5.1.2

Disaccharides & Polysaccharides

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Disaccharides and Polysaccharides

When two monosaccharides join via a condensation reaction, they form a disaccharide. When more than two monosaccharides join together, they form a polysaccharide chain.

Examples of disaccharides

Examples of disaccharides

  • Glucose + glucose → maltose.
  • Glucose + fructose → sucrose.
  • Glucose + galactose → lactose.
Functions of disaccharides

Functions of disaccharides

  • Sucrose is common table sugar.
  • Lactose is the sugar found in milk.
    • Lactose intolerance is a common problem where the body is unable to digest lactose.
Polysaccharides

Polysaccharides

  • Polysaccharides are made up of three or more monosaccharides joined together by glycosidic bonds.
  • The chain may be branched or unbranched.
  • The chain may contain different types of monosaccharides.
  • Starch, glycogen, cellulose and chitin are examples of polysaccharides.

Benedict's Test for Sugars

Benedict’s solution (also known as Benedict's reagent or the Benedict’s test) can be used as a test for reducing and non-reducing sugars.

Reducing sugars

Reducing sugars

  • All monosaccharides are reducing sugars.
    • E.g. Glucose, galactose and fructose.
  • Some disaccharides are reducing sugars.
    • E.g. Lactose and maltose.
Test for reducing sugars

Test for reducing sugars

  • Benedict's solution can be reduced by reducing sugars.
  • Benedict's solution is a clear blue liquid that changes colour and gives a precipitate depending on how much it is reduced.
    • Step 1: Place 2 ml of the substance in a boiling tube (substance must be in liquid form).
    • Step 2: Add 10 drops of Benedict's solution.
    • Step 3: Place in a boiling water bath for 3-5 minutes.
Results of the Benedict's test

Results of the Benedict's test

  • Blue solution → no reducing sugar.
  • Green/yellow precipitate → traces of reducing sugar.
  • Orange/red precipitate → moderate amounts of reducing sugar.
  • Brick red precipitate → large amount of reducing sugar.
Non-reducing sugars

Non-reducing sugars

  • Non-reducing sugars will show a negative result to the Benedict’s test. A second test is needed to determine if non-reducing sugar is present.
  • Sucrose is a non-reducing sugar. It is a disaccharide made up of glucose and fructose joined by a glycosidic bond.
Test for non-reducing sugars

Test for non-reducing sugars

  • Step 1: Boil in dilute HCl (to hydrolyse the non-reducing sugar).
  • Step 2: Neutralise the solution by adding sodium hydrogen carbonate.
  • Step 3: Repeat the Benedict’s test.
    • The result will now be positive if a non-reducing sugar is present.
    • If the solution remains blue, then no sugar is present.
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Practice questions on Disaccharides & Polysaccharides

Can you answer these? Test yourself with free interactive practice on Seneca — used by over 10 million students.

  1. 1
    What are the products of the hydrolysis of sucrose?Multiple choice
  2. 2
    Which of the following is true of polysaccharides?True / false
  3. 3
    What can the Benedict’s test be used to test?Fill in the list
  4. 4
    Test for Non-Reducing SugarsPut in order
  5. 5
    Which of the following describes Benedict's solution?Multiple choice
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Monosaccharides

Triglycerides & Phospholipids