4.1 Essential ideas

4.1.3 Carbon cycling

Energy flows and nutrients cycle. Carbon cycles through ecosystems in two general forms: as inorganic free carbon dioxide (CO2) and as fixed organic carbon compounds, especially carbohydrates like glucose, C6H12O6, or gases like methane, CH4. Further detail on the carbon cycle is given below:

Figure 4.1.3a – Carbon cycle
A carbon store is so named because these are places where carbon accumulates and is stored, either long term (i.e. in the lithosphere), or short term (i.e. in biomass).

Major processes of the carbon cycle

  1. Carbon dioxide makes up about 0.04% of the atmosphere.
  2. It is also present in aquatic ecosystems, dissolved in gaseous form, or as hydrogen carbonate ions (HCO3-).
  3. CO2 diffuses from the atmosphere (or from water) into the bodies of autotrophs, such as plants or cyanobacteria. These organisms convert inorganic carbon dioxide into organic carbon compounds, such as carbohydrates.
  4. Under anaerobic conditions, organic carbon compounds are digested by bacteria to produce methane gas. Methane accumulates in the ground or is diffused into the atmosphere.
  5. When exposed to oxygen, methane is oxidised to form carbon dioxide and water, which is returned to the atmosphere or water bodies by diffusion.
  6. In aerobic conditions, organic carbon compounds are digested by other heterotrophs, especially decomposers, to produce carbon dioxide, a product of respiration.
  7. A portion of the digested organic compounds is incorporated into the bodies of organisms in various forms. For example, reef-building corals and molluscs have shells composed of calcium carbonate (CaCO3).
  8. Organic matter that is not fully decomposed is converted to peat if it is exposed to anaerobic or acidic conditions in waterlogged soils.
  9. Partially decomposed organic matter is converted over long periods of time to form gas, oil and coal that accumulate in porous rocks. Hard-bodied organisms such as molluscs and corals become fossilised in limestone.
  10. Carbon dioxide is also added to the atmosphere by human activity, especially burning fossilised matter, coal, oil, gas, and biomass (i.e. fossil fuels).

Activity 1

1a: Annotate your diagram

  • Download the Carbon cycle activity (PDF) and print this out.
  • Annotate your diagram using the numbers 1-10 to indicate where each of the corresponding processes occurs.
  • Check your answers by scrolling over yellow dots on Figure 4.1.3a above.

1b: Skill - Practise drawing a simplified diagram of the carbon cycle

  • Place carbon compounds in boxes and use arrows to represent processes.
  • Label arrows with these processes: photosynthesis, carbon fixation, feeding, death, combustion, oxidation, cellular respiration, anaerobic respiration, decomposition, fossilisation.
  • Check your answers >

Carbon flux

There are two types of process involved with the carbon cycle:

  1. Those that remove carbon dioxide from the atmosphere and oceans, including photosynthesis and fossilisation.
  2. Those that add carbon dioxide to the atmosphere and oceans, especially cellular respiration and combustion of fossil fuels.

The amount of carbon that is transferred by different processes is called carbon flux.

carbon fluxesFigure 4.1.3b – Annual carbon fluxes in natural systems (green arrows) and anthropogenic systems (red arrows)
One gigatonne is equal to one billion metric tonnes.

Activity 2

Using information from Figure 4.1.3b:

  • List the carbon stores in order of decreasing size
  • Estimate the amount of carbon that accumulates as a result of human activities
  • Comment on the balance of carbon flux in the natural systems compared with the human systems.

Course link

Learn about cycling of two other important nutrients, and the vulnerability of agricultural soils in 14.1.6 Nitrogen and phosphorus cycles.

Language note

  • Bacteria that produce methane gas from carbon dioxide are called methanogenic archaeans. Some methanogenic bacteria can metabolise other carbon compounds as well.
  • When carbon is converted from a gaseous form into more complex organic compounds, this is called carbon fixation. Once it’s fixed, carbon remains fixed until it is released as gas, either carbon dioxide or methane. 

methanogenic bacteriaFigure 4.1.3c – Methanogenic archaeans have recently been found at Mount Everest base camp1
Scientists think that melting ice caps on Everest are to blame for uncovering this source of greenhouse gas. In this photo, you can see methane gas bubbles accumulating under the ice.

Study tip

There are many more processes that put carbon dioxide into the atmosphere, like rising ocean temperatures, deforestation and increased weathering of limestone due to acid rain. You do not need to include them all in your drawing of the carbon cycle. The processes listed above are sufficient.

weathering limestoneFigure 4.1.3d – Weathering limestone
This limestone has a distinctive weathering pattern. As limestone weathers naturally, it releases carbon dioxide into the atmosphere.

International mindedness

Biofuels are made with what have been traditionally food crops, such as maize and cane sugar. What are the ethical implications of diverting food crops for fuel? Are biofuels an ecological alternative to fossil fuels?

Nature of Science

In order to understand the carbon cycle, making accurate, quantitative measurements of atmospheric greenhouse gases is very important for reliability of the evidence.


  1. "Psychrophilic Methanogenic Bacteria Found at Everest Base Camp: What Are the Implications for Global Warming?" Solar Cities. N.p., 2 June 2011. http://solarcities.blogspot.jp/2011/06/psychrophilic-methanogenic-bacteria.html