Digestion and Absorption

Digestion and Absorption

1. Peristalsis

   • contraction of circular and longitudanal small intestine muscles to mix food and enzymes
   • gut muscles
     • smooth
     • short cells
     • continuous moderate force with periods of vigorous contraction
   • circular muscles prevent food from going back
   • longitudanal muscles push the food along the gut
   • unconsciously performed
   • away from mouth
   • one wave down esophagus

2. Pancreatic juice

   • pancreas secrets enzymes into lumen of small intestine
   • pancreas contains 2 types of gland tissue
   • small groups of cells secret glucagon and insulin; rest secret digestive enzymes
   • enteric nervous system = mediation of hormones synthesized and stomach secretions
   • ducts = small group of cells clustered around tubes into which enzymes are secreted
   • sythesized on ribosomes in rER
   • processed by Golgi apparatus and secreted by exocytosis
   • ducts merge into pancreatic duct; 1L of pancreatic juice secreted to lumen per day
   • enzymes
     • amylase for starch
     • lipase for trigylcerides and phospholipids
     • protease for proteins and peptides

3. Digestion in the small intestine

   • enzymes digest macromolecules in food to monomers
   • hydrolysis reactions
     • starch to maltose by amylase
     • triglycerides to fatty acids and glycerol or fatty acids and monoglycerides by lipase
     • phospholipids to fatty acids, gycerol and phosphate by phospholipase
   • proteins and polypeptides to shorter peptides by protease
   • wall of small intestines produce enzymes to digest more
     • DNA and RNA to nucleotides by nucleases
     • maltose to glucose by maltase
     • lactose to glucose and galactose by lactase
     • sucrose to glucose and fructose by sucrase
     • dipeptides to amino acids by dipeptidases
     • peptide to dipeptide by exopeptidases
   • most immobilized enzymes in plasma membrane of epithelium cells; some in intestinal juice
   • takes hours due to length
   • some not synthesized eg. cellulose

4. Villi and the surface area for digestion

   • increases epithelium surface area for increased rate of absorption
   • absorption = taking substances into cells and blood
   • in small intestine (7m long; 25-30mm wide)
   • folds on inner surface for increased surface area
   • villi = small finger like projections of mucosa lining inner wall of intestine
   • villus 0.5-1.5 mm long (40/sq mm)
   • increase surface area 10x

5. Absorption by villi

   • absorbs monomer, vitamins and mineral ions
     • glucose, fructose, galactose, other monosaccharides
     • all 20 amino acids
     • fatty acids, monoglycerides, glycerol
     • bases from digestion of nucleotides
     • calcium, potassium, sodium
     • ascorbic acid (Vit C)
   • epithelium cover: barrier for harmful substances; permeable to allow useful nutrients to enter
   • harmful substances that enter removed from blood, detoxified by liver
   • bacteria removed by phagocytic cells
   • useless substances in urine

6. Methods of absorption

   • nutrients small intestine lumen --> capillaries/lacteals in villi
   • absorbed through plasma membrane of epithelium cells
   • detailed example of triglycerides and glucose in textbook
   • plasma membrane surface area increased by microvilli
   • nutrients exit through plasma membrane facing inward towards capillaries/lacteals