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  2. 2. Digestion is the process by which  insoluble food, consisting of large molecules is broken down into soluble compounds
  3. 3. What are starch, proteins & fats broken down into? enzyme STARCH Glucose enzyme PROTEIN Amino acids enzyme FAT Fatty acids & glycerol
  4. 4. Why must food be digested? 1 Food in intestine 2 Blood Food in intestine enzyme Blood Wall of intestine 3 Food in intestine Blood To be small enough to be absorbed through the wall of intestine.
  5. 5. Digestion starts at the mouth and ends at the anus ……………….
  6. 6. Food is broken down by two actions: 1. PHYSICAL /MECHANICAL 2. CHEMICAL
  7. 7. 1. PHYSICAL DIGESTION by:  teeth  peristalsis of the alimentary canal Chewing (mastication)
  8. 8. Why is it important to break down food physically? To increase the surface area for enzyme action.
  9. 9. Question: SEP, 2009 Give TWO reasons why food must be masticated before swallowed. (4) 1. Increased surface area for enzymes to act quickly. 2. Easier to manipulate by tongue to form a bolus.
  10. 10. 2. CHEMICAL DIGESTION by:  enzymes Digestive enzymes are the chemicals that break large insoluble food molecules into smaller soluble molecules.
  11. 11. Four stages in feeding & digestion: 2. Digestion 3. Absorption into the bloodstream 4. Egestion 1. Ingestion
  12. 12. Digested food is absorbed & then assimilated by the cells Absorption Assimilation Digestive system Body cell Circulatory system
  13. 13. Digestive tract is also called:  Alimentary canal / tract  Gut
  14. 14. Mucus is produced throughout the gut:  For lubrication  To protect gut wall from digestive enzymes
  15. 15. Fig. 1 The human digestive system.
  16. 16. The 1st stage of nutrition: 1. Ingestion 2. Digestion 3. Absorption 4. Egestion
  18. 18.  salivary glands produce SALIVA (pH = 7-8)
  19. 19. Salivation is a reflex action which is triggered by:  Sight  Smell  Thinking of food
  20. 20. Saliva contains:  mucus  water  sodium hydrogen carbonate  salivary amylase (ptyalin [old term])
  21. 21. Functions of mucus in saliva: 1. lubricates the food 2. sticks the food particles together bolus
  22. 22. Salivary amylase
  23. 23. In swallowing, food is directed into the gullet / oesophagus
  24. 24. A person cannot breathe while swallowing. Why? 1. The soft palate closes the nasal cavity . 2. The epiglottis closes the trachea.
  25. 25. Swallowing
  26. 26. Peristalsis:  wave-like motions of the alimentary canal which move the food forwards Digestive tract Bolus 1 Wave of relaxation Bolus moves 2 2 Wave of contraction
  27. 27. Peristalsis:
  28. 28. Peristalsis is possible as gut wall is muscular  contraction of circular & longitudinal muscles alternate to push food
  29. 29. Food is pushed when: Circular muscle above bolus CONTRACTS What does the longitudinal muscle do at the same time? Relaxes
  30. 30. Food reaches the stomach
  31. 31. The stomach stores food for some time & releases it slowly  Food stays for 2-6 hours in the stomach Chyme: creamy liquid leaving the stomach
  32. 32. Food remains inside stomach due to contraction of two sphincters Oesophagus Cardiac sphincter Pyloric sphincter Lining of mucus Duodenum Churning due to: Layers of muscle
  33. 33. Which sphincter is contracted? Cardiac sphincter: Pyloric sphincter: relaxes to let chyme into the duodenum contracted
  34. 34. Gastric glands secrete gastric juice
  35. 35. What happens to food inside the stomach?
