Autotrophs and heterotrophsSpec C4.2.6, C4.2.7, C4.2.8, C4.2.9
In short
Autotrophs are organisms that use external energy sources to synthesise carbon compounds from simple inorganic substances. Photoautotrophs use light; chemoautotrophs, such as iron-oxidising bacteria, use energy from oxidation reactions. Heterotrophs use carbon compounds obtained from other organisms to synthesise the carbon compounds they need. Both release energy by oxidising carbon compounds in cell respiration.
- Autotroph
- An organism that uses an external energy source to synthesise carbon compounds from simple inorganic substances.
- Heterotroph
- An organism that uses carbon compounds obtained from other organisms to synthesise the carbon compounds that it requires.
Autotrophs need energy for two jobs: carbon fixation (turning carbon dioxide into organic compounds) and the anabolic reactions that build macromolecules such as starch, cellulose, proteins and nucleic acids.
| Photoautotrophs | Chemoautotrophs | |
|---|---|---|
| External energy source | Light | Energy released by oxidation reactions of inorganic chemicals |
| Examples | Plants, algae, cyanobacteria | Iron-oxidising bacteria such as Acidithiobacillus ferrooxidans; sulfur bacteria at hydrothermal vents |
Oxidation reactions release energy, so they are useful to living organisms. Iron-oxidising bacteria oxidise dissolved iron(II) ions to iron(III) ions (Fe²⁺ → Fe³⁺) and use the energy released to fix carbon dioxide. They are found where iron-rich water meets air, for example in mine drainage, and leave orange iron oxide deposits.
Heterotrophs
Heterotrophs take in complex carbon compounds such as proteins, carbohydrates, lipids and nucleic acids. These are digested into smaller molecules, either internally (in the gut of animals) or externally (fungi and many bacteria secrete enzymes onto their food). The products are absorbed and assimilated: used to build the heterotroph's own carbon compounds, for example its own proteins from absorbed amino acids.
Releasing energy
Both autotrophs and heterotrophs release energy from carbon compounds by oxidising them in cell respiration, producing ATP. Plants respire all the time, in the dark and in the light.
Written and checked against the IB Biology SL specification · Updated October 2026