Nutrient Absorption

Question 1

absorption of macronutrients occurs at what site in the GI tract

Answer 1

small intestine

Question 2

exocrine salivary enzymes

Answer 2

*salivary amylase
*lingual lipase

Question 3

exocrine gastric enzymes

Answer 3

*pepsin
*gastric lipase

note - these are the ONLY enzymes that function at low pH (acidic pH of the stomach)

Question 4

exocrine pancreatic enzymes

Answer 4

*pancreatic amylase
*trypsin
*chymotrypsin
*carboxypeptidase
*LIPASE-COLIPASE (only source of high-efficiency lipase)
*phospholipase A2
*cholesterol esterase

note - almost all pancreatic enzymes are secreted as proenzymes that need to be activated by enterokinase

Question 5

enterocyte membrane enzymes

Answer 5

*enterokinase
*DISACCHARIDASES (maltase, sucrase, lactase, etc)
*peptidases

note - the enterocytes are the only source of disaccharidases

Question 6

digestion of disaccharides

Answer 6

*BRUSH BORDER digestion via disaccharidases splits disaccharides into monosaccharides
1) lactase
2) sucrase
3) maltase

Question 7

digestion of starches

Answer 7

*LUMINAL DIGESTION to break 1:4 linkages via:
1) salivary & pancreatic alpha-amylase
2) enterocyte glucoamylase

Question 8

digestion of glycogen

Answer 8

*enterocyte alpha-dextrinase breaks 1:6 linkages of glycogen (in addition to the amylases, which break the 1:4 linkages)

Question 9

monosaccharide transport

Answer 9

*fructose enters the enterocyte via facilitated diffusion (GLUT5)
*glucose & galactose utilize secondary active transport (SGLT1) to enter the enterocyte [driven by the electrochemical gradient created by the Na+/K+ ATPase]
*glucose, fructose, & galactose exit the enterocyte via facilitated diffusion (GLUT2)

Question 10

GLUT5 transporter

Answer 10

*uses facilitated diffusion to help fructose enter the enterocyte

Question 11

SGLT1 transporter

Answer 11

*uses secondary active transport (driven by the electrochemical gradient created by the Na+/K+ ATPase) to help glucose & galactose enter the enterocyte

Question 12

GLUT2 transporter

Answer 12

*uses facilitated diffusion to help fructose, glucose, and galactose exit the enterocyte into the interstitial space/blood

Question 13

overview of protein digestion

Answer 13

1. luminal digestion in stomach (gastric pepsin)
2. luminal digestion in small intestine (pancreatic & enterocyte peptidases)
3. brush border digestion
4. intracellular digestion (di- and tri-peptidases)
5. transporters (amino acid specific)

Question 14

why is it more challenging to digest and absorb LIPIDS compared to proteins & carbs?

Answer 14

lipids have a LOW AQUEOUS SOLUBILITY

Question 15

overview of lipid digestion & absorption

Answer 15

1. luminal digestion in stomach (lingual & gastric lipases release some fatty acids; gastric mixing forms emulsion particles)
2. luminal digestion in small intestine (PANCREATIC LIPASE releases large amounts of FA; hydrolysis products form mixed micelles [BILE])
3. brush border - passive diffusion
4. intracellular events (re-esterification to triglycerides & chylomicron assembly/secretion)

Question 16

triglyceride digestion/absorption: gastric emulsification

Answer 16

*lingual and gastric lipase hydrolyze dietary fats, liberating fatty acids, mono- and di-glycerides
*these serve as lipid emulsifying agents

Question 17

triglyceride digestion/absorption: small bowel lumen

Answer 17

*pancreatic lipase-colipase attacks emulsion particle triglycerides
*fatty acids are solubilized in mixed micelles (requires bile acid)

Question 18

triglyceride digestion/absorption: absorption of micellar lipids at the brush border

Answer 18

*mixed micelles enter the membrane into the cytosol of the enterocyte

Question 19

path of carbohydrate absorption (simplified)

Answer 19

*glucose & galactose are absorbed by cotransport with Na+ ions via SGLT1
*fructose is absorbed by facilitated diffusion via GLUT5
*absorbed monosaccharides exit the basolateral side of the enterocyte via GLUT2 to the villus capillary blood, and then are transported to the liver by the portal vein

Question 20

path of protein absorption (simplified)

Answer 20

*amino acids, di-peptides, tri-peptides, and oligo-peptides are absorbed by cotransport with Na+ ions via a family of highly specific brush border transporters
*di-peptides, tri-peptides, and oligo-peptides undergo hydrolysis within the enterocyte
*amino acids exit the basolateral side of the enterocyte to the villus capillary blood, and then are transported to the liver by the portal vein

Question 21

path of lipid absorption (simplified)

Answer 21

*fatty acids and mono-glycerides enter the enterocyte by transmembrane diffusion
*long-chain fatty acids are re-esterified into TG and packaged into chylomicrons in the enterocyte
*chylomicrons exit the basolateral side of the enterocyte via golgi into the LACTEAL of the villi, then into the thoracic duct and finally into venous circulation
*short-chain fatty acids are absorbed directly into the portal circulation to the liver

Question 22

site of iron absorption

Answer 22

*duodenum mostly

note - if duodenum is bypassed, it is difficult to absorb iron

Question 23

site of B12 absorption

Answer 23

*terminal ileum ONLY

note - if terminal ileum is removed/diseased, you cannot absorb B12

Question 24

site of bile salt absorption

Answer 24

*terminal ileum ONLY

note - if terminal ileum is removed/diseased, you cannot absorb bile acids