Organisms need carbon to make all sorts of biological molecules including lipids, carbohydrates, and proteins. Just like we saw with nitrogen and phosphorus, carbon is transferred around ecosystems in a cycle called the carbon cycle.
The carbon cycle
The Earth’s atmosphere is about 0.04% carbon dioxide (CO2), which is where we will start the carbon cycle. For CO2 gas to be removed from the atmosphere, it can be used in photosynthesis by plants (and other producers such as algae) to produce glucose and other carbon compounds. It can also be dissolved into oceans.
Carbon compounds in producers are consumed by animals, so the carbon is transferred to carbon compounds in animals. When plants and animals die and produce waste, the dead and waste material is decomposed by saprobionts. Saprobionts also contain carbon compounds. All living organisms carry out respiration and return CO2 to the atmosphere.
Some dead plant and animal material is compressed over millions of years to produced fossil fuels such as coal and oil, and also other types of rock which contain carbon (e.g. limestone contains a lot of calcium carbonate). Human activity means that fossil fuels are combusted, which releases CO2 back into the atmosphere. Volcanic activity can return carbon from rocks into the atmosphere as CO2, and the weathering process can cause rocks to release ions such as hydrogencarbonate ions (HCO3–) into oceans and other waterways. These ions can end up as calcium carbonate in rocks and marine organisms with shells. Finally, CO2 can be released from oceans back into the atmosphere.
- CO2 can be removed from the atmosphere by photosynthesis and dissolving into oceans.
- CO2 can enter the atmosphere through respiration (by producers, consumers and decomposers), combustion of fossil fuels, volcanic activity, and release from oceans.
- Carbon is needed by organisms to make molecules such as lipids, carbohydrates and proteins.