MetabolismAQA GCSE Biology: Revision notes
Section 1
What is metabolism?
Metabolism is the sum of all the reactions that take place in a cell or in the whole body. It includes reactions that build up larger molecules and reactions that break molecules down, all powered by energy from respiration.
Section 2
Where does the energy for metabolism come from?
Respiration is an exothermic reaction that occurs continuously in living cells. The energy it transfers supplies all the energy needed for the body's living processes, including:
- Chemical reactions to build larger molecules
- Movement
- Keeping warm (in mammals and birds)
Without a continuous supply of energy from respiration, none of the other metabolic reactions listed below could take place.
Section 3
Which reactions build up larger molecules?
Metabolism converts small molecules into larger, useful ones:
- Glucose → starch, glycogen and cellulose (storage/structural carbohydrates)
- Glycerol + 3 fatty acids → lipid molecules (fats and oils, for storage)
- Glucose + nitrate ions → amino acids → proteins (for growth and repair)
In plants, glucose from photosynthesis is used for all these purposes, plus respiration.
A plant storing energy over winter converts excess glucose into insoluble starch, which does not affect the cell's water potential the way soluble glucose would.
Section 4
How does exercise show metabolism in action?
During and after vigorous exercise, if the body cannot supply enough oxygen, muscle cells respire anaerobically, producing lactic acid and building up an oxygen debt — the extra oxygen the body needs after exercise to break down accumulated lactic acid.
(HT) Blood flowing through the muscles carries this lactic acid to the liver, where it is converted back into glucose, showing metabolism linking different organs of the body together.
Section 5
What other reactions are part of metabolism?
Metabolism also includes:
- Respiration itself (releasing energy from glucose)
- Breakdown of excess proteins — the liver converts unwanted amino acids into urea, which is then excreted by the kidneys
This shows metabolism is not just about building molecules up, but also breaking them down and safely disposing of waste products.
Must Know
- Metabolism = the sum of all reactions in a cell or the body
- Respiration provides the energy for all metabolic reactions (building molecules, movement, keeping warm)
- Building-up reactions: glucose → starch/glycogen/cellulose; glycerol + fatty acids → lipids; glucose + nitrate ions → amino acids → proteins
- Breaking-down reactions: excess protein → urea (excreted by kidneys)
- Exercise example: anaerobic respiration → lactic acid → oxygen debt; liver converts lactic acid back to glucose (HT)
That's the notes covered.
Carry on to the next subtopic.