LEARNING OBJECTIVES
What you will be able to do
- name elements in major biological molecules
- relate large molecules to building blocks
- carry out five food tests
- describe DNA structure and complementary base pairing
AT A GLANCE
INTRODUCTION · THE BIG IDEA
The building blocks of carbohydrates, fats, proteins and DNA, and how food tests identify them.
Living organisms build a wide variety of structures from a small set of chemical building blocks. The arrangement of these units gives carbohydrates, fats, proteins and DNA their different roles.
Food tests turn invisible molecules into observable evidence. Reliable conclusions depend on the correct reagent, conditions, control and colour change.
SECTION 01
Molecules and their building blocks
Large biological molecules are assembled from smaller units. Their elements and arrangement determine their properties.
DETAILED EXPLANATION
- Starch, glycogen and cellulose are built from glucose; proteins from amino acids; fats and oils from fatty acids and glycerol.
- Starch stores carbohydrate in plants, glycogen in animals, and cellulose strengthens plant walls.
SECTION 02
Food tests
A valid food test uses the correct reagent, conditions and colour observation, with positive and negative controls where possible.
DETAILED EXPLANATION
- Benedict's solution plus heating: blue to green, yellow, orange or brick-red for reducing sugar.
- Biuret reagent: blue to lilac or purple for protein.
- Ethanol emulsion test: shake with ethanol, add water; a milky emulsion indicates fat or oil.
- DCPIP: blue solution is decolourised by vitamin C.
SECTION 03
DNA structure
DNA consists of two strands coiled into a double helix. Bases project inwards and bonds between complementary pairs hold the strands together.
DETAILED EXPLANATION
- The order of bases carries information; complementary pairing allows the opposite strand to be predicted.
- If one strand reads A-C-G-T-T-A, the complementary sequence is T-G-C-A-A-T.
STEP-BY-STEP EXAM EXAMPLE
Completing a complementary DNA strand
- Start with the sequence A–C–G–T–T–A.
- Pair A with T and C with G, one position at a time.
- Keep the bases in the same displayed order.
Answer: The complementary sequence is T–G–C–A–A–T.
QUICK CHAPTER SUMMARY
The ideas to carry forward
- Macromolecules are assembled from smaller units.
- Food tests require characteristic colour changes.
- DNA is a double helix with complementary base pairs.
- Base sequence stores genetic information.
QUICK REVISION CHECKLIST
Can you do each of these without your notes?
- name elements in major biological molecules
- relate large molecules to building blocks
- carry out five food tests
- describe DNA structure and complementary base pairing