  36. 36. Gastric juice contains: 1. HYDROCHLORIC ACID 2. PEPSIN 3. RENNIN What is the pH in the stomach? 1-2
  37. 37. Functions of hydrochloric acid: 1. creates the optimum pH for pepsin HCl Pepsinogen 2. (inactive form) Pepsin (active enzyme) 3. kills bacteria taken in with the food
  38. 38. Excessive production of gastric juice: burning sensation
  39. 39. Gastric juice enters oesophagus: not enough mucus to protect wall
  40. 40. Remedy for burning sensation in throat and chest: 1. Drink milk 2. Take antacids
  41. 41. Doctors can see the stomach lining by using an: oesophagus endoscope
  42. 42. Stomach ulcers due to: little mucus in stomach
  43. 43. Pepsin is a protease:  breaks: Proteins Polypeptides
  44. 44. Which conditions are needed for a piece of meat to be digested?  Pepsin in acidic conditions
  45. 45. Question: MAY,2009 The following diagram represents the protein found in egg white. Each circle represents an amino acid. Complete the diagram below to show how pepsin breaks down the protein found in egg white. [Idea is that short chains representing polypeptides are made.] (Do NOT accept individual amino acids represented as single circles or dipeptides represented as a pair of circles)
  46. 46. Question: MAY, 2012 List TWO reasons why amylase does not act on starch in the stomach. (2) i. pH in stomach is not optimum for amylase ii. there is no enzyme in the stomach to break starch
  47. 47. Some substances that are absorbed by the stomach:      ethanol vitamins certain salts and drugs some water glucose How did I get drunk so quickly?
  48. 48. Rennin  is an enzyme found in the stomach of young children  Function: Pepsin then acts on the insoluble protein Soluble protein in milk Changes into insoluble protein
  49. 49. In cheese-making, rennet [contains rennin] is added to milk
  50. 50. THE INTESTINES: Small intestine 7 m long Large intestine 1.5 m long
  51. 51. The small intestine is composed of: duodenum & ileum
  52. 52. Two functions of the small intestine: 1. digestion is completed 2. digested food is absorbed Bloodstream Products of digestion
  53. 53. Duodenum  U-shaped  first part of small intestine (most digestion occurs here)  about 25 cm long
  54. 54. The duodenum receives secretions from the: Liver [bile via the bile duct] Pancreas [pancreatic juice via the pancreatic duct]
  56. 56. Where is bile: i) made? ii)stored?
  57. 57. Made in liver Stored in gall bladder
  58. 58. Bile:  has no enzymes  consists of:  water  sodium chloride  bile salts  bile pigments  pH = 8
  59. 59. Functions of Bile:  dilutes contents from stomach  creates optimum pH for pancreatic enzymes to work in duodenum  contains bile salts which emulsify fats Bile salts
  60. 60. Bile salts emulsify fats i.e.:  break large fat globules into smaller globules What is the benefit of emulsification? The surface area where lipase can act is increased. Emulsification of fats
  61. 61. A person had his gall bladder removed. Explain why the doctor told him to limit fats in the diet. Fats cannot be emulsified; difficult to digest fats by lipase.
  62. 62. Three enzymes in pancreatic juice: Pancreas Pancreatic amylase Trypsin Lipase Action of each enzyme:
  63. 63. amylase maltose starch molecule trypsin proteins & polypeptides peptides lipase fat molecule + glycerol fatty acids
  64. 64. Question: MAY, 2012 Bile and pancreatic juice are involved in digestion of fats. a) Describe the role of both secretions in this process. (4) b) Explain why bile must be released before pancreatic juice for the efficient digestion of fats. (2)
  66. 66. The wall of the ileum has intestinal glands that produce intestinal juice
  67. 67. Enzymes in intestinal juice SUBSTRATE ENZYME END PRODUCT Peptides Lipids Peptidase Lipase Maltose Sucrose Lactose Maltase Sucrase Lactase Amino acids Fatty acids & glycerol Glucose + glucose Glucose + fructose Glucose + galactose
  68. 68. Remember: Proteins Pepsin Polypeptides Trypsin Peptides Peptidase Amino acids
  69. 69. Remember: Digestion of starch Starch Salivary Amylase (In Mouth) Maltose No digestion of starch in stomach (amylase is denatured) Maltose (in ileum) Maltase Glucose Pancreatic Amylase
  70. 70. Digestion is complete in the ileum. What happens to the digested food?
