Carbohydrate Digestion

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  • food in the mouth is broken down by teeth, giving a large surface area 
  • saliva containing amylase is realeased from the salivary glands 
  • this hydrolyses the glycosidic bonds in starch and forms maltose 
  • the stomach pH (acidic) then denatures the amylase enzyme 
  • more amylase is realeased from the pancreas
  • in the small intestine maltase is realeased from the epithelial lining
  • this hydrolyses the maltose into alpha gluclose 
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Sucrase and Lactase


  • the epithelial lining of the small intestine releases sucrase 
  • this hydrolyses the glycosidic bond in sucrose 
  • this forms the monosaccharides glucose and fructose 


  • the epithelial lining of the small intestine releases lactase 
  • this hydrolyses the glycosidic bond in lactose
  • this forms the monosaccharides glucose and galactose 
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  • Monosaccharides are the basic molecules carbohydrates and proteins are made of 
  • Carbohydrates and proteins are polymers

The structure of alpha glucose is: 

  • alpha glucose is linked by glycosidic bonds formed by condensation to make maltose and starch
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Lactose Intolerance

  • the production of lactase reduces in adult life 
  • in some people it reduces too much and they may not produce any at all 
  • this means they do not produce enough lactase to digest lactose 
  • when undigested lactose reaches the large intestine it is broken down by microorganisms 
  • this creates a large volume of gas, leading to nausea, diarrhoea and cramps 
  • it is not life threatening but can lead to insufficient calcium in the diet 
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Tests for reducing sugars

Benedicts Test; for reducing sugars: 

  • add Benedicts reagent (copper sulphate) to a food sample in liquid form 
  • heat the mixture in boiling water for 5 minutes 
  • if the solution turns orange, then there is a reducing sugar present 

and non-reducing sugars: 

  • if the sloution does not turn orange there is no reducing sugar present 
  • add the food sample to hydrochloric acid and place back in the water bath for 5 minutes 
  • then add sodium hydrogencarbonate to neutralise the acid 
  • re test the solution with Benedicts reagent in a water bath for 5 minutes 
  • if a non reducing sugar was present the solution will now turn orange 
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Test for Starch

Iodine Test: 

  • easy to detect
  • can be doen in a test tube or in a spotting tile 
  • add two drops of iodine solution to the food sample 
  • the presence of starch is indicated by the iodine turning black/blue 
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