Biochemical Tests for Molecules

Benedict's Test for Reducing + Non-Reducing Sugars.

Iodine Test for Starch.

Biuret Test for Proteins.

Emulsion Test for Lipids.


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  • Created on: 01-06-10 00:03
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Unit 2.1.1. OCR
Biochemical Tests For Molecules
Benedict's Test for Sugars
Sugar ­ Monosaccharides + disaccharides.
Classified as reducing or non-reducing.
Test for sugars ­ Benedict's Test
All Monosaccharides (e.g. glucose) + some disaccharides (e.g.
1. Add Benedict's reagent (blue) to a sample.
2. Heat ­ but don't boil.
Positive ­ form a coloured precipitate (solid particles suspended n the
Colour changes:
Blue to green to yellow to orange to brick red
The higher the concentration, the further along the colour changes.
This can show the amount of reducing sugar in a solution.
More accurate ­ filter solution + weigh precipitate.
To test for a N-RS, E.g. sucrose, they have to be broken down into
1. Carry out the reducing sugar test ­ if negative use this test.
2. Boil solution with dilute hydrochloric acid.
3. Neutralise with sodium hydrogencarbonate.
4. Carry out Benedict's test for reducing sugars again.
5. If positive ­ form a coloured precipitate.
Iodine Test for Starch
NOTE: always say "iodine in potassium iodine solution" not just "iodine".
Add iodine dissolved in potassium iodine solution to sample.
Starch Present ­ Colour change from browny-orange to blue-black.
No Starch ­ Stays browny-orange.
Biuret Test for Proteins
1. Add sodium hydroxide solution ­ turns alkaline.

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Unit 2.1.1. OCR
2. Add copper(II) sulfate solution.
Protein Present ­ purple layer forms.
No Protein ­ Stays blue
Emulsion Test for Lipids
1. Add ethanol.
2. Shake.
3. Pour into water.
Lipid present ­ turns milky ­ the more lipid, the more noticeable.
No lipid ­ solution stays clear.
A machine used to determine the concentration of a glucose
Measures strength of a coloured solution - sees how much light
passes through it.…read more


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