  71. 71. Model of the ileum showing numerous villi Villus: a finger-like projection
  72. 72. Villi contain blood & lymph vessels
  73. 73. Structure of a villus
  74. 74. A villus is adapted for absorption
  75. 75. The ileum is adapted to absorb digested food  by providing a LARGE surface area:  Folded walls  Has villi  Has microvilli
  76. 76. Ileum is adapted for absorption: 1. Ileum is long:  more time for digestion  a greater surface area for absorption 2. Villi:  increase surface area for absorption 3. Microvilli in epithelium:  increase surface area for absorption 4. Epithelium is very thin:  soluble products of digestion pass through quickly 5. Dense capillary network:  removes the digested food
  78. 78. The large intestine is composed of: 4. Colon 1.Caecum 2. Appendix 3. Rectum Anus
  79. 79. Material in the large intestine consists of:     water mucus dead cells undigested matter Most of the undigested matter is cellulose. Why is this so?
  80. 80. Function of the: Colon: absorbs water Rectum: stores FAECES before being expelled through the ANUS
  81. 81. The 4th stage of feeding & digestion: 4. Egesting
  82. 82. I really have to go to the bathroom! Egestion / Defaecation  the process of removing faeces
  83. 83. Caecum & Appendix are  Vestigial in humans: [small & no function] Caecum Appendix  Large in herbivores
  84. 84. Question: SEP, 2011 Explain why the caecum / appendix in humans is small and non-functional. (4) Human Rabbit Humans are omnivores. Do not depend only upon vegetation for nutrients. A large caecum is found in herbivores and is full of bacteria that produce cellulase to digest cellulose.
  85. 85. Ingestion (2 L) Salivary gland secretions (1 L) Gastric secretions (2 L) Bile (0.7 L) Small intestine secretions (2 L) 1% in faeces Pancreatic secretions (1.2 L) Although 8.9 L of water are ingested or secreted daily, the faeces are dry. Explain.
  86. 86. Ingestion (2 L) Salivary gland secretions (1 L) Gastric secretions (2 L) Bile (0.7 L) Small intestine secretions (2 L) 1% in faeces Pancreatic secretions (1.2 L) Absorbed in the small intestine & colon.
  87. 87. ASSIMILATION OF DIGESTED FOOD FOOD USE STORE Glucose Respiration Glycogen or Fat Fats Respiration, Fat to build cell structure, as a store of energy Build proteins for cell CANNOT BE STORED – ARE DEAMINATED structures, enzymes & USED IN RESPIRATION Amino acids
  88. 88. Deamination occurs in the liver AMINO ACID is broken into two: Part with nitrogen forms: AMMONIA UREA The rest is used for energy
  89. 89. Question: MAY, 2007 Write the correct term for each of the following processes: i) the breakdown of excess amino acids Deamination ii) rhythmical muscular contractions that push food along the digestive system Peristalsis
  90. 90. iii) the elimination of indigestible food from the body Egestion / Defaecation iv) the passage of digested food through the gut wall in the blood stream. Absorption
  92. 92. Blood supply to and away from the liver HEART HEPATIC VEIN to heart HEPATIC PORTAL VEIN Blood from digestive system LIVER HEPATIC ARTERY GUT Oxygenated blood from heart
  93. 93. Write an account of a controlled experiment you would perform to show the action of the enzyme amylase on starch. (6) [MAY, 1997] 1 Two spotting tiles are prepared with a drop of iodine solution in each hole. 2 An equal volume of starch is placed into two separate test tubes, labelled A and B.
  94. 94. 3 An equal volume of amylase is placed into two separate test tubes, labelled C and D. Test tube C is boiled for 5 minutes to denature the enzyme and so act as a control. 4 All four test tubes are placed in a water bath at 37C for 5 minutes to acclimatise.
  95. 95. Starch in test tube A is poured into amylase in test tube D, mixed and the stop watch is started. 5 6 After 30 seconds, a drop of the mixture is taken and added to one of the drops of iodine solution on the spotting tile. This is repeated until a yellow colour appears.
  96. 96. 7 8 Starch in test tube B is poured into amylase in test tube C, mixed and the same method is repeated. Contents in test tube: i) D give a yellow colour with iodine solution after a few minutes ii) C continues to give a blueblack colour after 30 minutes.
  97. 97. 9 10 The results show that amylase breaks down starch. If Benedict’s solution is added to the contents of test tubes C and D and heated for a few minutes, a brick red colour is obtained only in test tube D [unboiled amylase]. This shows that when starch breaks down, reducing sugars are formed. C D
  98. 98. THE END