Structure of Biological Molecules (Edexcel IGCSE Biology)

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Chemical Elements

  • 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

Chemical elements table, IGCSE & GCSE Biology revision notes

Structure of Carbohydrates, Proteins & Lipids

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, IGCSE & GCSE Biology revision notes

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)

Structure of a triglyceride, IGCSE & GCSE Biology revision notes

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, IGCSE & GCSE Biology revision notes

Amino acids join together to form proteins

Examiner Tips and Tricks

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.

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Lára Marie McIvor

Author: Lára Marie McIvor

Expertise: Biology Lead

Lára graduated from Oxford University in Biological Sciences and has now been a science tutor working in the UK for several years. Lára has a particular interest in the area of infectious disease and epidemiology, and enjoys creating original educational materials that develop confidence and facilitate learning.