Bioenergetics · GCSE Biology
Metabolism
Define metabolism for GCSE Biology as the sum of all reactions in a cell or organism: respiration, synthesis of proteins, starch, glycogen and lipids, and the breakdown reactions that transfer energy.
Metabolism is not a synonym for digestion or for “burning fat”. It is every chemical reaction in the body or cell, including respiration and the building and breaking of large molecules.
The important bits
What you need to know
- 1
Metabolism is the sum of all the chemical reactions in a cell or the organism.
- 2
Respiration (aerobic and anaerobic) is a central metabolic pathway: it transfers energy so other reactions can happen.
- 3
Synthesis reactions (anabolic, though you may not need the word): glucose → starch (plants) or glycogen (animals); glucose → cellulose; amino acids → proteins; glucose and other intermediates → lipids; glucose + nitrate → amino acids in plants.
- 4
Breakdown reactions (catabolic): starch or glycogen → glucose; proteins → amino acids; lipids → fatty acids and glycerol; these can then be respired or reused.
- 5
Liver metabolism includes converting lactic acid (oxygen debt), breaking down excess amino acids (urea for the kidney topic), and storing glycogen.
- 6
Enzyme control is the practical meaning of metabolism: each step has an enzyme with an active site, so temperature and pH that denature enzymes disrupt metabolism.
- 7
Thyroxine (from the thyroid) increases metabolic rate; adrenaline prepares the body for action by increasing heart rate and transferring more glucose to muscles. These hormones appear on the homeostasis paper but belong in the metabolism definition.
- 8
Do not write that metabolism is “the rate you burn calories” as your only definition. Use reactions in cells, then give two named examples.
Quotations worth analysing
Short evidence. Real method.
“Metabolism is the sum of all the reactions in a cell or organism.”
Then give examples: respiration, protein synthesis, conversion of glucose to glycogen or starch. A one-word “metabolism = energy” fails.
“Energy transferred from respiration is used to make large molecules from smaller ones.”
This links aerobic respiration to synthesis. Name a large molecule: protein, starch, glycogen, lipid, cellulose.
“Glucose is converted to glycogen for storage in liver and muscle.”
Glycogen is the animal starch analogue. Plants store starch. Do not mix the two stores.
Go deeper
What examples should I have ready for a two-mark definition?
Definition: sum of all chemical reactions in a cell or organism. Example 1: aerobic respiration in mitochondria transferring energy from glucose. Example 2: synthesis of proteins from amino acids at ribosomes. Example 3: conversion of glucose to starch in a leaf or glycogen in the liver. Example 4: breakdown of lactic acid after exercise. Two named reactions plus the definition usually scores. “Metabolism is how fast you lose weight” is magazine language. If the question is about plants, use cellulose, starch and nitrate to amino acids. If it is about humans, use glycogen, proteins, lipids and urea formation. The liver is a metabolism hub: it sits on respiration, oxygen debt, blood glucose and the kidney topic all at once.
Go deeper
How do enzymes and temperature sit inside metabolism?
Every metabolic pathway is a chain of enzyme-catalysed steps. That is why a fever that is too high, or hypothermia that is too low, threatens life: collisions slow or enzymes denature, so respiration and synthesis stall. Homeostasis of temperature and pH exists to protect metabolism. Thyroxine raises metabolic rate by increasing how fast these reactions run, which transfers more heat — useful in a cold environment, harmful if overactive (weight loss, restlessness). In the exam, a question that looks like enzymes and a question that looks like hormones may both want the word metabolism. Write “enzyme-controlled reactions that together make up metabolism” when you need to join the topics.
Go deeper
Where does metabolism appear in required practicals and data?
Respirometers (Triple) measure oxygen uptake as a proxy for metabolic rate in germinating seeds or maggots: soda lime absorbs CO₂, so a drop of liquid moves as O₂ is used. Warmer seeds (below denaturation) show faster movement because respiration enzymes collide more often. Pondweed bubble counts are photosynthetic rate, not metabolic rate as a whole, because photosynthesis is only one set of reactions. Compost-heap temperature rises because decomposer metabolism is exothermic. When a graph shows oxygen use per gram, you are looking at metabolic rate. State whether conditions are aerobic. Anaerobic metabolism still counts as metabolism, but a respirometer measuring O₂ will miss it — a neat evaluation point.
See the idea in action
Define metabolism and give two examples in a plant cell. Metabolism is the sum of all chemical reactions in the cell. Example 1: photosynthesis in chloroplasts building glucose. Example 2: aerobic respiration in mitochondria transferring energy from that glucose. Example 3: conversion of glucose to starch for storage and to cellulose for the cell wall. A human example would swap starch for glycogen in liver and muscle, and add protein synthesis from amino acids. “Metabolism means digestion” would not score.
Exam technique
Turn knowledge into marks
Learn the definition word for word, then attach two named reactions. Use glycogen for animals and starch for plants. Link energy transfer from respiration to synthesis of large molecules.
Common mistakes
Do not give these marks away
- 01
Defining metabolism only as digestion, exercise, or “burning fat”.
- 02
Saying plants have no metabolism at night, or mixing glycogen with starch in the wrong organism.
- 03
Forgetting that respiration is part of metabolism, not a separate unconnected idea.
Which definition of metabolism is correct?
AThe movement of air into the lungs
BThe sum of all chemical reactions in a cell or organism
CThe breakdown of starch by amylase only
DThe transport of water in xylem
Show the answer
The sum of all chemical reactions in a cell or organism. Metabolism includes respiration, synthesis and breakdown of large molecules. Breathing, a single enzyme reaction, or xylem transport are not the definition.
Quick questions
If this is the bit you searched
What is metabolism in GCSE Biology?
The sum of all the chemical reactions in a cell or organism, including respiration and the making and breaking of large molecules such as starch, glycogen, proteins and lipids.
Is respiration part of metabolism?
Yes. Respiration is a core metabolic pathway that transfers energy so other reactions can take place.
What is the difference between starch and glycogen?
Both are insoluble carbohydrate stores made from glucose. Plants store starch; animals store glycogen in liver and muscle.
How is metabolic rate linked to temperature?
Most metabolic reactions are enzyme-controlled, so rate rises with temperature up to an optimum and falls if enzymes denature. Thyroxine also increases metabolic rate.