Digestion and Absorption

Question 1

describe digestion in the mouth

Answer 1

• starts with carbohydrate digestion by the enzyme salivary α-amylase that cleaves α1-4 glycosidic bonds of starch and glycogen and it forms branched oligosaccharides
• lingual lipase for ipid digestion is released in the oral cavity but is mainly swallowed

Question 2

describe the purpose of HCl

Answer 2

• HCl denatures food for quicker digestion and also destroys pathogens
• HCl alters the conformation of pepsinogen in order for it to cleave itself, producing the active protease pepsin (autocatalytic activation)
  
  • pepsin is an endopeptidase and cleaves peptide bonds within a protein chain
  • pepsin has a optimum pH of 2

Question 3

describe the digestion in the stomach

Answer 3

• carb digestion stopped due to denatured salivary α-amylase
• protein and lipid digesiton start in the stomach due to acid-stable enzymes
• pepsin degrades proteins to large peptides
• lingual and gastric lipases degrade TAGs with medium-chain FAs (long chain FA degraded later)
  
  • medium-chain FAs go directly to the liver and available for energy metabolism
  • does not need bile salts/pancreatic enzymes

Question 4

describe the purpose of secretin and CCK

Answer 4

• released from endocrine cells of the duodenum when the acidic chyme reaches
• secretin leads to secretion of bicarb and water from the pancreas
  
  • bicarb used to change the acidic pH to a neutral pH in duodenum
• CCK leads to release of pancreatic enzymes and bile into the duodenum

Question 5

describe cholecystokinin

Answer 5

• activates release of bile from gallbladder
• inhibits gastric motility which allows neutralization of the acidic chyme that entered the duodenum
• activates enteropeptidase in the duodenum
• activates secretion of pancreatic enzymes, zymogens ad proteins

Question 6

describe the activation of pancreatic zymogens

Answer 6

• trypsinogen is released by the pancreas and is activated to trypsin after it reaches the lumen of the duodenum
  
  • this separation prevents pancreatic damage since trypsin is a powerful protease that activates all other pancreatic zymogens

Question 7

what does trypsin activate?

Answer 7

• zymogens to active enzymes
• proenzymes to active enzymes
• lipid digestion:
  
  • procolipase to colipase (protein, not an enzyme)
  • prophospholipase A2 to phospholipase A2

Question 8

describe where in the sequence pancreatic proteases cleave proteins

Answer 8

• trypsin: cleaves after arg/lys residues
• chymotrypsin: cleaves after bulky/aromatic residues
• elastase: cleaves after glycine, alanine, serine residues
• carboxypeptidase A or B (exopeptidases) cleave amino acids from the carboxyl-end

Question 9

which pancreatic enzymes are cleaved after dietary proteins are digested?

Answer 9

• pancreatic lipase
• pancreatic α-amylase
• pancreatic phospholipase A2
• all formed amino acids are taken up into the intestinal mucosal cells
• the final uptake of amino acids is larger than the intake of dietary amino acids (dietary AAs + digestive AAs)

Question 10

describe the absorption of dietary amino acids

Answer 10

• the uptake of dietary amino acids is performed by secondary active transport with cotransport of sodium ions
• the trasporters are specific for a group of amino acids which can be overlapping
• the release into the portal vein is by facilitated transport

Question 11

describe the big picture of digestion and absorption of carbs

Answer 11

Question 12

describe the oligo- and disaccharide digestion by brush border enzymes

Answer 12

Question 13

describe carb digestion in the small intestine

Answer 13

• pancreatic α-amylase acts on polysaccharides and oligosaccharides (which were formed in the mouth by salivary α-amylase) to generate: maltose, isomaltose and some small oligo-saccharides (dextrin)
  
  • these sugars are finally degraded to glucose by the sucrase-isomaltase complex as well as by maltase-glucoamylase
• enzymatic degradation of dietary disaccharides:
  
  • sucrase degrades sucrose to glucose and fructose
  • lactase degrades lactose to glucose and galactose

Question 14

describe lactose intolerance

Answer 14

• lactose intolerant individuals cannot properly digest lactose in the small intestine and lactose passes into the large intestine
• bacteria degrade lactose to osmotically active compounds resulting in large volumes of hydrogen has and CO2 which causes abdominal cramps, diarrhea and flatulence
  
