Digestion and absorption flashcards

Digestion and absorption of nutrients

  1. understand the processes of digestion and absorption of carbohydrates

CARBOHYDRATE DIGESTION

  • most consumed macromolecule

~ mixture of disaccharides and polysaccharide

~ disaccharides include sucrose, lactose and maltose

~ polysaccharides include starch, glycogen and cellulose

  • only monosaccharides can be absorbed —> so disaccharides and polysaccharides have to be digested into monosaccharides

  • starch, glycogen (—digest to/amylase→) maltose, sucrose, lactose

  • maltose (-maltase→) 2 glucose

  • sucrose (-sucrase→) 1 glucose + 1 fructose

  • lactose (-lactase→) 1 glucose + 1 galactose

CARBOHYDRATE ABSORPTION

  • at apical membrane (upper side)…

~ glucose and galactose are absorbed by secondary active transport

~ cotransported with Na+ through the symporter SGLT

~ fructose is transported by facilitated diffusion via GLUT5

  • at basolateral membrane (lower side)…

~ GLUT2 transports glucose, galactose and fructose

~ Na+/K+ pump maintains sodium gradient which drives the symporter SGLT

  1. understand the processes of digestion and absorption of proteins

PROTEINS

  • proteins that are digested include…

~ dietary proteins

~ secretions into the GI tract lumen

~ cell debris from the GI tract cell lining

  • amino acids, dipeptides and tripeptides can be absorbed

~ produced by proteases

TYPES OF PROTEASES

  1. endopeptidases

:: catalyses breakdown of an internal peptide bond

:: produces small peptide fragments

  1. exopeptidases

:: catalyses the breakdown of terminal peptide bond

:: produces amino acids

PROTEIN DIGESTION

  • pancreatic proteases

~ trypsin (endopeptidase)

~ chymotrypsin (endopeptidase)

~ carboxypeptidase (exopeptidase specific to carboxy terminal COO-)

  • brush border enzymes

~ aminopeptidase (exopeptidase specific to the amino termal NH2)

~ enterokinase (endopeptidase that activates trypsin)

PROTEIN ABSORPTION

  • amino acids are actively transported into the intestinal epithelial cells by cotransport with sodium across the apical membrane

  • 4 different types of carriers

  • dipeptides and tripeptides have different carriers

  • once inside the epithelial cell, dipeptides and tripeptides are further broken down by proteases into amino acids

  • these amino acids then cross the basolateral membrane and are absorbed into the blood

  1. understand the processes of digestion and absorption of fatty acids

LIPID DIGESTION & ABSROPTION

  • special problem for digestion beacuse they are insoluble in water

  • lipases can only act on the surface of fat droplets

  • bile salts increase surface area of droplets by emulsifying larger droplets into smaller droplets

  1. bile salts from liver coat fat droplets

  2. pancreatic lipase and colipase break down fats into monoglycerides and fatty acids stored in micelles

  3. monolgycerides and fatty acids move out of the micelles and enter cells by diffusion

  4. cholesterol is transported into cells by a membrane transporter

  5. absorbed fats combine with cholesterol and proteins in the intestinal cells to form chylomicrons

  6. chylomicrons are released into the lymphatic system

SUMMARY

  • carbohydrates are digested into monosacchoride to be absorbed into the blood

  • protein breakdown products are amino acids, dipeptides and tripeptides

  • lipid digestion and transport is complex and requires bile salts to emulsify fat droplets