Ad Banner Placeholder

Cambridge IGCSE Biology · 0610

Chapter 4: Biological molecules

Chemical elements

Biological molecules are essential for building structures and carrying out metabolic reactions in organisms.

  1. Carbohydrates: Contain carbon, hydrogen, and oxygen.
  2. Fats (lipids): Contain carbon, hydrogen, and oxygen.
  3. Proteins: Contain nitrogen, carbon, hydrogen, oxygen, and sulphur.

Synthesis of large molecules

Large molecules are formed when many smaller molecules bind together in chains.

  1. Carbohydrates: Made from glucose molecules.
    • Starch: An energy store in plants.
    • Glycogen: An energy store in animals.
    • Cellulose: Used for structure in plant cell walls.
  2. Proteins: Made from amino acids binding together in a chain.
  3. Fats and oils: Made from fatty acids and glycerol.
Monomers such as glucose and amino acids joining to form polymer chains including starch and protein
Diagram 1: Synthesis of biological molecules. Individual small units (monomers) such as glucose or amino acids bind together to form long chains (polymers) such as starch or protein.

Exam Traps

  • Starch and glycogen store energy; cellulose provides structural support in plant cell walls — do not swap their functions.

Chemical food tests

The following tests are used to identify specific biological molecules in a sample:

  1. Starch: Add iodine solution.
    • Positive result: Colour change to blue-black.
  2. Reducing sugars (e.g., glucose): Add Benedict’s solution and heat to 80°C.
    • Positive result: Colour change from blue to brick red (caused by insoluble copper (I) oxide).
  3. Proteins (Biuret test): Add sodium hydroxide solution and a few drops of dilute copper (II) sulphate solution.
    • Positive result: Colour change from blue to purple.
  4. Fats and oils (emulsion test): Shake the sample with ethanol to dissolve fats, then add water and shake again.
    • Positive result: The solution turns cloudy white.
  5. Vitamin C: Add the sample to blue DCPIP solution and shake gently.
    • Positive result: The solution becomes colourless.
Test tubes showing reagent colours and positive result colours for starch, reducing sugars, proteins, fats, and vitamin C food tests
Diagram 2: Food test colour changes. A series of test tubes showing the initial reagent colour and the final positive result colour for each of the five tests: starch, reducing sugars, proteins, fats and oils, and vitamin C.

Exam Traps

  • Blue-black is iodine for starch only; do not use it as the positive colour for any other food test.

Structure of DNA

DNA is the molecule that contains genetic information.

Double helix
DNA consists of two strands coiled around each other to form a double helix.
Backbone and bases
Each strand has a sugar backbone with chemicals called bases attached.
Base pairing
Bonds between pairs of bases hold the two strands together as cross-links.
Complementary pairing
The four types of bases (A, T, C, and G) always pair in the same way:
  • A always pairs with T.
  • C always pairs with G.
DNA double helix showing sugar-phosphate backbones and complementary base pairs A-T and C-G
Diagram 3: DNA double helix. The twisted ladder structure shows sugar-phosphate backbones forming the sides and base pairs forming the rungs. Bases are labelled to show A pairing with T and C pairing with G.

0/15

Ad Banner Placeholder