Biology 2.2.12: Practical Biochemistry 1: Qualitative Tests For Biological Molecules

Question 1

how do you test for starch?

Answer 1

iodine solution to a sample, if starch is present there is a colour change of yellow-brown to blue-black.

Question 2

how do you test for reducing sugars?

Answer 2

place sample of food into b oiling tube and add Benedict’s solution, then heat in water bath at 80°C for 3 mins, if solution creates an organ-red precipitate then reducing sugars are present.

Question 3

how do you test for non-reducing sugars?

Answer 3

the bond must be hydrolysed first to free up reducing groups.
start by doing a test sample for reducing sugars to check there are none to begin with, then take a separate sample and boil it with HCl to hydrolyse the sucrose into glucose and fructose, cool solution and use sodium hydro carbonate solution to neutralise it and test for reducing sugars again, a positive result of green-yellow-orange-red indicates that non-reducing sugars were present in the original sample

Question 4

how do you test for lipids?

Answer 4

using the emulsion test, a sample is taken and mixed thoroughly with ethanol as lipids are not soluble in water, then filter it and pour solution into water in a clean test tube, a cloudy white emulsion will indicate the presence of lipids.

Question 5

how do you test for proteins?

Answer 5

using the biuret test, if a protein is present the colour will change from light blue to lilac

Question 6

why do you have to hydrolyse the non-reducing sugar?

Answer 6

non-reducing sugars will not change the colour of the solution so it must be broken down to monosaccharides to prove they are non-reducing