OCR A Level Biology

Revision Notes

6.5.5 The Carbon Cycle

The Carbon Cycle

The role of organisms in the Carbon Cycle

  • Carbon is constantly being recycled around the biosphere so that the number of carbon atoms in the biosphere is essentially constant; carbon atoms merely swap from one compound to another by the various processes in the carbon cycle
  • Carbon is stored in various forms:
    • In the atmosphere (as CO2)
    • In sedimentary rocks
    • In fossil fuels like coal, oil, and gas; coal is almost pure carbon
    • In soil and other organic matter
    • In vegetation (e.g. as cellulose)
    • Dissolved in the oceans (as CO2)


  • Autotrophs use the energy of sunlight to ‘fix’ carbon dioxide, turning its carbon into sugars and other organic molecules
  • This removes carbon from the atmosphere
  • The Calvin cycle is where CO2 is fixed, by the enzyme Rubisco, which carboxylates RuBP
  • Terrestrial plants use gaseous CO2 directly from the air
  • Aquatic organisms use CO2 dissolved in water
  • As much CO2 is fixed from ocean microorganisms, as from terrestrial plants


  • Plants that die are not fully decomposed by saprobionts; their bodies form layers of sediment that can accumulate over millions of years, locking carbon into the ground
  • This sediment is a store of energy and can form fossil fuels like peat and coal
  • Aquatic organisms that die also form sediments on the sea bed; these can go on to form other fossil fuels like oil and gas
  • Shells and other calcium-containing body parts can form sedimentary rocks such as limestone
  • The existence of life forms over billions of years has shaped the biosphere, in that their remains are still being recycled


  • All life forms respire, including autotrophs
  • Heterotrophs rely on respiration for all their energy needs
  • Respiration puts CO2 into the atmosphere, in the opposite direction to photosynthesis
  • CO2 is released in the Link Reaction and the Krebs Cycle of aerobic respiration
  • Anaerobic respiration also releases CO2 into the atmosphere, via fermentation by yeast, moulds and bacteria


  • Carbon is passed from autotroph to heterotroph during feeding
  • Carbon is also passed from primary consumer to secondary consumer
  • Biomass transfer always includes the transfer of carbon, the main element in biomass

Decay & Decomposition

  • Dead plants and animals are fed upon by detritivores and decayed by saprophytes
    • Releasing carbon into the surroundings
    • Supplying carbon to the detritivores
    • Supplying carbon to the saprophytes
  • Waste matter such as faeces and urine is used by decaying saprobionts
  • Such processes can release CO2 back into the air

The Roles of Organisms in the Carbon Cycle_1. downloadable AS Level & A Level Biology revision notes

The roles of organisms in the carbon cycle

Burning fossil fuels

  • Since the mid 19th century, humans have extracted and burned increasing amounts of fossil fuels from the Earth
  • CO2 is being returned to the atmosphere faster than it can be absorbed by plants and aquatic producers
  • The CO2 level in the atmosphere is approximately double that of 800,000 years ago
  • Warmer temperatures mean that less CO2 can be dissolved in the oceans, so is released into the air
  • This has caused dramatic climate change and affected many other species, mainly through changing habitats

Join Save My Exams

Download all our Revision Notes as PDFs

Try a Free Sample of our revision notes as a printable PDF.

Join Now
Already a member?
Go to Top