B3: Organisation and the Digestive System


B3.1: Tissues and Organs

  • a tissue is a group of cells with similar structure and function.
  • organs are collections of tissues performing specific functions.
  • organs are organised into organ systems, which work together to form organisms. 
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B3.2: The Human Digestive System

  • organ systems of organs that perform specific functions in the body.
  • the digestive system in a mammal is an organ system where several organs work together to digest and absorb food.
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B3.3: The Chemistry of Food

  • carbohydrates are made up of units of sugar.
  • simple sugars are carbohydrates that only contain one or two sugar units. they turn blue Benedict's solution brick red on heating. 
  • complex carbohydrates contain long chains of simple sugar molecules bonded together. starch turns yellow-red iodine blue black in its presence. 
  • lipids consist of three molecules of fatty acid bonded to a molecule of glycerol. the ethanol test indicates the presence of a lipid in solutions.
  • protein molecules are made up of long chains of amino acids. Biuret reagent turn blue to purple in the presence of proteins 
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B3.4: Catalysts and Enzymes

  • catalysts increase the rate of chemical reactions without chemically changing.
  • enzymes are biological catalysts and catalyse specific reactions in living organisms due to the shape of their active site. This is the lock and key theory of enzyme action. 
  • enzymes are proteins. the amino acid chains are folded to form the active site and match the shape of a specific substrate molecule.
  • the substrate binds to the active site and the reaction catalysed by the enzyme.
  • metabolism is the sum of all the reactions in a cell or the body. 
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B3.5: Factors Affecting Enzyme Action

  • enzyme activity is affected by temperature and pH.
  • high temperatures denature the enzyme, changing the shape of the active site.
  • pH can affect the shape of the active site of an enzyme and make it work very efficiently or stop it working. 
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B3.6: How the Digestive System Works

  • digestion involves the breakdown of large insoluble molecules into soluble substances that can be absorbed into the blood across the wall of the small intestine.
  • digestive enzymes are produced by specialised cells in glands and in the lining of the digestive system. 
  • carbohydrases such as amylase catalyse the breakdown of carbohydrates into simple sugars.
  • proteases catalyse the breakdown of proteins into amino acids.
  • lipases catalyse the breakdown of lipids into fatty acids and glycerol. 
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B3.7: Making Digestion Efficient

  • the protease enzymes of the stomach work the best in acid conditions. the stomach produces hydrochloric acid which maintains a low pH.
  • the enzymes made in the pancreas and small intestine work best in alkaline conditions. 
  • bile produced in the liver, stored in the gall bladder and released through the bile duct neutralises acid and emulsifies fat. 
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