The Carbon Cycle (AQA GCSE Combined Science: Synergy: Life & Environmental Sciences): Revision Note

Cycling of Materials

Carbon stores

• The element carbon is found in many forms on Earth:

  • As carbon dioxide (CO₂) in the atmosphere

  • Dissolved in the water of the oceans

  • As calcium carbonate (CaCO₃) in sea shells and limestone rocks

  • In fossil fuels such as coal, oil and natural gas

  • As carbohydrates and other large molecules in all living organisms

Carbon cycle processes

• Carbon cycles through the environment by the following processes:

  • Photosynthesis

  • Respiration

  • Combustion

  • Decay

  • Industrial uses of limestone

Photosynthesis

• Plants and algae absorb CO₂ from the atmosphere and convert it to carbohydrates

• Photosynthesis uses carbon dioxide and water to produce glucose and oxygen in the presence of chlorophyll and light

• The equation for photosynthesis is:

6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

Respiration

• Respiration is the production of energy in living things

• During this process, animals, plants and microorganisms release CO₂ back into the atmosphere

• The equation for respiration is:

C₆H₁₂O₆ + 6O₂ → 6CO₂  + 6H₂O 

Combustion

• Combustion involves burning wood, fossil fuels and other fuels to release the energy contained within them

• During this process, CO₂ is released back into the atmosphere

• An example equation is the combustion of methane:

CH₄ + 2O₂ → CO₂ + 2H₂O 

Decay

• During decay, waste products or dead organisms are broken down by microorganisms

  • The materials they contain (such as carbon and mineral ions such as iron, magnesium and nitrates) are returned to the environment

• When decomposing microorganisms respire, CO₂ is returned to the atmosphere

Industrial uses of limestone

• Limestone is calcium carbonate, CaCO₃

• One of its most important industrial uses is making cement and concrete

• During these processes, limestone is heated and releases CO₂ to the atmosphere:

CaCO₃ → CaO + CO₂

The Carbon cycle

Biological Processes in the Carbon Cycle

• Life depends on photosynthesis in green plants, which make carbohydrates from carbon dioxide in the air

• Animals feed on plants, passing carbon compounds along food chains

• Animals and plants respire, releasing carbon dioxide back into the atmosphere

• When organisms die, microorganisms decompose the dead material

  • Microorganisms such as bacteria and fungi act as decomposers

• As decomposers respire, carbon is returned to the atmosphere as carbon dioxide and mineral ions are returned to the soil