Light is absorbed and converted into usable chemical energy (ATP) via the process of photosynthesis

This energy is used to make organic compounds (e.g. glucose) from inorganic molecules (CO2 and H2O)

The organic compounds can be used as building blocks to create a range of macromolecules needed by the cells and tissues

All green plants are photosynthetic, along with certain algae (e.g. seaweeds) and prokaryotes (e.g. cyanobacteria)

These inorganic sources may include ammonia, hydrogen gas, hydrogen sulfide or iron oxide

Such communities may be located near deep sea hydrothermal vents or in underground caves where no light can penetrate

The specific chemical processes deployed by these organisms may vary depending on the inorganic chemical used as an energy source

Iron-oxidising bacteria found in the soil are an example of a chemoautotroph – they use the electrons from iron (oxidation) to produce ATP

Carbohydrates (glucose) serve as the primary fuel source, but other compounds (lipids, proteins) may be used

The organic compounds can either be produced by the organism itself (autotrophy) or obtained from other organisms (heterotrophy)

Cell respiration can occur anaerobically (no oxygen = low ATP yield) or aerobically (oxygen = high ATP yield)

The release of energy involves oxidation reactions (high energy electrons are used in the production of ATP)