Biochemical Tests: Proteins (AQA A Level Biology)

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Biochemical Tests: Proteins

Test for protein

  • A liquid solution of a sample is treated with sodium or potassium hydroxide to make the solution alkaline
  • A few drops of copper (II) sulfate solution (which is blue) is added to the sample
    • Biuret ‘reagent’ contains an alkali and copper (II) sulfate

  • If a colour change is observed from blue to lilac/purple, then protein is present.
    • The colour change can be very subtle, it’s wise to hold the test tubes up against a white tile when making observations)

  • If no colour change is observed, no protein is present
    • For this test to work, there must be at least two peptide bonds present in any protein molecules, so if the sample contains amino acids or dipeptides, the result will be negative

  • The Biuret test is qualitative - it does not give a quantitative value as to the amount of protein present in a sample

The Biuret test for protein, IGCSE & GCSE Biology revision notes

A positive colour change produces a lilac/purple colour.

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Lára

Author: Lára

Expertise: Biology Lead

Lára graduated from Oxford University in Biological Sciences and has now been a science tutor working in the UK for several years. Lára has a particular interest in the area of infectious disease and epidemiology, and enjoys creating original educational materials that develop confidence and facilitate learning.