CoachAn organism is autotrophic when it builds its own organic matter from mineral carbon (carbon dioxide) using an external energy source — light for the photoautotrophs (plants, algae, cyanobacteria), the oxidation of mineral compounds for the chemoautotrophs. It is heterotrophic when it must take in organic matter made by other organisms, which supplies both its carbon and its energy (animals, fungi, most bacteria).
Examples
Example 1.4 (The rabbit and the oak)
The rabbit’s hundred grams of grass supply, once digested, about of chemical energy and the carbon of every molecule it will build; the oxygen it breathes lets it oxidise that food, and the carbon dioxide it exhales is the same carbon leaving. The oak’s intake is the reverse: carbon dioxide in, oxygen out, the energy coming as light — some falling on its crown at midday, of which it stores under one percent as sugar. Both release almost all the energy they process as heat: the rabbit’s fur is warm, and the oak’s leaves are cooled by the evaporation that its own light absorption drives.