tract, they have a tendency to glob together into large fat droplets. In order for really robust lipid digestion to occur - these big lipid droplets that form must be converted to large numbers of smaller droplets - a process called _____________. The benefit of this breaking the large droplets into numerous smaller ones is that this substantially increases the ____________ at the lipid-water interface and it is here that many of the lipid-digesting enzymes must work because many of them have enough hydrophilic character they cannot easily penetrate into the core of the lipid droplets. Spell check Spell check Spell check Spell check Spell check Spell check The muscular churning of the stomach and the segmenting, mixing contractions of the small intestine help to emulsify lipid droplets. However, once big lipid droplets are emulsified, if the emulsification is not stabilized in some way to prevent the newly-formed small droplets from re-associating and dissolving back into one another, that is exactly what they would do. To stabilize emulsified lipids in the human digestive tract, the liver secretes ____________ into the duodenum. The main organic molecules found in this secretion are ____________ which liver cells make by modifying ____________ precursors. These, along with phospholipids, some of which are also secreted by the liver, are _____________ molecules, meaning they have, within the same molecule, regions which are ___________ - or water-loving, and other regions which are __________ - or water-fearing. The former can dissolve in water, while the latter are very soluble in lipids. These molecules coat the surfaces of the small lipid droplets by dissolving their lipid-soluble ends in the droplets and projecting their water-soluble regions outward into the surrounding aqueous (water-based) fluids. Thus, amphipathic molecules act like ___________ - increasing the solubility of the little lipid droplets in water. The lipid-digesting enzymes then adhere to the stabilized droplets and digest the lipids within. Spell check Spell check Spell check Spell check Spell check Spell check Spell check Bile acids aggregate with one another in specific fashion by orienting all of their hydrophobic regions toward the center of the aggregates where they all dissolve in another (because ___________ dissolves like), and they orient their water-soluble ends toward the outer, peripheral margins of the aggregates. These aggregates of bile acids are called ______________. Many of the end products of lipid digestion, including cholesterol molecules, free long-chain fatty acids, and 2-monoglycerides and lysophospholipids are very hydrophobic so, once released from the lipid droplets, are more soluble in the lipid-dense core of the micelles than in the aqueous fluid in the duodenum. Consequently these hydrophobic end products of lipid digestion accumulate in high concentrations in the cores of the micelles. The micelles then function to ________ these hydrophobic end products of lipid digestion through the unstirred __________ layer that overlies and hydrates the epithelial cells lining the small intestine and deliver them in bulk quantities to the intestinal epithelial cells (i.e. enterocytes). Spell check Spell check Spell check Spell check Once the end products of lipid digestion are taken up by the enterocytes that line the intestine, they are moved to the ______________ within these cells. Inside these subcellular organelles, the newly absorbed lipids are recombined to make new lipids. For instance, some free long-chain fatty acids that were absorbed are added to lysophospholipids that were absorbed to generate phospholipids the body needs. Similarly, some newly absorbed free fatty acids are added onto newly absorbed 2-monglycerides to form triglycerides that the body needs. However, the enterocytes cannot just pass all of these newly formed lipids across their basolateral membranes and into the interstitial fluids because the lipids are highly hydrophobic and not readily soluble in interstitial fluid. So, within the enterocyte endoplasmic reticulum, the newly formed lipids are coated with apolipoproteins to form lipoprotein particles called ________________. Many of the amino acid residues in the apolipoproteins used are charged, which gives the proteins polar character and makes them, and hence the lipids they are surrounding, more soluble in watery fluids (like interstitial fluid and blood). Thus, it is in the form of these lipoprotein chylomicron structures that the newly absorbed dietary lipids leave the gut. Chylomicrons are relatively large molecules and cannot enter the small ____________ vessels that drain blood away from the gut. Rather, they enter the ____________ vessels and are carried away from the gut by the lymph - not by the blood. Thus, the ___________ does NOT get the same 'first crack' advantage of taking up and using or storing the hydrophobic lipids that are absorbed in the gut that it gets for taking up and using products of sugar or protein digestion. Spell check Spell check Spell check Spell check Spell check

