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CIE AS Biology (9700) 2019-2021 Revision Notes

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2. Biological Molecules

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The Emulsion test for lipids forms a milky colour

The biuret test for proteins

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

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