Biology A-Level Chapter 2

Summary:

The notes cover the methods for testing the presence of four types of biological molecules: reducing sugars, starch, lipids, and proteins. For reducing sugars, the Benedict’s test is used. By adding Benedict’s reagent to a sample solution and heating it, a color change from blue to brown/brick-red indicates the presence of reducing sugars. The iodine test is used for starch. Adding iodine in potassium iodide solution to the sample produces a blue-black color if starch is present. For lipids, the emulsion test is used. Adding ethanol to the sample and shaking it with water produces a milky color if lipids are present. For proteins, the biuret test is used.

Excerpt:

Biology A-Level Chapter 2

2.1 Testing for Biological Molecules 
CIE A Level Biology (9700) exams from 2022 Resources

REVISION NOTES TOPIC QUESTIONS PAST PAPERS

CONTENTS
2.1.1 Biological Molecule Tests
2.1.2 The Benedict’s Test
2.1.3 Testing for Non-Reducing Sugars

2.1.1 BIOLOGICAL MOLECULE TESTS
Testing for Key Biological Molecules
There are a number of tests that can be carried out quickly and easily in a lab to determine if a sample contains one of the key biological molecules (carbohydrates, proteins and lipids).

The following tests are qualitative – they do not give a quantitative value as to how much of each type of molecule may be present in a sample.

The Benedict’s test for reducing sugars:

• Add Benedict’s reagent (which is blue as it contains copper (II) sulfate ions) to a sample solution in a test tube.
• Heat the test tube in a water bath or beaker of water that has been brought to a boil for a
  few minutes.
• If a reducing sugar is present, a coloured precipitate will form as copper (II) sulfate is reduced
  to copper (I) oxide which is insoluble in water.
• A positive test result is, therefore, a colour change somewhere along a colour scale from blue
  (no reducing sugar) to brown/brick-red (a high concentration of reducing sugar).
  • This test is semi-quantitative as the degree of the colour change can give an indication of how much (the concentration of) reducing sugar present.