Edexcel IGCSE Biology

Revision Notes

2.3.1 Carbohydrates, Fats & Proteins

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


  • Carbohydrates contain the elements carbon, hydrogen and oxygen
  • A monosaccharide is a simple sugar e.g. glucose (C6H12O6) or fructose
    • Glucose molecules contain lots of energy which can be released in respiration by breaking the bonds between the carbon atoms
  • A disaccharide is made when two monosaccharides join together
    • Maltose is formed from one glucose molecules
    • Sucrose is formed from one glucose and one fructose molecule
  • A polysaccharide is formed when lots of monosaccharides join together
    • Starch, glycogen or cellulose are all formed when lots of glucose molecules join together
    • Polysaccharides are insoluble and therefore useful as storage molecules

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

Glycogen, cellulose and starch are all made from glucose molecules


  • Most fats (lipids) in the body are made up of triglycerides
  • Their basic unit is one glycerol molecule chemically bonded to three 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 notes

The structure of a triglyceride


  • Proteins are formed from long chains of amino acids
  • There are 20 different amino acids
  • When amino acids are joined together a protein is formed
  • Amino acids can be arranged in any order, resulting in hundreds of thousands of different proteins
    • Examples of proteins include enzymes, haemoglobin, ligaments and keratin

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

Amino acids join together to form proteins

Protein shape

  • Different proteins have different amino acid sequences resulting in them being different shapes
  • Even a small difference in the amino acid sequence will result in a completely different protein being formed
  • 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
  • The shape of a protein determines its function
  • For example:
    • Enzymes have a specifically shaped active site – this is where a specific substrate molecule fits 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
    • Antibodies are proteins produced by certain types of white blood cells that 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

Enzyme substrate specificity, IGCSE & GCSE Biology revision notes

Every enzyme has a different shaped active site-specific to one substrate

Exam Tip

You should be able to explain the importance of sugars, amino acids, fatty acids and glycerol in the synthesis and breakdown of carbohydrates, proteins and lipids. There will be many examples of each of these molecules throughout the course.

Author: Ruth

Ruth graduated from Sheffield University with a degree in Biology and went on to teach Science in London whilst also completing an MA in innovation in Education. With 10 years of teaching experience across the 3 key science disciplines, Ruth decided to set up a tutoring business to support students in her local area. Ruth has worked with several exam boards and loves to use her experience to produce educational materials which make the mark schemes accessible to all students.

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