  • the osmotic diarrhea results from water coming from the mucosal cells that enters the lumen of the large intestine

Question 15

describe congenital lactase deficiency

Answer 15

• lactase activity is normally the highest after birth
• congenital lactase deficiency leads to severe osmotic diarrhea and dehydration in the baby
  
  • the baby cries due to painful bloating
• these neonates need lactose-free formula

Question 16

describe primary lactose intolerance

Answer 16

• the amount of lactase is strongly reduced (this is normal) at about 7 years of age in the majority of humans

Question 17

describe secondary lactose intolerance

Answer 17

• can occur due to loss or damage of intestinal mucosal cells (food poisoning)
• severe diarrhea or gastroenteritis due to rotavirus can lead to intestinal injury
• Celiac disease is immune-mediated causing damage to the intestinal mucosa in response to gluten (protein in grains)

Question 18

describe the absorption of dietary sugars

Answer 18

• the absorption of dietary glucose and galactose is performed by SGLT-1
  
  • SGLT-1 uses secondary active transport and cotransport with Na ions
• dietary fructose enters the intestinal mucosal cells via facilitated transport by GLUT-5
• all 3 monosaccharides are released by facilitated transport by GLUT-2 into the portal vein

Question 19

describe the big picture of DNA and RNA digestion and absorption

Answer 19

Question 20

describe the overview of lipids

Answer 20

Question 21

describe the main functions of bile

Answer 21

• transport of free cholesterol and conjugated bilirubin from the liver into the duodenum for eventual release of free cholesterol in feces
• delivery of conjugated bile salts and phosphotidylcholine to the small intestine for emulsification of dietary lipids for digestion
• formation of mixed micelles for uptake of dietary lipids into the intestinal mucosal cells
• release of 5% of bile salts into feces

Question 22

contrast bile acids and bile salts

Answer 22

• 2 differences:
  
  • A negative charge is needed for the hepatic ABC transporter
    
    • the conjugation of bile acids with glycine or taurine changes the pKa to 4 or 2, respectively
  • improves emulsification of dietary fat in the intestines

Question 23

explain the difference between a primary and secondary bile acid

Answer 23

• primary bile acids are synthesized only in the liver which uses free cholesterol
  
  • cholic acid
  • chenodeoxycholic acid
• secondary bile acids are formed by modification of primary bile acids by bacteria in the ileum
  
  • deoxycholic acid
  • lithocholic acid

Question 24

list the steps of bile production process in the liver

Answer 24

• synthesizes primary bile acids from free cholesterol (cholesterol 7-α-hydroxylase)
• conjugates primary or secondary bile acids with glycine or taurine
• releases conjugated bile salts and free cholesterol into the bile canaliculi via the ABC-transporter

Question 25

describe pancreatic lipase

Answer 25

• pancreatic lipase is secreted with procolipase by the pancreas and reaches the duodenum
  
  • procolipase is cleaved to colipase in the duodenum by trypsin
• digestion of dietary triacylglycerols by pancreatic lipase needs release of bile salts stimulated by CCK
• pancreatic lipase is not synthesized as a zymogen
  
  • it cannot act in the healthy pancreatic cells since the substrate (TAGs), bile salts and colipase are not present

Question 26

the special _____ pathway of _____ synthesis is found in ______ cells

Answer 26

the special MAG pathway of TAG synthesis is found in intestinal mucuosal cells

• pancreatic lipase forms MAG and 2 fatty acyl CoAs which can be directly linked to form TAGs
• TAGs and cholesteryl ester and lipid soluble vitamins are released inside of chylomicrons into the lymph

Question 27

describe the fate of chylomicrons in the body

Answer 27

Question 28

describe 3 possible causes of steatorrhea

Answer 28

• lack of conjugated bile salts
  
  • due to liver damage including liver cirrhosis or bile duct obstruction by gallstones or tumor
• defects related to pancreatic juice
  
  • deficiency of enzymes, lack of transport of enzymes into the intestines like seen in CF or lack of bicarb secretion which would impair pH adjustment in duodenum
• defective mucosal cells related to uptake of nutrients
  
  • a shortened bowel could also lead to steatorrhea