Biological Molecules (Testing)

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Reducing Sugars

A Reducing Sugar is a sugar that is able to act as a reducing agent (an electron donor, so is oxidised). This is because they have a free aldehyde or ketone group.

This includes:

  • All monosaccharides
  • The disaccharides - Maltose and Lactose
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Reducing Sugars

Benedict's Reagent is used to test for Reducing Sugars. It is composed of Copper Sulphate in Aqueous Alkali.

Testing:

  • Add 3cm cubed of sample into a test tube. (If the sample is not already a liquid, grind it up with water).
  • Add an equal volume of Benedict's Reagent.
  • Heat the solution for 5 minutes in a gently boiling water bath.

Positive:

The solution forms an insoluble orange/brick red precipitate of copper (II) oxide.

Negative:

The solution stays a deep blue colour.

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Non-Reducing Sugars

A Non-Reducing Sugar is a sugar that is not able to act as a reducing agent as it cannot be oxidised.

This includes:

  • The disaccharide - Sucrose

To detect a non- reducing sugar, it must first be hydrolyzed into its monosaccharide components by hydrolysis.

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Non-Reducing Sugars

Benedict's Reagent is also used to test for Non-Reducing Sugars. It is composed of Copper Sulphate in Aqueous Alkali.

Testing:

  • The sample must be in liquid form.
  • Add 3cm cubed of sample into a test tube and add an equal volume of dilute HCl.
  • Heat the solution for 5 minutes in a gently boiling water bath.
  • Slowly add 3cm cubed of sodium hydrogen carbonate to the test tube to neutralise the HCl. (Test with pH paper to make sure it is alkaline).
  • Add 3cm cubed of Benedict's Reagent to test tube and heat tube for 5 minutes in boiling water.

Positive:

The solution forms an insoluble orange/brick red precipitate of copper (II) oxide.

Negative:

The solution stays a deep blue colour.

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Starch

Testing:

  • Put 2cm cubed of sample into a test tube.
  • Add 2 drops of iodine in potassium iodide solution and shake.

Positive:

Colour change from yellow- orange to blue- black.

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Lipids Emulsion Test

Testing:

  • Take a dry, clean test tube and put 2cm cubed of the sample in it.
  • Add 5cm cubed of ethanol and put a bung on top.
  • Shake the test tube thoroughly to dissolve the lipid in the sample.
  • Add 5cm cubed of water to the sample and shake gently.

Positive:

If a lipid is present a cloudy/milky white colour appears.

Negative:

The solution stays clear and colourless.

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Proteins

The Biuret Test is used to detect the presence of peptide bonds.

Testing:

  • Add 2cm cubed of the sample to a test tube.
  • Add 2cm cubed of Sodium hydroxide (NaOH) to the sample and gently shake.
  • Add 2cm cubed of copper (II) sulphate to the sample then gently shake.

Positive: 

The sample should form a purple/lilac layer on top.

Negative:

The sample  should remain a blue colour.

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