Edexcel A (SNAB) A Level Biology:复习笔记5.3.9 Carbon Cycle & Reduction of Atmospheric Carbon Dioxide

• The atmospheric carbon dioxide that contributes to the greenhouse effect is part of the global carbon cycle
• The term carbon cycle refers to the many processes by which carbon is transferred and stored, e.g.
  • Carbon is found in the biomass of living organisms e.g. in carbohydrates and proteins
  • Carbon is transferred when one organism consumes and digests another
  • Carbon is found in the atmosphere as carbon dioxide and in the oceans as e.g. hydrogen carbonate ions
  • Carbon in fossil fuels is present in the form of hydrocarbons

The carbon cycle

• The following events occur during the carbon cycle
  • Carbon is present in the atmosphere in the form of carbon dioxide
  • Carbon dioxide is removed from the atmosphere by producers during photosynthesis
    • Producers incorporate carbon into their biomass in the form of carbohydrates and other biological molecules
  • Carbon is transferred to and between consumers as a result of feeding
  • Carbon is transferred back into the atmosphere by both plants and animals as a result of respiration
    • Respiration releases carbon dioxide as a product
  • Carbon dioxide can also be removed from the atmosphere by dissolving in the oceans
  • Dissolved carbon can be taken in by marine plants when they photosynthesise or by other marine organisms as they build calcium carbonate exoskeletons
  • When living organisms die their tissues are broken down by decomposers such as bacteria and fungi
    • When these organisms respire, they too release carbon dioxide back into the atmosphere
  • Any living tissue that is not fully decomposed can go towards the formation of peat or fossil fuels over millions of years; carbon can be stored in these sinks for long periods
  • The combustion of peat and fossil fuels releases carbon dioxide back into the atmosphere
    • The combustion of biomass such as wood also returns carbon to the atmosphere

The carbon cycle includes the locations in which carbon is stored, shown here as 'carbon pools', and the processes by which it is transferred, shown here as 'carbon fluxes'

• A good understanding of the carbon cycle is essential in the fight against global warming
  • It is possible to see the points at which carbon enters the atmosphere; reducing the carbon transfer at these points will prevent further increases in atmospheric carbon dioxide e.g.
    • Reducing the combustion of fossil fuels
    • Reducing the combustion of biomass
    • Reducing disturbance of carbon pools such as soils and peat bogs
  • We can also see the points at which carbon is removed from the atmosphere; increasing the transfers here could help to reduce the greenhouse effect e.g.
    • Increasing rates of photosynthesis by planting trees