The big idea: Metabolism is the whole web of chemical reactions that happen inside a living cell.
Almost every one of these reactions is catalysed by an enzyme, which is why metabolism is described as the cell's enzyme-catalysed reactions.
These reactions fall into two types:
those that build larger molecules (anabolism), and those that break molecules down (catabolism).
- Metabolism
- All of the enzyme-catalysed chemical reactions that take place inside a living organism.
- Anabolism
- The reactions of metabolism that BUILD larger molecules from smaller ones. Anabolism uses (requires) energy.
- Catabolism
- The reactions of metabolism that BREAK larger molecules into smaller ones. Catabolism releases energy.
- Enzyme
- A biological catalyst — a protein that speeds up a metabolic reaction without being used up.
A name to remember it by: Anabolism = building up (think of an athlete building muscle).
Catabolism = breaking down (think of a catastrophe tearing things apart).
Both are part of metabolism, the cell's full set of enzyme-controlled reactions.
The key difference between the two halves of metabolism is the direction of the reaction and what happens to energy.
Anabolic reactions join small subunits into larger molecules — and this costs energy.
Catabolic reactions split large molecules into smaller ones — and this releases energy.
Anabolism — building, using energy: In anabolism, small subunits (monomers) are joined to make larger molecules (polymers and other macromolecules).
This building work requires energy.
Most anabolic reactions are condensation reactions (they join subunits and release water).
Examples: making glycogen or starch from glucose; building a protein from amino acids; photosynthesis building glucose.
Catabolism — breaking down, releasing energy: In catabolism, larger molecules are broken down into smaller subunits.
This breaking-down releases energy that the cell can use.
Most catabolic reactions are hydrolysis reactions (they add water to split bonds).
Examples: aerobic respiration breaking down glucose; digestion breaking down food; hydrolysis of macromolecules into monomers.
| Feature | Anabolism | Catabolism |
|---|---|---|
| What it does | Builds larger molecules from smaller ones | Breaks larger molecules into smaller ones |
| Direction | Building up (synthesis) | Breaking down (degradation) |
| Energy | Uses (requires) energy | Releases energy |
| Typical reaction | Condensation (joins subunits, releases water) | Hydrolysis (adds water, splits bonds) |
| Examples | Protein synthesis; making glycogen or starch; photosynthesis | Respiration; digestion; hydrolysis of macromolecules |
Anabolism
- Builds larger molecules from smaller ones
- Uses (requires) energy
- Usually a condensation reaction
- e.g. making glycogen, protein synthesis, photosynthesis
Catabolism
- Breaks larger molecules into smaller ones
- Releases energy
- Usually a hydrolysis reaction
- e.g. respiration, digestion, hydrolysis of macromolecules
A memory hook: Anabolism = Add subunits, Absorbs energy (building up).
Catabolism = Cuts molecules, Creates free energy (breaking down).
If a process makes something bigger, it is anabolic; if it breaks something down, it is catabolic.
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How this is tested: On Paper 1A a multiple-choice question commonly gives a list of cell processes and asks you to select the one that is an example of catabolism — the process that breaks a molecule down (such as respiration or the hydrolysis of a macromolecule).
Another frequent Paper 1A format is a classification task: each option pairs a process with a label, and you must choose the row where every process is sorted correctly — for example glycogen formation is anabolic while hydrolysis of macromolecules is catabolic.
On Paper 2 you may be asked to distinguish between anabolism and catabolism, or to state which type a named reaction belongs to.
IB-style question — classify each process
For each process, state whether it is anabolic or catabolic: (i) glucose molecules being joined to form glycogen, and (ii) a protein being hydrolysed into amino acids during digestion. [2]
How to score both marks
- Process (i) — glycogen formation. Many small glucose subunits are joined into a larger molecule (glycogen). Building a larger molecule from smaller ones means this is anabolic.
- Process (ii) — protein hydrolysis. A large protein is broken down into smaller amino-acid subunits using water. Breaking a larger molecule into smaller ones means this is catabolic. (Mark 1: (i) anabolic. Mark 2: (ii) catabolic.)
Final answer
(i) Anabolic — small glucose subunits are joined into a larger molecule (glycogen). (ii) Catabolic — a large protein is broken down into smaller amino-acid subunits.
✓ How to decide every time: Ask one question: does the molecule get bigger or smaller?
Bigger (subunits joined) → anabolic. Smaller (molecule split) → catabolic.
You do not need to know any numbers — just the direction of the change.
| Process | Anabolic or catabolic? | Why |
|---|---|---|
| Glycogen being made from glucose | Anabolic | Small subunits (glucose) joined into a larger molecule — building up |
| Hydrolysis of macromolecules in digestion | Catabolic | A large molecule split into smaller subunits — breaking down |
| Protein synthesis (amino acids → protein) | Anabolic | Subunits joined into a larger molecule, using energy |
| Aerobic respiration (glucose broken down) | Catabolic | A large molecule broken down, releasing energy |