digestion and absorption

• digestion is when large biological molecules are hydrolysed into smaller molecules that can be absorbed across the cell membrane
• enzymes that hydrolyse carbohydrates are:

  amylase = salivary glands and pancreas

  membrane bound disaccharidases = duodenum/ ileum
• digestion of carbohydrates:

  • MOUTH = [amylase] hydrolyses polysaccharides into disaccharide maltose by hydrolysing glycosidic bonds
  • STOMACH = acidic so denatures amylase prevent anymore hydrolysis
  • SMALL INTESTINE/ PANCREAS = [pancreatic amylase] continuous hydrolysis of any remaining starch into maltose
  • DUODENUM + ILEUM = [membrane bound disaccharidases] hydrolyses disaccharide into monosaccharides (epithelium lining produces the enzyme)

  maltose ---maltase---> alpha glucose + alpha glucose

  sucrose ---sucrase---> glucose + fructose

  lactose ---lactase---> glucose + galactose
• proteins are large polymer molecules that can be hydrolysed by a group of enzymes called peptidases ( endopeptidases, exopeptidases, membrane bound dipeptidases )
• endopeptidases hydrolyses the peptide bond between amino acids in the central region/ middle of the polymer chain
• exopeptidases hydrolyse peptide bonds on the terminal/ ends of amino acid chains (sometimes of those formed by endopeptidases)
• membrane bound dipeptidases hydrolyse peptide bond between dipeptides (2 amino acids)

  (on the epithelial cell lining the ileum
• enzymes that hydrolyse lipids:

  lipase in the pancreas
• protein digestion starts in the stomach and ileum
• digestion of lipids:

  1. [LIPASE] Emulsification of lipids by bile salts = The fats and oils are split up into tiny droplets increasing the surface area of the lipids for faster hydrolysis of lipids (hydrolyse the ester bond) by lipase enzymes (made in pancreas, secreted into small intestine)
  2. [MICELLES] the monoglycerides and fatty acids stick with the bile salts to form tiny structures called micelles. Micelles help the products of lipid digestion to be absorbed.
• there are 2 ways in which digestion of lipids occurs:

  physical digestion = emulsification to form micelles

  chemical digestion = lipase enzyme
• micelles made of fatty acids, monoglycerides and bile salts. They deliver fatty acids and monoglycerides to the epithelium of ileum via diffusion
• the ileum is adapted to absorb products of digestion as:

  • thin walled = short diffusion distance
  • network of capillaries = maintain diffusion gradient
  • villi and microvilli = increase surface area for diffusion
  • contain muscle = maintain diffusion gradient as it mixes contents ensure all products are absorbed
• monosaccharides and amino acids are absorbed via active transport and co-transport alongside sodium ions as usually higher concentration is epithelial cells
• lipid absorption:

  1. micelles encounter ILEUM EPITHELIAL CELLS = diffuse across cell surface membrane due to non polar nature of fatty acids and monoglycerides
  2. ENDOPLASMIC RECTICULUM = modified back into triglycerides
  3. GOLGI APPARATUS = combined with proteins to form chylomicrons
  4. GOLGI VESICLE = chylomicrons released in vesicle where it transports it towards end of epithelial cell and leaves via exocytosis
  5. LACTEAL = chylomicron is absorbed and transported around eventually draining into lymphatic system

  triglycerides in the chylomicrons are hydrolyse by an enzyme where they diffuse into cells