IB DP Biology: SL复习笔记4.2.3 Carbon Cycle: Methane


  • Methane (CH4) is a simple hydrocarbon
  • It is present as a gas in the atmosphere, and underground, and is the main component of natural gas fossil fuel
  • Methane can be produced by the naturally occurring process known as methanogenesis by organisms known as methanogens
    • Before methanogenesis can occur, a series of bacteria convert organic matter into a compound called acetate, as well as carbon dioxide and hydrogen
    • A group of single-celled organisms called archaeans then carry out methanogenesis via two different mechanisms:

CO2 + 4H2 → CH4 + 2H2O


    • CH3COOH is acetic acid, a compound that can be formed from acetate
  • Archaeans carry out methanogenesis in a range of environments
    • Waterlogged mud
      • E.g. in naturally occurring wetlands, or in man-made rice fields
    • The guts of ruminant mammals such as cattle
    • Landfill sites containing organic matter such as food waste
    • Anaerobic digesters used for the break down of organic waste
  • Depending on the location of methanogenesis, the methane is either released directly into the atmosphere or into the surrounding soil
    • Methane can accumulate in the ground, but it can gradually make its way to the surface where it is released into the atmosphere
  • Human activities such as livestock farming and landfill disposal of waste food are leading to an increase in the release of methane into the atmosphere
    • Note that anaerobic digesters are sealed units for the production of biogas, and the methane produced is collected and used for fuel

Exam Tip

Note that you don't need to know the chemical processes by which methane is produced. Make sure that you know that methane is produced by methanogenic archaeans and released into the atmosphere or into underground stores.

Oxidation of Methane

  • The lifetime of methane gas once it reaches the atmosphere is around 10-12 years
  • This is because methane is oxidised in the atmosphere
    • This oxidation involves a reaction with molecules called hydroxyl radicals (·OH)
      • Hydroxyl radicals are highly reactive molecules that form in the presence of sunlight from other gases in the atmosphere e.g. nitrous oxides
    • Hydroxyl radicals react with methane in a series of reactions that produce carbon dioxide and water
  • Methane oxidation keeps levels of methane in the atmosphere relatively constant
    • Recent increases in atmospheric methane have led to concern that levels of hydoxyl radicals in the atmosphere may be declining