Photoautotrophs use light as their external energy source, whereas chemoautotrophs obtain energy from exothermic reactions involving inorganic substances. Both are autotrophs because they synthesize organic carbon compounds from inorganic carbon sources such as carbon dioxide.
Level: HL only: Subtopic 2.2, Energy and biomass in ecosystems (understanding 2.2.22).
The Mechanism
Autotrophs require energy to convert inorganic carbon into energy-rich organic compounds that can become biomass. The key difference is where that energy originates.
Photoautotrophs capture light energy and convert it into chemical energy through photosynthesis. Plants, algae and cyanobacteria are examples. Because they introduce energy and biomass into many ecosystems, they commonly form the first trophic level.
Chemoautotrophs obtain energy through chemosynthesis. They oxidize inorganic substances, such as ammonia, hydrogen sulfide or iron compounds, in exothermic reactions. Certain nitrifying bacteria in soils and sulfur-oxidizing microorganisms around hydrothermal vents are examples.
| Feature | Photoautotroph | Chemoautotroph |
|---|---|---|
| External energy source | Light | Exothermic inorganic chemical reactions |
| Carbon source | Inorganic carbon, usually carbon dioxide | Inorganic carbon, usually carbon dioxide |
| Main process | Photosynthesis | Chemosynthesis |
| Example | Green plant or cyanobacterium | Nitrifying or sulfur-oxidizing bacterium |
| Typical environment | Ecosystems receiving sufficient light | Soils, deep oceans and other environments where suitable inorganic substances occur |
A common misconception is that chemoautotrophs obtain energy by consuming organic matter. They do not: that describes heterotrophic nutrition. Chemoautotrophs use inorganic chemical reactions for energy while producing their own organic carbon compounds.
Exam Technique
For a distinguish question, state the contrasting external energy sources directly: light versus exothermic inorganic chemical reactions. Do not write only that both “make their own food,” because this does not identify the examinable difference. A strong answer may also state that both use inorganic carbon and connect them to biomass production at the start of food chains.
