IGCSE Biology 0610
4.1 Biological molecules
Chemical Elements of Biomolecules
- Carbohydrates : Carbon, Hydrogen, Oxygen
- Proteins: Carbon, Hydrogen, Oxygen, Nitrogen, Sulfur
- Carbon, Hydrogen, Oxygen
Tip: You can tell the chemical elements that make up carbohydrates by their name. Carbo = Carbon, hydr = Hydrogen, and ate = Contains 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
- 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 - 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 - Protein test
Add 5-6 drops of biuret solution to the test-tube with extract.
Original colour – Blue
Protein present – Purple - 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 - 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)