Structure of Biological Molecules (Edexcel IGCSE Biology: Double Science)

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

Carbohydrates

• Carbohydrates contain the elements carbon, hydrogen and oxygen
• Carbohydrates can be small, simple sugars or more complex larger molecules
  • A monosaccharide is a simple sugar e.g. glucose (C₆H₁₂O₆) or fructose
  • A disaccharide is made when two monosaccharides join together, e.g. maltose is formed from two glucose molecules
  • A large polysaccharide is formed when lots of monosaccharides join together
    • Starch, glycogen or cellulose are all formed when lots of simple glucose molecules join together
    • Polysaccharides are insoluble and therefore useful as storage molecules

Glycogen, cellulose and starch are all made from glucose molecules

Lipids

• Most 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)

The structure of a triglyceride: glycerol and three fatty acids

Proteins

• Proteins are formed from long chains of 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
  • Different proteins have different amino acid sequences resulting in them having different shapes; the shape of a protein determines its function
  • Examples of proteins include enzymes, haemoglobin, ligaments and keratin

  

Amino acids join together to form proteins