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06102026–2028 syllabus

BIOLOGY · CHAPTER 4

Biological molecules

The building blocks of carbohydrates, fats, proteins and DNA, and how food tests identify them.

Core + Supplement3 connected sectionsNotes only

LEARNING OBJECTIVES

By the end of this chapter, you should be able to:

  • name elements in major biological molecules
  • relate large molecules to building blocks
  • carry out five food tests
  • describe DNA structure and complementary base pairing

THE BIG IDEA

The building blocks of carbohydrates, fats, proteins and DNA, and how food tests identify them.

This chapter builds each process from its structures, inputs and outputs. Follow the connections rather than memorising isolated facts, then use precise biological terms when explaining cause and effect.

01

SECTION 01

Molecules and their building blocks

Large biological molecules are assembled from smaller units. Their elements and arrangement determine their properties.

DETAILED EXPLANATION

  • Carbohydrates and fats contain carbon, hydrogen and oxygen. Proteins also contain nitrogen and may contain sulfur.
  • 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.
LABELLED STUDY DIAGRAMBuilding larger molecules
1glucose → starch • glycogen • cellulose→
2amino acids → proteins→
3fatty acids + glycerol → fats and oils
02

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

  • Iodine: orange-brown to blue-black for starch.
  • 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.
03

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

  • A always pairs with T, and C always pairs with G.
  • 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.
LABELLED STUDY DIAGRAMComplementary pairing
1A ═ T→
2C ≡ G→
3two strands → double helix

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.
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