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Macronutrient Absorption: Comprehensive Test Questions and Solutions, Exams of Biology

North Carolina State University (NCSU)Biology

A comprehensive set of test questions and detailed answers covering the absorption of carbohydrates, proteins, and lipids. It explores the roles of various enzymes, transporters, and cellular processes involved in nutrient uptake in the gastrointestinal tract. Key topics include the function of m cells, the digestion of proteins, the role of trypsin and enterokinase, and the processes of lipid emulsification and chylomicron formation. This resource is designed to test and reinforce understanding of the complex mechanisms underlying macronutrient absorption, making it an invaluable tool for students studying physiology and nutrition. Useful for university students.

Typology: Exams

2024/2025

Available from 06/03/2025

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PHY 503: CHO, PROTEIN, AND LIPID ABSORPTION: TEST
QUESTIONS WITH COMPLETE SOLUTIONS
What are M cells of Peyer's patches of the lymphoid follicles in the lamina propria? --
Answer โœ”โœ” package ingested whole proteins in clathrin-coated vesicles that are then
secreted across the basolateral membrane into the lamina propria, where
immunocompetent cells process the protein antigens to initiate immune responses
What are the three locations for digestion of proteins to amino acids? -- Answer โœ”โœ” 1)
GI lumen by way of pepsin in the stomach and endo/exopeptidases from the small
intestine
2) brush border to break down oligopeptides and dipeptides
3) in the cytoplasm of intestinal mucosal cells to break down remaining peptides into
individual amino acids
What is the function of trypsin? -- Answer โœ”โœ” activates other pancreatic protease
zymogens and trypsinogen proteins after being activated itself
What is enterokinase (enteropeptidase)? -- Answer โœ”โœ” a serine protease in the brush
border (mainly duodenum) that activates trypsinogen
What do endopeptidases do? -- Answer โœ”โœ” cleave internal peptide bonds (60-70% of
luminal amino nitrogen is converted to oligopeptides this way)
What are the three endopeptidases and what do they do? -- Answer โœ”โœ” trypsin cleaves
at basic residues, chymotrypsin cleaves at aromatic residues, and elastase cleaves at
neutral residues
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PHY 503: CHO, PROTEIN, AND LIPID ABSORPTION: TEST

QUESTIONS WITH COMPLETE SOLUTIONS

What are M cells of Peyer's patches of the lymphoid follicles in the lamina propria? -- Answer โœ”โœ” package ingested whole proteins in clathrin-coated vesicles that are then secreted across the basolateral membrane into the lamina propria, where immunocompetent cells process the protein antigens to initiate immune responses What are the three locations for digestion of proteins to amino acids? -- Answer โœ”โœ” 1) GI lumen by way of pepsin in the stomach and endo/exopeptidases from the small intestine

  1. brush border to break down oligopeptides and dipeptides
  2. in the cytoplasm of intestinal mucosal cells to break down remaining peptides into individual amino acids What is the function of trypsin? -- Answer โœ”โœ” activates other pancreatic protease zymogens and trypsinogen proteins after being activated itself What is enterokinase (enteropeptidase)? -- Answer โœ”โœ” a serine protease in the brush border (mainly duodenum) that activates trypsinogen What do endopeptidases do? -- Answer โœ”โœ” cleave internal peptide bonds (60-70% of luminal amino nitrogen is converted to oligopeptides this way) What are the three endopeptidases and what do they do? -- Answer โœ”โœ” trypsin cleaves at basic residues, chymotrypsin cleaves at aromatic residues, and elastase cleaves at neutral residues

