Digestion and absorption of nutrients

Carbohydrates

  • most consumed macromolecules

  • mixture of disaccharides and polysaccharides

  • only monosaccharides can be absorbed

  • disaccharides and polysaccharides needs to be digested before being absorbed

  • amylase breaks down starch and glycogen into maltose, sucrose and lactose

  • maltose is broken down by maltase

  • sucrose is broken down by sucrase

  • lactose is broken down by lactase

monosaccharide absorption

At apical membrane

  • glucose and galactose absorbed by secondary active transport

  • cotransport with Na+ through symporter SGLT

  • fructose is transported by facilitated diffusion via GLUT5

basolateral membrane

  • GLUT2 transports glucose, galactose and fructose

  • Na+/K+ pump maintains sodium gradient- driving the symporter SGLT

proteins

  • digested proteins include dietary proteins, secretions into the GI tract lumen, cell debris from the GI tract lining

  • Amino acids, dipeptides and tripeptides can be absorber- produced by proteases

  • endopeptidases- type of protease the catalyses the breakdown of an internal peptide bond- producing small peptide fragments

  • exopeptidases- catalyses the break down of end termina peptide bond- producing amino acids

  • produced by pancreas- trypsin, chymotrypsin (endopeptidases), carboxypeptidases (exopeptidases carboxy terminal)

  • brush border enzymes- aminopeptidase (exopeptidase at amino terminal), enterokinase (endopeptidase activating trypsin)

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

  • 4 different types of carriers

  • dipeptides and tripeptides have different carriers

  • once in epithelial cells they are further broken down by proteases into amino acids which are then absorbed into the blood

lipids

  • insoluble in water

  • lipases can only act on the surface of fat droplets

  • bile salts increase the surface area of droplets by emulsifying large droplets into smaller droplets.

  • bile salts from liver coat fat droplets

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

  • monoglycerids and fatty acids move out of micelles and enter cells by diffusion

  • cholesterol is transported into cells by a membrane transporter

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

  • chylomicrons are released into the lymphatic system