IGCSE Biology 0610

4.1 Biological molecules

Chemical Elements of Biomolecules

  • Carbohydrates : Carbon, Hydrogen, Oxygen
  • Proteins: Carbon, Hydrogen, Oxygen, Nitrogen, Sulfur
  • Carbon, Hydrogen, Oxygen

Large and Small Molecules

Large molecules are made from smaller molecules:

  • Starch, glycogen, and cellulose from glucose
  • Proteins from amino acids
  • Fats and oils from fatty acids and glycerol

Chemical Tests for Biological Molecules

  1. Starch test
    Add 2-3 drops of iodine solution onto the extract/into the test-tube. If starch present (Positive result), colour turns from yellow-brown to blue-black
  2. Reducing Sugars test
    Using Benedict’s solution, add it to a test-tube with extract and heat gently in a water bath.
    Original colour – Bright blue
    Traces of reducing sugar – Green/yellow
    Moderate amount of reducing sugar – Orange
    Large amount of reducing sugar – Brick-red
  3. Protein test
    Add 5-6 drops of biuret solution to the test-tube with extract.
    Original colour – Blue
    Protein present – Purple
  4. Fat test
    Chop or grind testing material, do not add water yet. Place the extract into the test-tube. Add enough ethanol to cover the test-tube. Add a stopper and shake the test-tube. Add distilled water, shake again.
    Original – Clear
    Fat present – Cloudy/milky
  5. Vitamin C test
    Add liquid extract to DCPIP solution in test-tube drop one drop at a time.
    Original colour – blue
    Vitamin C present – Colour disappears

DNA Structure

  • Two strands coiled together to form a double helix
  • Each strand contains chemicals called bases
  • Bonds between pairs of bases hold the strands together
  • The bases always pair up in the same way: A with T, and C with G (full names are not required)

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