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

Introduction to Biological Molecules

  • Biological molecules are essential for the structure and function of living organisms.

  • Three main categories of biological molecules: carbohydrates, fats (lipids), proteins.

  • These molecules are termed organic because they contain carbon.

Chemical Elements in Biological Molecules

  • Key elements in biological molecules include:

    • Carbon (C)

    • Hydrogen (H)

    • Oxygen (O)

    • Nitrogen (N) (present only in proteins)

    • Sulfur (S) (present in some proteins)

  • Importance of understanding that large molecules are formed from smaller ones.

Carbohydrates

  • Carbohydrates are large molecules composed of simple sugars.

  • Monosaccharides: Simplest form of sugar; example: glucose.

  • Disaccharides: Formed by two monosaccharides; example: maltose (two glucose molecules).

  • Polysaccharides: Formed by many monosaccharides; examples: starch, glycogen, cellulose.

  • Visual representation of carbohydrate types:

    • Monosaccharide: one sugar molecule.

    • Disaccharide: two linked sugar molecules.

    • Polysaccharide: multiple linked sugar molecules.

Fats (Lipids)

  • Fats are large molecules formed from smaller units: fatty acids and glycerol.

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

  • Liquid fats at room temperature are called oils.

  • Visual representation of a fat molecule showing its structure.

Proteins

  • Composed of long chains of smaller units called amino acids.

  • Approximately 20 different amino acids exist, which can be arranged in various orders to form different proteins.

  • Different sequences of amino acids result in different protein structures and functions.

  • Visual representation depicts how amino acids combine to form proteins.

Food Tests for Biological Molecules

  • Different tests can identify the presence of specific biological molecules in food samples:

Test for Starch

  • Use iodine solution;

  • Positive result: color changes from orange-brown to blue-black (e.g., test with potato).

Test for Reducing Sugars

  • Use Benedict's solution;

  • Heat at 60-70 degrees Celsius for 5 minutes;

  • Positive result: color changes from blue to orange or brick red.

Test for Proteins

  • Add Biuret solution to the sample;

  • Positive result: color changes from blue to violet or purple.

Test for Fats and Oils

  • Mix sample with ethanol, then combine with cold distilled water;

  • Positive result: formation of a cloudy or milky emulsion.

Test for Vitamin C

  • Add DCPIP solution (blue) to the sample;

  • Positive result: blue color disappears confirming the presence of vitamin C.

Structure of DNA Molecule

  • DNA (deoxyribonucleic acid) contains genetic instructions for growth and development of organisms.

  • Structure: enters a double helix formed by two strands.

  • Strands held together by hydrogen bonds between base pairs:

    • A (adenine) pairs with T (thymine)

    • C (cytosine) pairs with G (guanine)

  • Visual representation showing the DNA structure and base pairing.

Conclusion

  • Summary of key concepts in Chapter 4: Biological Molecules.

  • Encouragement to subscribe for more biology revision resources.