Digestion and Lactose Intolerance

Revision cards for AS Biology AQA on Carbohydrates, digestive system, lactose-intolerance, reducing sugars and non-reducing sugars and Starch - including Iodine test.

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  • Most carbohydrates are polymers
  • Contain elements Carbon, Hydrogen and Oxygen
  • Carbohydrates are made from Monosaccharides:

Formula for Glucose, Galactose and Fructose --- C6H12O6

  • Glucose is a hexose sugar (6-Carbon) - this goes onto make larger chains such as Cellulose and Starch
  • Alpha and beta forms of Glucose
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You need to know a-glucose:

This is an alpha-glucose structure:(http://www.science.marshall.edu/murraye/340/glucose.gif)

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Two monosaccharides join together to form a disaccharide.

  • Glucose + Fructose --> Sucrose

(sucrose is just regular table sugar) + hydrolysed down again by Sucrase

  • Glucose + Galactose ---> Lactose

(lactose is the sugar in Milk - you can be intolerant to it) + hydrolysed by Lactase

  • Glucose + Glucose --> Maltose

(Maltose is a product of starch digestion, broken down by the enzyme amylase)

+ hydrolysed to its monosaccharides again by maltase

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Lactose Intolerance

  • You are lactose intolerant if you don't make enough of the enzyme Lactase
  • Lactase hydrolyses Lactose into its monosaccharides - glucose and galactose
  • Undigested lactose is fermented by bacteria in the large intestine and can go onto cause intestinal complaints such as...

- Stomach cramps

- Excessive flatuence due to large volume of gas

- Diarrhoea

... And obviously these are really unpleasant! So, before lactose-intolerant people drink milk, you can add lactase to it to pre-digest it for you.

  • Lactose intolerance is bad for babies because their source of food is milk, so they need special Formula milk to be made for them.
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Benedict's Test for Reducing Sugars

Test for reducing sugars: Reducing sugars = monosaccharides & some dissaccharides such as Maltose.

  • You add Benedict's Reagent - Copper (II) sulfate which is blue
  • You heat the sample
  • If the sample contains reducing sugars such as glucose, galactose, fructose or maltose then the solution should turn brick red. (a red precipitate forms)

Test for non-reducing sugars:

  • You boil the solution in dilute HCL (Hydrochloric Acid) and then neutralise with sodium hydrogencarbonate.
  • You then do the Benedict's Test for the reducing sugars which would now have been hydrolysed into their constituent monosaccharides
  • Then the same result- but compared with another sample of the same with just the reducing sugar test one on its own - so you can prove that it is non-reducing sugars.
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Starch is made up of a mixture of two polysaccharide types - amylose and amylopectin

  • Both are long chains of alpha-glucose
  • Linked by glycosidic bonds
  • Formed in condensation reactions

Starch-> Salivary Amylase and Pancreatic Amylase break it down into maltose

Maltase (from the intestinal epithelium) then breaks down maltose into alpha-glucose molecules

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iodine test

The test for Starch uses iodine dissolved in potassium iodide solution.


When starch is present:

Brown-orange changes to Blue-Black colour.

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Very good for revision as the notes are staright to the point.



The diagram for the alpha-glucose has the -OH group on the wrong side of the carbon-1 atom, it shows beta-glucose, but really good revision notes, clear and concise which I like :)

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