Material cycles — Edexcel GCSE Combined Science

Revision notes

4 short notes, in the order of the specification. Each one in short:

1. Cycling of materials

   Materials such as carbon, nitrogen and water are cycled through an ecosystem, moving between abiotic components (air, water and soil) and biotic components (living organisms). Plants build materials into biomass, which passes to animals when they feed. Decomposers break down dead organisms and waste, returning materials to be used again. Cycling is needed because supplies are limited.

2. The carbon cycle

   The carbon cycle returns carbon to the atmosphere as carbon dioxide, so plants can use it again in photosynthesis. Plants and algae take in carbon dioxide by photosynthesis, and carbon compounds pass along food chains. Respiration by all organisms, including decomposers breaking down dead material, and combustion of fuels release carbon dioxide back into the air.

3. The water cycle and potable water

   The water cycle keeps supplying fresh water to the land through evaporation, transpiration, condensation and precipitation. Potable water is water that is safe to drink. In areas of drought, desalination removes salt from sea water by distillation or by reverse osmosis through a partially permeable membrane. Desalination needs large amounts of energy, so it is expensive.

4. The nitrogen cycle

   The nitrogen cycle makes nitrates available to plants, which need them to make amino acids and proteins. Decomposers turn proteins and urea into ammonia, nitrifying bacteria convert ammonium ions into nitrates, and nitrogen-fixing bacteria turn nitrogen gas into nitrogen compounds. Denitrifying bacteria turn nitrates back into nitrogen gas. Farmers add fertilisers or use crop rotation.