7.5. The Carbon Cycle

The Carbon Cycle.001



The atmosphere

The carbon dioxide content in the atmosphere is slightly more than 0,04 %

The carbon cycle.The carbon cycle.


Plants, algae and som bacteria "harvest" sunlight. Carbon is assimiliated in glucose.

Word formula:

  • Carbon dioxide + water + light → glucose + oxygen

Chemical formula:

  • 6CO2 + 6H2O + hν → C6H12O6 + 6O2

Cellular respiration

Animals eat plants.

Glucose is used (in animals as well as plants):

  • As building material in the cell.
  • As energy reserve: It is oxidized (combusted), and energy is released. The animal "harvests" the energy released.

Word formula:

  • Glucose + oxygen → carbon dioxide + water + energy

Chemical formula:

  • C6H12O6 + 6O2 → 6CO2 + 6H2O + energy

The carbon dioxide is exhaled, returned to the atmosphere.


Animals, plants die.

  • Decomposers break down dead organisms.
  • Carbon dioxide is returned to the atmosphere through cellular respiration.

Fossil fuels

Dead organisms may be packed under high pressure under millions of years.

  • Oil, coal, natural gas is formed (fossil fuels)

When combusted, for example in combustion engines, carbon dioxide forms again, and is returned to the atmosphere.


Carbon dioxide may be dissolved in water. Carbonic acid, H2CO3, is formed:

  • CO2 + H2O → H2CO3

Carbonic acid gives rise to carbonates:

  • Hydrogen carbonate, HCO3
  • Carbonate, CO32–

Carbonates, spec. calcium carbonate (CaCO3) is used in animals' shells and corals.

  • Shells & corals may pack under high pressure and long time ⇒ Limestone

Limestone weathers:

  • Carbonates are re-formed.
  • Carbon dioxide may be returned to the atmosphere.

Limestone is important for the ecology on land!