Testing for Biological Molecules (Cambridge (CIE) AS Biology): Flashcards
Exam code: 9700
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Describe the iodine test for starch.
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Describe the iodine test for starch.
Add iodine solution (iodine in potassium iodide) to the sample.
A positive result is a colour change from orange-brown to blue-black.
Describe the emulsion test for lipids.
Mix the sample with ethanol, then add an equal volume of water.
A positive result is a white (milky) emulsion.
Describe the biuret test for proteins.
Add biuret reagent to the sample.
If protein (peptide bonds) is present, the colour changes from blue to purple/violet.
Describe the Benedict's test for reducing sugars.
The Benedict's test: add Benedict's solution and heat.
A positive result is a colour change from blue to a brick-red precipitate.
The test is used to identify the presence of lipids.
The emulsion test is used to identify the presence of lipids.
True or False?
The biuret test turns purple when protein is present.
True.
Biuret reagent goes from blue to purple/violet when peptide bonds (protein) are present.
How can the Benedict's test be made semi-quantitative?
Standardise the test, then either time how long it takes for the first colour change, or compare the final colour against colour standards.
This gives an estimate of the concentration of reducing sugar.
A positive Benedict's test gives a precipitate.
A positive Benedict's test gives a brick-red precipitate.
How do you carry out the Benedict's test for reducing sugars?
Add Benedict's solution to the sample and heat in a water bath.
If a reducing sugar is present, a coloured precipitate forms.
What range of colours can the Benedict's test produce, from low to high concentration of reducing sugar?
Blue (none) → green → yellow → orange → brick-red (high concentration).
How can the Benedict's test be made semi-quantitative?
Standardise the test (same volume, concentration, temperature and time).
Then estimate the concentration using the time to first colour change, or by comparing to colour standards.
A reducing sugar produces a precipitate with Benedict's solution.
A reducing sugar produces a brick-red precipitate with Benedict's solution.
True or False?
A green colour with Benedict's solution indicates a higher concentration of reducing sugar than an orange colour.
False.
Orange (and brick-red) indicate a higher concentration than green.
How do you test for a non-reducing sugar?
Do a Benedict's test first — it stays blue (negative).
Boil a fresh sample with dilute acid (hydrolysis), then neutralise with alkali.
Repeat the Benedict's test — a brick-red result confirms a non-reducing sugar.
Why does acid hydrolysis allow a non-reducing sugar to give a positive Benedict's result?
The acid hydrolyses the glycosidic bond, releasing reducing monosaccharides.
These then react with Benedict's solution.
Why must the solution be neutralised before the second Benedict's test?
Benedict's solution only works in alkaline conditions.
The acid used for hydrolysis must be neutralised or the test will not work.
A non-reducing sugar must be broken down by acid before it will react with Benedict's solution.
A non-reducing sugar must be broken down by acid hydrolysis before it will react with Benedict's solution.
True or False?
A non-reducing sugar gives a positive Benedict's result without any further treatment.
False.
It must first be hydrolysed (with acid, then neutralised) before it will react with Benedict's solution.
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