Autotrophs & Heterotrophs (HL) (DP IB Environmental Systems & Societies (ESS)): Revision Note

Autotrophs & Heterotrophs

What are autotrophs and heterotrophs?

• All living organisms can be classified into two groups based on how they obtain carbon compounds: autotrophs and heterotrophs

  • Autotrophs synthesise carbon compounds from inorganic sources like carbon dioxide and water

  • Heterotrophs obtain carbon compounds by consuming other organisms (either plants, animals, or decomposing organic matter)

Types of autotrophs

Photoautotrophs

• Photoautotrophs use light as their external energy source to produce organic compounds through photosynthesis

  • Examples include: green plants, algae, and some bacteria, like cyanobacteria

• They convert carbon dioxide and water into glucose and oxygen using light energy

Chemoautotrophs

• Chemoautotrophs use energy from exothermic inorganic chemical reactions to produce organic compounds through chemosynthesis

  • Examples include: some bacteria and archaea, especially those found in extreme environments, such as hydrothermal vents or deep-sea ecosystems

• They can oxidise substances like hydrogen sulphide or ammonia to obtain energy

  • For example, bacteria living near hydrothermal vents use sulphur compounds to produce energy in environments without sunlight

• The giant tube worm (Riftia pachyptila) lives on or near hydrothermal vents deep in the ocean

  • Giant tube worms have a symbiotic relationship with chemosynthetic bacteria living inside them

  • The bacteria use hydrogen sulphide from the vent and oxygen and carbon dioxide from the surrounding water to produce sugars through chemosynthesis

  • The tube worms rely on the bacteria for food because they lack a digestive system

  • This allows them to survive in extreme environments where light and traditional food chains do not exist

• Chemoautotrophs are the primary producers in these extreme environments

  • This means they create energy-rich compounds like sugars that support other organisms in these food webs

Types of heterotrophs

• Herbivores: obtain carbon compounds by consuming plants

• Carnivores: obtain carbon compounds by consuming other animals

• Omnivores: obtain carbon compounds by consuming both plants and animals

• Decomposers: obtain carbon compounds by breaking down dead organic material, returning nutrients to the ecosystem

  • Examples include: fungi, bacteria, and earthworms