Metabolism (AQA GCSE Combined Science: Trilogy) : Revision Note

Metabolism: basics

• Metabolism can be defined as:

the sum of all the reactions in a cell or the body

• Energy released during respiration allows enzyme-controlled reactions to occur inside cells, e.g. to produce new molecules

Synthesis & breakdown of molecules

• Organisms gain organic molecules, either by consuming food, e.g. in animals, or carrying out photosynthesis, e.g. in plants

• The molecules gained can then be broken down and used to produce, or synthesise, new molecules

• Metabolic reactions include the synthesis and breakdown of:

  • carbohydrates; complex carbohydrates are built from sugars, e.g. glucose

  • lipids; these are built from fatty acids and glycerol

  • proteins; synthesised from amino acids

Examples of metabolic reactions

• Carbohydrates

  • Glucose is used in the synthesis of:

    • starch; energy storage in plants

    • glycogen; energy storage in animals

    • cellulose; strengthens plant cell walls

  • Glucose is broken down during respiration to release energy

• Lipids

  • Glycerol is combined with three fatty acids in the synthesis of lipids, which can be used in energy storage

• Proteins

  • Glucose and nitrates are involved in the production of amino acids

  • Amino acids are used in the synthesis of proteins

  • Excess proteins are broken down to produce urea, which is excreted from the body