(3.3) Digestion & Absoprtion

Digestion

process in which large molecules are hydrolysed by enzymes into small molecules which can be absorbed & assimilated

Physical Breakdown

if food is large, broken into smaller pieces via teeth - makes it possible to ingest & increases surface area for chemical digestion

muscles in stomach wall contract to churn it, physically breaking it up

Chemical Digestion

hydrolyses large insoluble molecules -> small soluble molecules

carbohydrases = carbohydrates -> monosaccharides

via enzymes

lipases = lipids -> glycerol & fatty acids

proteases = proteins -> amino acids

Carbohydrate Digestion

amylase produced in mouth & pancreas

hydrolyses alternate glycosidic bonds -> maltose

maltase produced in ileum lining

hydrolyses maltose -> alpha glucose

saliva contains salivary amylase hydrolyses starch in mouth -> maltose. contains mineral salts (maintains pH neutral) food enters stomach & acid denatures amylase. food enters small intestine mixes with pancreatic juice. contains pancreatic amylase hydrolyse remaining starch -> maltose. food pushed along ileum, maltase on CSM of epithelial cells, hydrolyses maltose -> alpha glucose

sucrase hydrolyses single glycosidic bond in sucrose -> glucose & fructose

lactase hydrolyses single glycosidic bond in lactose -> glucose & galactose

Lipid Digestion

lipases produced in pancreas & hydrolyse ester bonds

triglycerides -> glycerol & fatty acids

emulsification = lipids split up into micelle droplets by bile salts (produced by liver)

increases lipids SA to speed up lapses

Protein Digestion

peptidases hydrolyse large, complex molecules

endopeptidases = hydrolyse peptide bonds between amino acids in central region of protein -> peptide molecules

exopeptidases = hydrolyse peptide bonds on terminal amino acids of peptide molecules formed by endopeptidases -> dipeptides & amino acids

dipeptidases = hydrolyse bond between 2 amino acids (membrane bound to CSM of ileum's epithelial cells)

Structure of the Ileum

villi = foldings of ileum wall, thin walls & network of blood capillaries (increase SA of ileum & increase absorption rate)

Adaptations of villi

situated between lumen of intestines & blood/tissues of body

increase SA for diffusion

thin walls = reduce diffusion distance

contain muscle & can move = as products are absorbed, new material replaces it (increases efficiency)

well supplied with blood vessels = blood carries away absorbed molecules, maintains diffusion gradient

maintains diffusion gradients

microvilli = finger like projections on CSM of epithelial cells (further increase SA)

Absorption

Amino acids & Carbohydrates

  1. Na+ out of epithelial cells (active transport) by NaK Pump into blood. takes place in protein carrier molecule in CSM of epithelial cells
  1. maintains higher conc Na+ in lumen of intestine than inside epithelial cells
  1. Na+ diffuse into epithelial cells down this conc gradient by co-transport protein in CSM. Na+ carry either amino acid/glucose into cell with them
  1. amino acid/glucose pass into blood plasma by facilitated diffusion using another carrier

amino acids/glucose move against their own conc gradient using Na+ conc gradient = indirect form of active transport

Triglycerides

  1. monoglycerides & FA still associated with bile salts from digestion = micelles form
  1. micelles come into contact with epithelial cells of villi, break down & release monoglycerides & FA, they're non-polar & diffuse across CSM -> epithelial cells
  1. monoglycerides & FA epithelial cells -> endoplasmic reticulum (form triglycerides)
  1. triglycerides endoplasmic reticulum -> Golgi apparatus, associate with cholesterol & lipoproteins -> form chylomicrons

chylomicrons = particles adapted for lipid transport

  1. chylomicrons mover epithelial cells -> lymphatic capillaries (lacteals) via exocytosis -> bloodstream -> cells