What do exopeptidases do? -- Answer โœ”โœ” cleave C-terminal peptide bonds, resulting in individual amino acids (30-40% of ingested protein is cleaved this way) What are the two exopeptidase and what do they do? -- Answer โœ”โœ” carboxypeptidase A cleaves at aromatic and neutral aliphatic amino acids, and carboxypeptidase B cleaves at basic amino acids What is PepT1? -- Answer โœ”โœ” the oligopeptide transporter that allows for di- and tripeptides to be taken up by enterocytes What happens to di- and tripeptides after uptake by PepT1? -- Answer โœ”โœ” most are hydrolyzed to single amino acids, but some show up in the portal vein (especially those containing proline because they are more resistant to hydrolysis) What drives the process of oligopeptide uptake? -- Answer โœ”โœ” a proton gradient that is fueled by an Na+ gradient Why is an Na+ gradient required for oligopeptide uptake? -- Answer โœ”โœ” because in order to ensure that there are enough protons in the lumen, anything that comes in has to be pumped back out by the Na+/H+ exchanger What is the apparent advantage of peptide absorption by PepT1 over individual amino acid absorption? -- Answer โœ”โœ” it is faster; if you give amino acids as a peptide, you will observe a higher level of these amino acids in the blood compared to if you gave the same amino acids in monomer form Is pepsin activity required for complete protein digestion? -- Answer โœ”โœ” no, because as long as pancreatic function is intact, proteins can still be assimilated What happens to protein assimilation if a patient has pancreatic insufficiency? -- Answer โœ”โœ” absorption will be deceased, resulting in increased stool nitrogen What does congenital absence of trypsin do? -- Answer โœ”โœ” yields a loss of protein assimilation, because trypsin is required to activate the other pancreatic proteases

How do products of TAG breakdown enter the intestina lumen and what happens when they get there? -- Answer โœ”โœ” as vesicles, mixed micelles, and monomers; pancreatic lipase and biliary bile salts coat the emulsion droplets to begin chemical digestion How are lipid emulsion droplets digested? -- Answer โœ”โœ” from outside to inside What are the barriers that fatty acids and glycerol must cross in order to reach the inside of an enterocyte? -- Answer โœ”โœ” 1) mucus layer that lines intestinal epithelium

  1. unstirred water layer
  2. apical membrane (contiguous with water layer)
  3. enterocyte membrane What can happen to fatty acids in the acidic environment adjacent to the brush border? -- Answer โœ”โœ” they can become protonated and leave the mixed micelle by direct diffusion, by colliding with the membrane and becoming incorporated into it, or through carrier transport via a fatty acid translocase protein (FAT/CD36) What happens to lipids that become incorporated into chylomicrons? -- Answer โœ”โœ” they enter the lymphatic circulation and are then distributed throughout the body when the lymph rejoins the bloodstream What is the process of chylomicron breakdown? -- Answer โœ”โœ” 1) chylomicrons are exocytosed to the interstitial space and enter the central lacteal of villi to be transported to the venous system through the thoracic duct
  4. LPL works alongside ApoC to break down TAGs into their constituents again
  5. glycerol and FFAs can be used as metabolic substrates, or can be resynthesized to TAGs for storage and future use
  6. chylomicron remnants are taken up by liver cells to be broken down What happens to digested fats when they enter the enterocytes? -- Answer โœ”โœ” the enterocyte re-esterifies and assembles lipolytic products into emulsion-like chylomicrons, which are exported to the lymph for delivery to other organs through circulation

What are the 5 blood lipoproteins? -- Answer โœ”โœ” chylomicrons, VLDLs, LDLs, intermediate-density lipoproteins, and HDLs How does fatty acid length dictate processing by the enterocyte? -- Answer โœ”โœ” glycerol, short-chain, and medium-chain fatty acids can all pass directly through the enterocytes to the blood; chylomicrons and VLDLs enter the lymphatic circulation via the central lacteal of the villi and are sent to the blood that way What are the detailed steps of chylomicron formation and breakdown? -- Answer โœ”โœ”

  1. long-chain fatty acids are converted back to TAGs, phospholipids, and cholesterol esters in the smooth ER
  2. fat droplets form in the smooth ER
  3. apolipoproteins are synthesized in the rough ER, and then move to the smooth ER to associate with fat droplets (except ApoA1, which associates with chylomicrons in the Golgi)
  4. nascent chylomicrons and VLDLs arrive at the cis-face of the Golgi, and the apolipoproteins are glycosylated here
  5. vesicles carrying chylomicrons or VLDLs bud off from the trans-Golgi and move to the basolateral membrane
  6. vesicles fuse with the basolateral membrane and release their chylomicrons/VLDLs
  7. chylomicrons/VLDLs pass through the large interendothelial channels of the lymphatic capillaries and enter the lymph
  8. glycerol, short-chain, and medium-chain fatty acids pass through the enterocyte and enter the blood capillary What happens to most of the bile acids that are secreted into the duodenal lumen? -- Answer โœ”โœ” they are reabsorbed in the distal SI and proximal colon and recycled to the liver for re-secretion through the enterohepatic circulation What is the basal rate of bile synthesis sufficient for? -- Answer โœ”โœ” to replace the ~5% of bile acids that are excreted in the stool What happens to bile acids that are secreted into the duodenum? -- Answer โœ”โœ” they are conjugated to glycine or taurine to form bile salts, which then enter the portal circulation in the distal ileum and are taken up across the apical membrane by a Na+- coupled transporter

