Qualitative Tests for Biological Molecules

Question 1

Reducing sugars – Benedict’s Reagent:

Answer 1

Reducing sugars – Benedict’s Reagent:

All monosaccharides and some disaccharides are reducing sugars as they reduce (give electrons too) other molecules. Heat the sample with Benedict’s reagent, there is a colour change from blue to green to yellow to orange to brick red. The Cu2+ ions in Benedict’s are reduced, forming red copper (I) oxide precipitate. Test strips: dip the strip into sample solution and compare with calibration card, tells you whether a reducing sugar is present of absent. They are often used to test for glucose in urine of diabetic patients.

Question 2

Non-reducing sugars:

Answer 2

Non-reducing sugars: To test non-reducing sugars, test for a reducing sugar, then boil in HCl to hydrolyse the sucrose into glucose. Then cool the solution and use sodium hydrogencarbonate to neutralise it, and then test for reducing sugars with benedict’s solution. If the test is positive, the original solution had non-reducing sugars present.

Question 3

Starch - iodine:

Answer 3

Starch - iodine:
Add iodine solution (in potassium iodide) to a sample. If starch is present, colour change of yellow-brown to blue-black. Iodine disrupts amylose helix causing colour change.

Question 4

Lipids – emulsion test:

Answer 4

Lipids – emulsion test:
Mix sample with ethanol - any lipid will go into solution in ethanol (remember: lipids not soluble in water). Filter it and pour into test tube containing water. A cloudy white emulsion indicates the presence of lipids. This happens due to tiny lipid droplets coming out of solution when mixed with water.

Question 5

Proteins – biuret test:

Answer 5

Proteins – biuret test:
Place sample on a spotting tile and add biuret reagent. If a protein is present, the colour changes from light blue to lilac/mauve. The colour is formed by a complex between the nitrogen atoms in the peptide chain and Cu2+ ions – the test detects the presence of peptide bonds.