Carbon Cycle: Carbon Dioxide in the Atmosphere (DP IB Biology: SL)

• All living cells carry out some form of cellular respiration, from bacteria, to plants, to animals
• Many organisms carry out aerobic respiration
• Aerobic respiration requires the uptake of oxygen and produces carbon dioxide as a waste product

Aerobic respiration produces carbon dioxide as a waste product

• Because carbon dioxide is being constantly produced inside cells, a concentration gradient between the inside and the outside of cells is maintained
  • Carbon dioxide leaves cells by diffusion

• This carbon dioxide is eventually released into the environment; either air or water
  • Single-celled organisms release carbon dioxide by diffusion at the cell surface
  • Terrestrial plants release carbon dioxide by diffusion from their stomata into the surrounding air
  • Animals release carbon dioxide into the surrounding air or water by diffusion via their gas exchange surfaces e.g. mammalian lungs or fish gills

NOS: Making accurate, quantitative measurements; it is important to obtain reliable data on the concentration of carbon dioxide and methane in the atmosphere

• The amount of carbon in the atmosphere is constantly changing due to seasonal fluctuations in rates of photosynthesis and due to human activities
  • Photosynthesis removes carbon dioxide from the atmosphere, meaning that atmospheric carbon dioxide levels decrease in whichever hemisphere is experiencing spring and summer
  • The combustion of fossil fuels by humans releases carbon dioxide into the atmosphere
  • Livestock such as cattle release methane into the atmosphere

Changes in atmospheric carbon dioxide levels measured at the Mauna Loa Observatory (ppmv = parts per million by volume).  The yearly fluctuations shown in red are due to seasonal changes in photosynthesis rates.

• Carbon can be returned to the atmosphere by the burning of fossil fuels and organic material; a process known as combustion
  • Complete combustion releases carbon dioxide and water as byproducts

• Fossil fuels form when animals and plants die in conditions where decomposing microorganisms are not present; the carbon in their bodies is converted, over millions of years and with significant pressure, into fossil fuels such as coal and oil
  • The combustion of fossil fuels releases carbon that has been stored for millions of years
  • Increased use of fossil fuels is contributing to an increase in the carbon dioxide content of the atmosphere

• Organic material, or biomass, burns when fires occur in e.g. forests or grasslands
  • Such fires can have natural causes e.g. lightning hitting hot, dry ground, or can be set by humans e.g. when clearing land for the purpose of farming
  • Biomass can also be burned as a fuel in e.g. wood fires or biomass boilers