Why isn't glucose a product of starch digestion by alpha amylases? -- Answer โœ”โœ” because they do not have activity against terminal alpha-1,4 linkages Why can't the intestine absorb starch digested by amylase? -- Answer โœ”โœ” because it needs to be in monosaccharide form for absorption, and amylase can only produce oligosaccharides What are the brush border disaccharidase enzymes? -- Answer โœ”โœ” 1) lactase - cleaves lactose to glucose + galactose

  1. glucoamylase (maltase) - splits maltase and maltotriose into glucose molecules
  2. sucrose-isomaltose-glucosidase enzyme - sucrase cleaves sucrose into glucose + fructose, and isomaltase splits branching alpha-1,6 linkages of alpha-limit dextrins What are the final percentage products of CHO digestion? -- Answer โœ”โœ” 80% glucose, 10% fructose, 10% galactose What is the rate-limiting step in CHO assimilation, and what is the only exception to this? -- Answer โœ”โœ” absorption, NOT hydrolysis; lactase is much slower than the absorption transporter, so lactose breakdown is the rate-limiting step for lactose assimilation How are glucose and galactose absorbed across the apical enterocyte membrane in the SI? -- Answer โœ”โœ” through secondary active transport that is coupled to Na+ via SGLT How does SGLT1 work? -- Answer โœ”โœ” as Na+ moves into the cell down its electrochemical gradient, glucose (or galactose) is pulled with it against its gradient into the cell, fueled by the Na+/K+ pump How is fructose absorbed across the apical enterocyte membrane of the SI? -- Answer โœ”โœ” through facilitate diffusion by GLUT- 5 How do the 3 monosaccharides exit across the basolateral enterocyte membrane? -- Answer โœ”โœ” using GLUT- 2 What are the two structural requirements for monosaccharide binding to SGLT1? -- Answer โœ”โœ” 1) hexose sugar must be in the D-conformation
  1. hexose must be in the form of a 6-membered pyranose ring Why can't SGLT1 absorb fructose? -- Answer โœ”โœ” because it is a 5-membered pyranose ring What would happen to glucose absorption if the Na+/K+ pump was inhibited? -- Answer โœ”โœ” it would be reduced because the Na+ gradient would be decreased, and SGLT1 does not bind well to glucose in the absence of Na+ so there will be reduced driving force for glucose absorption What happens to insulin secretion when fructose is included with glucose in the diet? -- Answer โœ”โœ” it is lower than when a similar amount of just glucose is ingested, because fructose does not stimulate insulin the same way that glucose does How does insulin release modulate food intake and how is this affected by fructose ingestion? -- Answer โœ”โœ” through leptin, so decreased insulin after fructose ingestion means decreased leptin release and less suppression of food intake What do high levels of fructose intake compared to glucose do to lipogenesis? -- Answer โœ”โœ” increase de novo TAG synthesis What are the 4 major pathways of protein assimilation in the GI tract? -- Answer โœ”โœ” 1) hydrolysis of proteins to absorbable amino acids by luminal proteases from the stomach and pancreas
  2. digestion of proteins to peptides by luminal proteases, followed by digestion of peptides to amino acids by brush border enzymes
  3. direct absorption of digested oligopeptides by enterocytes, followed by intracellular cytosolic enzyme digestion of oligopeptides to amino acids that can be moved across the basolateral membrane
  4. direct movement of digested oligopeptides into the blood How can newborns absorb proteins within the first 6 months of life? -- Answer โœ”โœ” absorb intact proteins from colostrum and milk during a developmentally regulated endocytosis process (mediated by corticosteroids)