Biochemical Tests – Practical Skills Ep 2

In A-Level biology, there are four biochemical tests to learn for four different biological molecules. This article gives full details of the tests, and a helpful colour summary at the bottom.

Test for reducing and non-reducing sugars

A sugar is a monosaccharide or disaccharide. The vast majority of sugars learnt about in A-Level biology (glucose, fructose, galactose, maltose, and lactose) are reducing sugars, but sucrose is a non-reducing sugar. The Benedict’s test can be used to test samples for each of these:

  1. Add an excess of Benedict’s reagent to the sample and use a water bath to boil the solution.
  2. If reducing sugars are present, a coloured precipitate will form and the test ends.
  3. If no reducing sugars are present, the solution will stay blue and the test can continue to test for non-reducing sugars.
  4. Add dilute hydrochloric acid and use a water bath to boil. This breaks down non-reducing sugars to monosaccharides (reducing sugars).
  5. Add sodium hydrogencarbonate to neutralise, then repeat step 1.
  6. If non-reducing sugars were present, a coloured precipitate now forms. If the solution still stays blue, there were no non-reducing sugars.

The colour of the precipitate gives an indication of the sugar concentration. The colour change goes from blue to green, yellow, orange, and finally brick red (the highest concentration).

The precipitate can be filtered, dried, and weighed to make more accurate comparisons between samples. It is also possible to use a colorimeter to quantify the concentrations. We will look at colorimetry in a separate article.

Test for starch

This is perhaps the simplest of the biochemical tests. Simply add a few drops of iodine in potassium iodide solution to the sample. If starch is present, the colour will turn from browny-orange to blue-black.

Test for proteins

The Biuret test can be used to look for the presence of proteins. These are the practical steps:

  1. Add a few drops of sodium hydroxide solution to the sample.
  2. Add copper (II) sulfate solution.
  3. If protein is present, the solution turns purple. If no protein is present the solution stays blue.

Test for lipids

The biochemical test for lipids is called the emulsion test. It is carried out as follows:

  1. Mix the sample with ethanol and shake until dissolved.
  2. Pour the mixture into water.
  3. If lipid is present, a milky emulsion forms. If no lipid is present, the mixture stays clear.


Summary of the colour changes

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