Nutrient Absorption Flashcards
absorption of macronutrients occurs at what site in the GI tract
small intestine
exocrine salivary enzymes
*salivary amylase
*lingual lipase
exocrine gastric enzymes
*pepsin
*gastric lipase
note - these are the ONLY enzymes that function at low pH (acidic pH of the stomach)
exocrine pancreatic enzymes
*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
enterocyte membrane enzymes
*enterokinase
*DISACCHARIDASES (maltase, sucrase, lactase, etc)
*peptidases
note - the enterocytes are the only source of disaccharidases
digestion of disaccharides
*BRUSH BORDER digestion via disaccharidases splits disaccharides into monosaccharides
1) lactase
2) sucrase
3) maltase
digestion of starches
*LUMINAL DIGESTION to break 1:4 linkages via:
1) salivary & pancreatic alpha-amylase
2) enterocyte glucoamylase
digestion of glycogen
*enterocyte alpha-dextrinase breaks 1:6 linkages of glycogen (in addition to the amylases, which break the 1:4 linkages)
monosaccharide transport
*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)
GLUT5 transporter
*uses facilitated diffusion to help fructose enter the enterocyte
SGLT1 transporter
*uses secondary active transport (driven by the electrochemical gradient created by the Na+/K+ ATPase) to help glucose & galactose enter the enterocyte
GLUT2 transporter
*uses facilitated diffusion to help fructose, glucose, and galactose exit the enterocyte into the interstitial space/blood
overview of protein digestion
- luminal digestion in stomach (gastric pepsin)
- luminal digestion in small intestine (pancreatic & enterocyte peptidases)
- brush border digestion
- intracellular digestion (di- and tri-peptidases)
- transporters (amino acid specific)
why is it more challenging to digest and absorb LIPIDS compared to proteins & carbs?
lipids have a LOW AQUEOUS SOLUBILITY
overview of lipid digestion & absorption
- luminal digestion in stomach (lingual & gastric lipases release some fatty acids; gastric mixing forms emulsion particles)
- luminal digestion in small intestine (PANCREATIC LIPASE releases large amounts of FA; hydrolysis products form mixed micelles [BILE])
- brush border - passive diffusion
- intracellular events (re-esterification to triglycerides & chylomicron assembly/secretion)