Types of Heterotrophs based on electron source

1. Organotrophs

• Organotrophs are heterotrophs that utilize organic compounds for energy as well as food.
• These organisms derive energy from the oxidation of organic compounds via internal respiration.
• In organotrophs, the organic compound acts as the source of an electron, which then passes through the electron transport chain to produce ATPs.
• Organotrophs include all animals and some bacteria. The organotrophic bacteria can be aerobic or anaerobic.

2. Lithotrophs

• Lithotrophs are heterotrophs that utilize inorganic compounds or geological processes as their source of electrons.
• The most common inorganic compounds that are used as electron sources include elemental sulfur and elemental gases.
• Most lithotrophs are smaller bacteria as the inorganic compounds do not produce enough energy, like in the case of larger organic sugars.
• These can oxidize electron donors to produce electrons that are then channeled into electron respiratory channels to form ATP.
• The only known lithotropic organisms are unicellular organisms; however, multicellular organisms might obtain energy from symbioses with the unicellular organisms.
• Most lithotrophs are found in areas like sea floors or underground water sources where chemical food sources are found.