Revision notes topic questions past papers

if a reducing sugar is present

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CIE AS Biology (9700) 2019-2021 Revision Notes

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2. Biological Molecules

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Page 3

The iodine test for starch

To test for the presence of starch in a sample, add a few drops of orange/brown iodine in

potassium iodide solution to the sample

The iodine is in potassium iodide solution as iodine is insoluble in water

If starch is present, iodide ions in the solution interact with the centre of starch molecules,

producing a complex with a distinctive blue-black colour

This test is useful in experiments for showing that starch in a sample has been digested by

enzymes

The emulsion test for lipids

Lipids are nonpolar molecules that do not dissolve in water but will dissolve in organic

solvents such as ethanol

Add ethanol to the sample to be tested, shake to mix and then add the mixture to a test tube

of water

If lipids are present, a milky emulsion will form (the solution appears ‘cloudy’); the more lipid

present, the more obvious the milky colour of the solution

If no lipid is present, the solution remains clear

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