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

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.
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
Examiner Tips and Tricks
All of the features of metabolism listed are covered in more detail in the relevant topics but you should be aware of how they all link together here.
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