4. Biological Molecules (Cambridge IGCSE Biology 0610 for exams in 2023, 2024 and 2025)

Introduction to Biological Molecules

  • The bodies of living organisms are composed of various chemicals divided into three main categories:

    • Carbohydrates

    • Fats (Lipids)

    • Proteins

  • Known as organic molecules because they contain carbon.

  • The fundamental chemical elements within these molecules are carbon (C), hydrogen (H), and oxygen (O). Only proteins contain nitrogen (N) and may also include sulfur (S).

Carbohydrates

  • Carbohydrates are large molecules derived from simpler sugar units.

  • Monosaccharides: The simplest form of sugar.

    • Example: Glucose

  • Disaccharides: Formed when two monosaccharides bond.

    • Example: Maltose (from two glucose molecules)

  • Polysaccharides: Formed from many monosaccharides joining together.

    • Examples: Starch, glycogen, cellulose

  • Visual illustration explanation:

    • Monosaccharide: Single sugar molecule

    • Disaccharide: Two sugar molecules linked

    • Polysaccharide: Many sugar molecules linked

Fats (Lipids)

  • Fats are large molecules composed of fatty acids and glycerol.

  • Basic structure: One glycerol molecule chemically bonded to three fatty acid chains.

  • Fats at room temperature are termed oils.

  • Visual description of fat molecule structure highlighting glycerol and fatty acid chains.

Proteins

  • Proteins are made of long chains of amino acids.

  • There are approximately 20 different amino acids, allowing for vast combinations leading to numerous proteins.

  • Even minor variations in amino acid order can create different proteins.

  • Visual demonstration showing amino acids forming a protein structure.

Food Tests

  • Various tests can determine the presence of starch, reducing sugars, proteins, fats/oils, and vitamin C in foods.

Starch Test

  • Add iodine solution to the food sample; a positive result changes color from orange-brown to blue-black.

  • Example: Potato test showing positive presence of starch.

Reducing Sugars Test

  • Mix food sample with Benedict's solution and heat in a water bath. A positive test turns the solution from blue to orange or brick red.

  • Diagrams illustrating the process of testing for reducing sugars.

Proteins Test

  • Add Biuret solution to food sample; a positive result is indicated by a color change from blue to violet/purple.

  • Illustration showing the test result for protein presence.

Fats and Oils Test

  • Combine food sample with ethanol and then mix with an equal volume of cold distilled water. A positive result is a cloudy or milky emulsion.

Vitamin C Test

  • Add DCPIP solution (blue) to the food sample; a positive result shows the blue dye disappearing.

  • Diagram illustrating positive vitamin C test.

DNA Structure

  • DNA: Deoxyribonucleic acid, contains genetic instructions essential for growth and development.

  • Structure consists of two strands in a double helix formation.

  • Each strand contains bases that pair specifically:

    • A (adenine) pairs with T (thymine)

    • C (cytosine) pairs with G (guanine)

  • Bonds between base pairs held together by hydrogen bonds.

  • Visual depiction of DNA's double helix and paired bases.

Conclusion

  • Summary of key points about biological molecules and their tests.

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