Explanation / Answer

Gastric and Lingual lipases are the acidic lipases responsible for the luminal digestion of triglyceride lipids that occurs in the oral cavity and/or the stomach. However, ~70% to 90% of the ingested triglycerides and most of the other ingested lipids (i.e. esterifies cholesterol molecules and phospholipids) reach the duodenum without having undergone any enzymatic digestion. Because lipids are more soluble in each other than in the aqueous, water-based fluids in the lumen of the GI tract, they have a tendency to glob together into large fat droplets. In order for really robust lipid digestion to occur - these big lipid droplets that form must be converted to large numbers of smaller droplets - a process called emulsification. The benefit of this breaking the large droplets into numerous smaller ones is that this substantially increases the surface area at the lipid-water interface and it is here that many of the lipid-digesting enzymes must work because many of them have enough hydrophilic character they cannot easily penetrate into the core of the lipid droplets.

The muscular churning of the stomach and the segmenting, mixing contractions of the small intestine are helpful to emulsify lipid droplets. However, once big lipid droplets are emulsified, if the emulsification is not stabilized in some way to prevent the newly-formed small droplets from re-associating and dissolving back into one another, that is exactly what they would do. To stabilize emulsified lipids in the human digestive tract, the liver secretes bile into the duodenum. The main organic molecules found in this secretion are bile acids which liver cells make by modifying cholesterol precursors. These, along with phospholipids, some of which are also secreted by the liver, are amphipathic molecules, meaning they have, within the same molecule, regions which are hydrophilic - or water-loving, and other regions which are hydrophobic - or water-fearing. The former can dissolve in water, while the latter are very soluble in lipids. These molecules coat the surfaces of the small lipid droplets by dissolving their lipid-soluble ends in the droplets and projecting their water-soluble regions outward into the surrounding aqueous (water-based) fluids. Thus, amphipathic molecules act like detergents - increasing the solubility of the little lipid droplets in water. The lipid-digesting enzymes then adhere to the stabilized droplets and digest the lipids within.

Bile acids aggregate with one another in specific fashion by orienting all of their hydrophobic regions toward the center of the aggregates where they all dissolve in another (because micelles dissolves like), and they orient their water-soluble ends toward the outer, peripheral margins of the aggregates. These aggregates of bile acids are called chylomicrons. Many of the end products of lipid digestion, including cholesterol molecules, free long-chain fatty acids, and 2-monoglycerides and lipophospholipids are very hydrophobic so, once released from the lipid droplets, are more soluble in the lipid-dense core of the micelles than in the aqueous fluid in the duodenum. Consequently these hydrophobic end products of lipid digestion accumulate in high concentrations in the cores of the micelles. The micelles then function to water these hydrophobic end products of lipid digestion through the unstirred ferry layer that overlies and hydrates the epithelial cells lining the small intestine and deliver them in bulk quantities to the intestinal epithelial cells (i.e. enterocytes).

Once the end products of lipid digestion are taken up by the enterocytes that line the intestine, they are moved to the endoplasmic reticuli within these cells. Inside these subcellular organelles, the newly absorbed lipids are recombined to make new lipids. For instance, some free long-chain fatty acids that were absorbed are added to lipophospholipids that were absorbed to generate phospholipids the body needs. Similarly, some newly absorbed free fatty acids are added onto newly absorbed 2-monglycerides to form triglycerides that the body needs. However, the enterocytes cannot just pass all of these newly formed lipids across their basolateral membranes and into the interstitial fluids because the lipids are highly hydrophobic and not readily soluble in interstitial fluid. So, within the enterocyte endoplasmic reticulum, the newly formed lipids are coated with apolipoproteins to form lipoprotein particles called chylomicrons. Many of the amino acid residues in the apolipoproteins used are charged, which gives the proteins polar character and makes them, and hence the lipids they are surrounding, more soluble in watery fluids (like interstitial fluid and blood). Thus, it is in the form of these lipoprotein chylomicron structures that the newly absorbed dietary lipids leave the gut. Chylomicrons are relatively large molecules and cannot enter the small capillary vessels that drain blood away from the gut. Rather, they enter the lymphatic vessels and are carried away from the gut by the lymph - not by the blood. Thus, the liver does NOT get the same 'first crack' advantage of taking up and using or storing the hydrophobic lipids that are absorbed in the gut that it gets for taking up and using products of sugar or protein digestion.

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