CIE A Level Biology (9700) 2019-2021

Revision Notes

2.2.1 Biological Molecules: Key Terms

Biological Molecules: Key Terms

  • The key molecules that are required to build structures that enable organisms to function are:
    • Carbohydrates
    • Proteins
    • Lipids
    • Nucleic Acids
    • Water

5 Biological molecules (1), downloadable IGCSE & GCSE Biology revision notes

5 Biological molecules (2), downloadable IGCSE & GCSE Biology revision notes

The key biological molecules for living organisms

  • Carbohydrates, proteins, lipids and nucleic acids contain the elements carbon (C) and hydrogen (H) making them organic compounds
  • Carbon atoms are key to the organic compounds because:
    • Each carbon atom can form four covalent bonds – this makes the compounds very stable (as covalent bonds are so strong they require a large input of energy to break them)
    • Carbon atoms can form covalent bonds with oxygen, nitrogen and sulfur
    • Carbon atoms can bond to form straight chains, branched chains or rings
  • Carbon compounds can form small single subunits (monomers) that bond with many repeating subunits to form large molecules (polymers) by a process called polymerisation
  • Macromolecules are very large molecules
    • That contain 1000 or more atoms therefore having a high molecular mass
    • Polymers can be macromolecules, however not all macromolecules are polymers as the subunits of polymers have to be the same repeating units


  • Carbohydrates are one of the main carbon-based compounds in living organisms
  • All molecules in this group contain C, H and O
  • As H and O atoms are always present in the ratio of 2:1 (eg. water H2O, which is where ‘hydrate’ comes from) they can be represented by the formula Cx (H2O)y
  • The three types of carbohydrates are monosaccharides, disaccharides and polysaccharides

Types of carbohydrate table

Types of carbohydrate table, downloadable AS & A Level Biology revision notes

Exam Tip

When discussing monomers and polymers, give the definition but also name specific examples eg. nucleic acids – the monomer is a nucleotide.

The Two Forms of Glucose

  • The most well-known carbohydrate monomer is glucose
  • Glucose has the molecular formula C6H12O6
  • Glucose is the most common monosaccharide and is of central importance to most forms of life
  • There are different types of monosaccharide formed from molecules with varying numbers of carbon atom, for example:
    • Trioses (3C) eg. glyceraldehyde
    • Pentoses (5C) eg. ribose
    • Hexoses (6C) eg. glucose
  • Glucose exists in two structurally different forms – alpha (α) glucose and beta (β) glucose and is therefore known as an isomer
    • This structural variety results in different functions between carbohydrates

The Two Forms of Glucose, downloadable AS & A Level Biology revision notes

Straight chain and ring structural formula of alpha & beta glucose

  • Different polysaccharides are formed from the two isomers of glucose

Structure of polysaccharides table

Table 4 The two forms of glucose, downloadable AS & A Level Biology revision notes


Exam Tip

You must be able to recognise and draw the isomers of α and β glucose.

Author: Catherine

Cate has over 20 years’ experience teaching Biology to IGCSE, IB and A-level students in seven different countries across Asia, Europe, North America and the Middle East. This has given her a fine appreciation of different cultures, places and teaching methods. Cate has a keen interest in producing Biology revision resources that will help students engage with the subject.

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