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Structure of Biological Molecules (Edexcel IGCSE Biology)
Revision Note
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
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
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
Examiner 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.
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