Nutrient Absorption Flashcards

1
Q

absorption of macronutrients occurs at what site in the GI tract

A

small intestine

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2
Q

exocrine salivary enzymes

A

*salivary amylase
*lingual lipase

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3
Q

exocrine gastric enzymes

A

*pepsin
*gastric lipase

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

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4
Q

exocrine pancreatic enzymes

A

*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

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5
Q

enterocyte membrane enzymes

A

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

note - the enterocytes are the only source of disaccharidases

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6
Q

digestion of disaccharides

A

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

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7
Q

digestion of starches

A

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

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8
Q

digestion of glycogen

A

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

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9
Q

monosaccharide transport

A

*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)

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10
Q

GLUT5 transporter

A

*uses facilitated diffusion to help fructose enter the enterocyte

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11
Q

SGLT1 transporter

A

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

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12
Q

GLUT2 transporter

A

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

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13
Q

overview of protein digestion

A
  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)
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14
Q

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

A

lipids have a LOW AQUEOUS SOLUBILITY

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15
Q

overview of lipid digestion & absorption

A
  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)
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16
Q

triglyceride digestion/absorption: gastric emulsification

A

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

17
Q

triglyceride digestion/absorption: small bowel lumen

A

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

18
Q

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

A

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

19
Q

path of carbohydrate absorption (simplified)

A

*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

20
Q

path of protein absorption (simplified)

A

*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

21
Q

path of lipid absorption (simplified)

A

*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

22
Q

site of iron absorption

A

*duodenum mostly

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

23
Q

site of B12 absorption

A

*terminal ileum ONLY

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

24
Q

site of bile salt absorption

A

*terminal ileum ONLY

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