Topic 6 - Human Physiology

• The digestive system is an example of an organ system in which several organs work together to digest and absorb food

  
• Digestion


  A process in which relatively large, insoluble molecules in food (such as starch and proteins) are broken down into smaller, soluble molecules that can be absorbed into the bloodstream and delivered to cells in the body

  
• Components of the human digestive system

  • Organs of the alimentary canal
  • Accessory organs

  
• Alimentary canal


  The channel or passage through which food flows through the body, starting at the mouth and ending at the anus

  
• Digestion occurs within the alimentary canal

  
• Accessory organs


  Produce substances that are needed for digestion to occur (such as enzymes and bile) but food does not pass directly through these organs

  
• Peristalsis


  1. Series of muscle contractions in the walls of the oesophagus or small intestine that pass like a wave along the alimentary canal

  2. Forces the bolus of food along the alimentary canal

  3. Controlled unconsciously by the autonomic nervous system

  
• Peristalsis


  • Controlled by circular and longitudinal smooth muscles
  • Circular muscles contract to reduce the diameter of the lumen
  • Longitudinal muscles contract to reduce the length of that section
• Chyme


  The less solid form of the food mass that continues on to the small intestine after reaching the stomach

  
• Mucus is produced to continually lubricate the food mass and reduce friction
• Pancreas


  A gland made up of two types of tissue: one that secretes hormones into the blood, and one that synthesises and secretes digestive enzymes into the lumen of the small intestine

  
• Secretion of pancreatic enzymes

  1. Enzymes are synthesised on the ribosomes of the rough endoplasmic reticulum, processed in the Golgi apparatus, and then secreted by exocytosis into the lumen of the small intestine

  2. Secretion is stimulated by the release of hormones into the stomach and intestines in response to ingestion of food

  
• Acinar cells


  • Specialised gland cells that synthesise and secrete pancreatic enzymes into the ducts
  • Ducts join together to form the pancreatic duct, which secretes the pancreatic juice into the duodenum of the small intestine

  
• Enzymes


  Required to carry out the hydrolysis reactions needed to digest large insoluble macromolecules into small, soluble, monomers

  
• Enzymes in pancreatic juice

  • Amylase
  • Lipase
  • Phospholipase
  • Protease

  
• Enzymes are also produced in the walls of the small intestine to break down the products of pancreatic enzyme digestion into monomers

  
• Integral proteins


  Enzymes that are immobilised and attached to the membrane of the epithelial cells in the small intestine

  
• Humans are unable to produce the enzymes required to digest cellulose

  
• Digestion of proteins


  1. Pancreatic juice contains endopeptidases and exopeptidases

  2. Endopeptidases hydrolyse peptide bonds within polypeptide chains to produce dipeptides

  3. Exopeptidases hydrolyse peptide bonds at the ends of polypeptide chains to produce dipeptides

  4. Dipeptidase enzymes in the cell surface membrane of epithelial cells hydrolyse dipeptides into amino acids

  
• Emulsification of lipids


  1. Bile (containing bile salts) is secreted and binds to fatty liquid, breaking the fatty droplets into smaller ones

  2. Emulsification increases the surface area of the fatty droplets for action of digestive enzymes

  
• Digestion of lipids


  1. Lipase enzymes break down lipids to glycerol and fatty acids

  2. Lipase enzymes are produced in the pancreas and secreted into the small intestine

  
• Digestion of starch


  1. Amylase hydrolyses the 1,4 glycosidic bonds in amylose and amylopectin, breaking starch down into maltose

  2. Amylase is unable to digest the 1,6 bonds in amylopectin, producing dextrins

  3. Maltase and dextrinase enzymes on the microvilli further digest the products into monosaccharides

  
• Digestion of nucleic acids

  Nucleases break down DNA and RNA into nucleotides by hydrolysing the phosphodiester bonds

  
• The products of digestion travel via the hepatic portal vein into the liver, where excess glucose is stored as glycogen

  
• Villi


  • Finger-like projections on the internal intestinal walls
  • Covered in microvilli, which are foldings of the cell surface membrane of the epithelial cells
  • Significantly increase the surface area of the ileum to maximise absorption

  
• Absorption


  The movement of digested food molecules, vitamins and mineral ions from the digestive system into the blood and lymph

  
• Products of digestion that are absorbed

  • Simple monosaccharides
  • Amino acids
  • Fatty acids, monoglycerides and glycerol
  • Nucleotide bases
  • Mineral ions
  • Vitamins

  
• The epithelium provides a barrier to prevent absorption of some harmful substances, but some can still pass into the blood
• Dialysis tubing can be used to model the epithelium of the small intestine and the absorption of the products of digestion

  
• Small numbers of bacteria are engulfed and digested by phagocytes in the blood
• Some other harmful substances are removed from the blood and broken down by the liver
• Dialysis tubing


  A non-living, partially permeable membrane made from cellulose
• Dialysis tubing


  • It is sometimes used to model the process of digestion and absorption that occurs in the small intestine
  • Pores in the membrane are small enough to prevent the passage of large molecules (such as starch and sucrose) but allow smaller molecules (such as glucose) to pass through by diffusion
  • It can be used to model the epithelium of the small intestine

  
• Dialysis tubing experiment


  1. Fill a section of dialysis tubing with a mixture of amylase solution and starch solution

  2. Suspend the tubing in a beaker of water for a set period of time at 40°C

  3. Take samples from the liquid outside the dialysis tubing at regular intervals and test for the presence of starch and glucose

  
• Iodine test


  Used to test for the presence of starch. A blue-black colour is produced in the presence of starch
• Benedict's reagent


  Used to test for the presence of glucose. An orange-red precipitate is formed in the presence of glucose when Benedict's solution is added and the solution is heated to 90°C or above
• Tube 1 (with amylase)

  1. Amylase breaks down starch into glucose

  2. Glucose diffuses across the partially permeable membrane

  3. No starch should be found in the liquid outside the dialysis tubing

  
• Tube 2 (without amylase)

  1. Starch is not broken down into glucose

  2. No glucose is present in the water outside the dialysis tube

  3. Starch molecules are too large to diffuse across the partially permeable membrane

  
• This test is qualitative, so does not show the rate of enzyme activity
• The rate of digestion can be investigated quantitatively by using the semi-quantitative Benedict's test