Autotrophs synthesize organic carbon compounds from inorganic sources, whereas heterotrophs obtain organic carbon compounds from other organisms. This distinction is core content for both SL and HL in IB ESS subtopic 2.2, Energy and biomass in ecosystems.
How They Differ
Autotrophs use an external energy source to convert inorganic carbon, usually carbon dioxide, into organic compounds such as glucose. They therefore form the producer level at the start of food chains.
Heterotrophs cannot synthesize all their required organic carbon compounds from inorganic carbon. Instead, they obtain these compounds by consuming producers, other consumers, or dead organic matter.
| Feature | Autotroph | Heterotroph |
|---|---|---|
| Carbon source | Inorganic carbon, usually carbon dioxide | Organic carbon compounds from other organisms |
| Ecological role | Producer | Consumer or decomposer |
| Typical examples | Plants, algae and some bacteria | Animals, fungi and many bacteria |
| Position in food chains | First trophic level | Subsequent trophic levels |
| Energy strategy | Uses an external energy source to synthesize carbon compounds | Gains chemical energy from consumed carbon compounds |
Most familiar autotrophs are photoautotrophs, which use light energy in photosynthesis. For example, grass produces organic matter and transfers stored chemical energy to a grazing herbivore, which is a heterotroph.
At HL, students also study chemoautotrophs, which use energy released by exothermic inorganic chemical reactions. Chemosynthetic bacteria at deep-sea hydrothermal vents support food webs without sunlight.
A common misconception is that autotrophs “make their own energy.” They do not create energy. In accordance with the first law of thermodynamics, they transform light or chemical energy into chemical energy stored in organic compounds. Both autotrophs and heterotrophs release usable energy from organic compounds through cellular respiration.