Biological Molecules Tests

Biological Molecule tests, positive results and explanations

Starch Test

1. Using a pipette, place a drop of starch solution in a depression in a spotting tile

2. Add a drop of iodine in potassium iodide solution

Starch positive result

• Blue-black colour formed

• Coloured polyiodide complex formed with starch

Reducing Sugar (glucose) test

1. Place about 2cm³ of glucose solution in test tube

2. Add Benedict’s solution in excess

3. Stand tube in 80-100˚C water bath

Reducing Sugar (glucose) positive result

• Brick-red precipitate is formed

• The reducing sugar reduces copper(II) ions in the Benedict’s copper(I) oxide

• Colour may vary, lower concentrations turn green, yellow and orange

Non-reducing sugar (sucrose) test

1. Place about 2cm³ sucrose solution in test tube

2. Add three drops of dilute HCl

3. Shake tube and place in 80-100˚C water bath for 5 minutes

4. Allow to cool

5. Add three drops of dilute NaOH and mix to neutralise acid

6. Repeat reducing sugar test

Non-Reducing Sugar (sucrose) positive result

• Brick-red precipitate formed

• Acid hydrolyses the sucrose into glucose and fructose

Protein test

1. Add about 5cm³ of protein solution into test tube

2. Add equal Biuret reagent

Protein positive result

• Lilac (mauve) solution

• Nitrogen atoms in peptide bonds form lilac complex with copper(II) ions in biuret reagent

Lipid test

1. Place lipid in test tube

2. Add about 5cm³ of ethanol and shake to dissolve lipid

3. Pour into test tube ¾ filled with cold water

Lipid positive result

• White emulation on surface of water

• Alcohol mixes with water, lipid forms emulsion of microscopic droplets suspended