Enzymes as catalysts and their role in metabolismSpec C1.1.1, C1.1.2, C1.1.3
In short
Enzymes are biological catalysts: they speed up chemical reactions in cells without being used up. Metabolism is the complex network of interdependent and interacting chemical reactions in an organism. Each enzyme is specific, so many enzymes are needed, and cells control metabolism by controlling enzymes. Anabolic reactions build macromolecules; catabolic reactions break them down.
An enzyme is a protein that acts as a biological catalyst: it increases the rate of a chemical reaction and is not changed or used up by the reaction, so one enzyme molecule can be used again and again.
Without enzymes, most reactions in cells would happen far too slowly at body temperature to keep the organism alive. Raising the temperature enough to speed them up would damage cell structures. Enzymes give fast rates at the moderate temperatures found in cells, and the cell can switch reactions on or off by making, activating or inhibiting particular enzymes.
Metabolism
Metabolism is the complex network of interdependent and interacting chemical reactions occurring in living organisms. The product of one reaction is often the substrate of the next, so reactions are linked into pathways, and pathways share molecules with each other.
Enzymes are specific: each one catalyses one reaction or one type of reaction. Living organisms therefore need many different enzymes, often thousands. Because every step depends on its own enzyme, control over metabolism can be exerted through these enzymes, for example by changing how much of an enzyme is made or how active it is.
Anabolic and catabolic reactions
| Anabolism | Catabolism | |
|---|---|---|
| What happens | Larger molecules built from smaller ones | Larger molecules broken down into smaller ones |
| Typical reaction | Condensation: monomers joined, water released | Hydrolysis (water used to split bonds) or oxidation |
| Energy | Needs an energy input (usually from ATP) | Often releases energy |
| Examples | Protein synthesis (amino acids joined by peptide bonds); glycogen formation from glucose; photosynthesis (carbon compounds made from CO₂) | Hydrolysis of macromolecules into monomers in digestion; oxidation of substrates such as glucose in respiration |
- Anabolism
- Reactions that build macromolecules from monomers, for example by condensation.
- Catabolism
- Reactions that break down macromolecules into monomers, for example by hydrolysis, or oxidise substrates as in respiration.
Photosynthesis is anabolic and respiration is catabolic. Do not describe digestion as anabolic just because it happens before new molecules are built.
Written and checked against the IB Biology HL specification · Updated October 2026