Practical: Testing for Biological Molecules

Carbohydrates, lipids and proteins are all biological molecules (found in living organisms) You need to know how you can test for them using different chemicals

Test for Sugars using Benedict’s Reagent

There’s lots of different types of sugar molecules. Due to their chemical properties, many sugars (e.g. glucose) are called reducing sugars.

1. Add Benedict’s reagent (which is blue) to a sample and heat it in a water bath that’s set to 75°C. If the test’s positive it will form a coloured precipitate (solid particles suspended in the solution)

2. The higher the concentration of reducing sugars, the further the colour change goes

The colour of the precipitate changes from: blue → green → yellow → orange → brick red

Biuret Test for Proteins

1. First add a few drops of potassium hydroxide solution to make the solution alkaline.

2. Then add some copper(II) sulphate solution (which is bright blue)

   No protein → solution will stay blue

   Protein is present → solution will turn purple

Emulsion Test for Lipids

1. Shake the test substance with ethanol for about a minute until it dissolves, then pour the solution into water

   Lipids present → they will precipitate out of liquid and show up as a milky emulsion

   The more lipid there is → more noticeable the milky colour will be

Test for Starch using Iodine

1. Just add iodine solution to the test sample

   Starch is present → sample changes from browny-orange to a dark, blue-black colour

   No starch → it stays browny-orange

Hazards

• Iodine is an irritant to the eyes - wear goggles

• Chemicals use in the biuret tests are dangerous - if you spill any of the chemicals on your skin, wash it off straight away

• Water Bath in the Benedict’s test - it can be hot so be careful around it