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Biochemical Tests

Aspects of Biochemistry

QuestionAnswer
Name the test used to detect proteins. Biuret test
What does the Biuret test detect? The presence of peptide bonds/links. A coloured complex forms where there a peptide bonds.
What would you observe in a positive test for protein? Change in colour of Biuret from blue to violet/purple/lilac.
What would you observe in a negative test for protein? No apparent change. Biuret solution remains blue.
Name the test used to detect lipids. Emulsion test
What is the basis of the emulsion test? Lipids are insoluble in water but soluble in ethanol.
What would you observe in a positive test for lipids? White cloudiness - an emulsion.
What would you observe in a negative test for lipids? No apparent change. Mixture remains colourless.
Name the test used to detect starch. Iodine test.
What would you observe in a positive test for starch? Iodine solution changes from yellow-orange to blue-black.
What is the basis of the iodine test? Iodine binds to the centre helix of amylose to form starch-iodine complex which has a blue black colour.
What would you observe in a negative test for starch? No apparent change. Iodine solution remains yellow-orange.
Name the test used to detect reducing sugars. Benedict's solution
What would you observe in a positive test for reducing sugar? Benedict's solution changes from blue to green/yellow/orange/red with precipitate.
What is the basis of the Benedict's test? Sugar reduces Cu2+ ions to Cu+ ions.
What would you expect to observe in a negative test for reducing sugar? No apparent reaction. Benedict's solution remains blue.
Why is Benedict's test for reducing sugars considered a semi-quantitative test? The colour change (green to red) gives an indication of the amount of reducing sugar present in the substance.
What is the difference between a qualitative test and a quantitative test? Qualitative - tells you whether or not something is present. Quantitative - tells you how much of something is present.
What test is used to detect a non-reducing sugar? An initial negative test with Benedict's solution followed by hydrolyzing the sugar to make it into a reducing sugar then testing again with Benedict's solution.
How can a non-reducing sugar be converted into a reducing sugar? Add dilute hydrochloric acid and heat. Add some sodium hydrogen carbonate afterwards to neutralize then test.
What would you expect to observe for a positive test for non-reducing sugar? No apparent reaction with Benedict's. After hydrolysis, it changes Benedict's from blue to green/yellow/orange/red.
What would you expect to observe for a negative test for non-reducing sugar? No apparent reaction with Benedict's before AND after hydrolysis.
Why is a blank sample necessary as a control in qualitative tests? To ensure that the colour change observed in the samples with analyte are accurate. To ensure that the reagents are not contaminated and there should be NO apparent reaction with the blank sample.
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