Disaccharides & Polysaccharides
Biology: IB Diploma Higher Level, topic 5.1.2
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
- Glucose + glucose → maltose.
- Glucose + fructose → sucrose.
- Glucose + galactose → lactose.
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 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
- 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
- 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
- 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 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
- 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.
Test yourself on Disaccharides & Polysaccharides
What are the products of the hydrolysis of sucrose?
Which of the following is true of polysaccharides?
What can the Benedict’s test be used to test?
Test for Non-Reducing Sugars
Which of the following describes Benedict's solution?