The carbon cycle

• Carbon is constantly being recycled around the biosphere so that the number of carbon atoms in the biosphere is essentially constant; carbon atoms merely swap from one compound to another by the various processes in the carbon cycle

• Carbon is stored in various forms:

  • In the atmosphere (as CO₂)

  • In sedimentary rocks

  • In fossil fuels like coal, oil, and gas; coal is almost pure carbon

  • In soil and other organic matter

  • In vegetation (e.g. as cellulose)

  • Dissolved in the oceans (as CO₂)

Photosynthesis

• Autotrophs use the energy of sunlight to 'fix' carbon dioxide, turning its carbon into sugars and other organic molecules

• This removes carbon from the atmosphere

• The Calvin cycle is where CO₂ is fixed, by the enzyme Rubisco, which carboxylates RuBP

• Terrestrial plants use gaseous CO₂ directly from the air

• Aquatic organisms use CO₂ dissolved in water

• As much CO₂ is fixed from ocean microorganisms, as from terrestrial plants

Sedimentation

• Plants that die are not fully decomposed by saprobionts; their bodies form layers of sediment that can accumulate over millions of years, locking carbon into the ground

• This sediment is a store of energy and can form fossil fuels like peat and coal

• Aquatic organisms that die also form sediments on the sea bed; these can go on to form other fossil fuels like oil and gas

• Shells and other calcium-containing body parts can form sedimentary rocks such as limestone

• The existence of life forms over billions of years has shaped the biosphere, in that their remains are still being recycled

Respiration

• All life forms respire, including autotrophs

• Heterotrophs rely on respiration for all their energy needs

• Respiration puts CO₂ into the atmosphere, in the opposite direction to photosynthesis

• CO₂ is released in the Link Reaction and the Krebs Cycle of aerobic respiration

• Anaerobic respiration also releases CO₂ into the atmosphere, via fermentation by yeast, moulds and bacteria

Feeding

• Carbon is passed from autotroph to heterotroph during feeding

• Carbon is also passed from primary consumer to secondary consumer

• Biomass transfer always includes the transfer of carbon, the main element in biomass

Decay & Decomposition

• Dead plants and animals are fed upon by detritivores and decayed by saprophytes

  • Releasing carbon into the surroundings

  • Supplying carbon to the detritivores

  • Supplying carbon to the saprophytes

• Waste matter such as faeces and urine is used by decaying saprobionts

• Such processes can release CO₂ back into the air

Burning fossil fuels

• Since the mid 19^th century, humans have extracted and burned increasing amounts of fossil fuels from the Earth

• CO₂ is being returned to the atmosphere faster than it can be absorbed by plants and aquatic producers

• The CO₂ level in the atmosphere is approximately double that of 800,000 years ago

• Warmer temperatures mean that less CO₂ can be dissolved in the oceans, so is released into the air

• This has caused dramatic climate change and affected many other species, mainly through changing habitats

Methane

• Produced when decomposers break down dead remains of organisms, and when microbes in primary consumers digest food that has been eaten

• Levels have more than doubled in the past 150 years due to:

  • More extraction of fossil fuels

  • More decaying waste

  • More cattle

  • Natural stores such as frozen ground thawed

The greenhouse effect

1. Heat from the sun is reflected from the Earth’s surface

2. When it reaches the atmosphere some rays pass through

3. Other rays are trapped in by green house gases (carbon dioxide)