Revision Notes

4.1.1 Chemicals & Life

Chemical Elements

  • Most of the molecules in living organisms fall into three categories: carbohydrates, proteins and lipids
  • These all contain carbon and so are described as organic molecules


Chemical elements table, IGCSE & GCSE Biology revision notes

Large Molecules are Made from Smaller Molecules


  • Long chains of simple sugars
  • Glucose is a simple sugar ( a monosaccharide)
  • When 2 glucose molecules join together maltose is formed (a disaccharide)
  • When lots of glucose molecules join together starch, glycogen or cellulose can form (a polysaccharide)


Glycogen, cellulose and starch are all made from glucose molecules, IGCSE & GCSE Biology revision notesGlycogen, cellulose and starch are all made from glucose molecules



  • Most fats (lipids) in the body are made up of triglycerides
  • Their basic unit is 1 glycerol molecule chemically bonded to 3 fatty acid chains
  • The fatty acids vary in size and structure
  • Lipids are divided into fats (solids at room temperature) and oils (liquids at room temperature)

Structure of a triglyceride, IGCSE & GCSE Biology revision notesStructure of a triglyceride



  • Long chains of amino acids
  • There are about 20 different amino acids
    They all contain the same basic structure but the ‘R’ group is different for each one
  • When amino acids are joined together a protein is formed
  • The amino acids can be arranged in any order, resulting in hundreds of thousands of different proteins
  • Even a small difference in the order of the amino acids results in a different protein being formed

General amino acid structure, IGCSE & GCSE Biology revision notesGeneral amino acid structure


Amino acids join together to form proteins, IGCSE & GCSE Biology revision notesAmino acids join together to form proteins

Extended Only

Protein Shape

  • There are thousands of different proteins in the human body and other organisms
  • Many of these proteins are different shapes and the shape often has an important effect on the function of the protein
  • For example:
    • Enzymes have an area in them known as the active site – this is important as this is the place where another molecule fits into the enzyme in order for a reaction to take place
    • If the shape of the active site does not match the shape of the molecule that fits into it, the reaction will not take place
    • Every enzyme has a different shaped active site
    • Antibodies are proteins produced by certain types of white blood cell to attach to antigens on the surface of pathogens
    • The shape of the antibody must match the shape of the antigen so that it can attach to it and signal it for destruction
  • The different sequences of amino acids cause the polypeptide chains to fold in different ways and this gives rise to the different shapes of proteins
  • In this way every protein has a unique 3-D shape that enables it to carry out its function

Author: Jenna

Jenna studied at Cardiff University before training to become a science teacher at the University of Bath specialising in Biology (although she loves teaching all three sciences at GCSE level!). Teaching is her passion, and with 10 years experience teaching across a wide range of specifications – from GCSE and A Level Biology in the UK to IGCSE and IB Biology internationally – she knows what is required to pass those Biology exams.

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