Testing for biological molecules

Non-reducing sugars

If there aren’t any reducing sugar you can test for non reducing sugars Eg. Sucrose

1) Add hydrochloric acid
2) Heat in 75*c water bath
3) Add Sodium hydrogen carbonate (to neutralise)
4) Add Benedict’s reagent

Positive result - coloured precipitate
Negative result- stays blue

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Reducing sugars

Simple sugars made of one or two units eg. Glucose, Maltose

1) Add Benedict’s reagent (blue)
2) Put in 75*c water bath

Positive result - coloured precipitate
Higher concentration of sugar = further colour change

Blue ~ Green ~ Yellow ~ Orange ~ Red

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Starch

Add iodine

Positive result - colour change to Blue-Black colour
Negative result- stays brown-orange

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Protein

1) Add sodium hydroxide (to make solution alkaline)
2) Add copper (2) sulfate - blue

Positive result - turns purple
Negative result - stays blue

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Lipids

1) Shake substance with Ethanol until it dissolves
2) Pour solution into water

Positive result - milky emulsion will show up

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