AQA GCSE Biology

Revision Notes

7.2.3 How Materials are Cycled

Recycling in an Ecosystem

  • Many different materials cycle through the abiotic and biotic components of an ecosystem
  • All materials in the living world are recycled to provide the building blocks for future organisms

Carbon Cycle

  • Elements such as carbon are not endless resources
  • There is a finite amount of each element on the planet
  • Elements need to be recycled in order to allow new organisms to be made and grow

The main processes of the carbon cycle

  • Carbon is taken out of the atmosphere in the form of carbon dioxide by plants and algae during photosynthesis (the carbon is used to make glucose, which can be turned into carbohydrates, fats and proteins – biomass – within plants and algae)
  • This carbon is passed on to animals (and microorganisms) when they feed on plants and algae
  • It is returned to the atmosphere in the form of carbon dioxide, during respiration by plants, animals and microorganisms (that release CO2 during decomposition)
  • If animals and plants die in conditions where decomposing microorganisms are not present, the carbon in their bodies can be converted, over millions of years and significant pressure, into fossil fuels
  • When wood or fossil fuels are burned (a process is known as combustion), the carbon within them combines with oxygen and carbon dioxide is released into the atmosphere

The carbon cycle, downloadable IGCSE & GCSE Biology revision notes

The carbon cycle

Exam Tip

The main parts of the carbon cycle involving living organisms are:

  • Carbon is taken out of the atmosphere by producers via photosynthesis
  • It is passed on to animals and decomposers by feeding
  • It is returned to the atmosphere by respiration; in plants, in animals and in decomposing microorganisms

Don’t forget – photosynthetic organisms like plants respire too, returning carbon dioxide to the atmosphere!

In addition, carbon dioxide is returned to the atmosphere by combustion of wood and fossil fuels.

You should be able to identify what each arrow represents in any diagram of the carbon cycle.

Water Cycle

  • The water cycle provides fresh water for plants and animals on land before draining into the seas
  • Water is continuously evaporated and precipitated
  • Water molecules move between various locations – such as rivers, oceans and the atmosphere – by specific processes:
  • Water enters the atmosphere as water vapour in one of two processes
    • Energy from the Sun heats the Earth’s surface and water evaporates from oceans, rivers and lakes
    • Transpiration from plants releases water vapour into the air
  • The warmer air of the lower atmosphere rises, taking the water vapour with it
    • The moist air cools down as it rises
    • Water vapour condenses back into liquid water, forming clouds
  • Water returns to Earth in the form of precipitation
    • As the water droplets in the cloud get bigger and heavier, they begin to fall as rain, snow and sleet
    • This is called precipitation

The-water-cycle, IGCSE & GCSE Biology revision notes

The water cycle

Main processes of the water cycle table

Main processes of the water cycle, downloadable IGCSE & GCSE Biology revision notes

Role of Microorganisms

  • Microorganisms play a vital role in cycling material through an ecosystem; they return carbon to the atmosphere as carbon dioxide and mineral ions to the soil
    • When living organisms produce waste products or organisms die, the waste products and dead organisms are broken down (digested) by microorganisms known as decomposers
    • Bacteria and fungi are the main groups of decomposers
  • Decomposition (also known as decay) is the process by which waste products or dead organisms are broken down and 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, carbon is returned to the atmosphere in the form of carbon dioxide

Author: Jenna

Jenna studied at Cardiff University before training to become a science teacher at the University of Bath specialising in Biology (although she loves teaching all three sciences at GCSE level!). Teaching is her passion, and with 10 years experience teaching across a wide range of specifications – from GCSE and A Level Biology in the UK to IGCSE and IB Biology internationally – she knows what is required to pass those Biology exams.
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