The Carbon Cycle (OCR Gateway GCSE Biology: Combined Science)

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The Carbon Cycle

  • Nutrients such as carbon and nitrogen are not endless resources, and so, they need to be recycled in order to allow new organisms to be made and grow 
  • Carbon is taken out of the atmosphere in the form of carbon dioxide by plants to be used for photosynthesis
  • It is passed on to animals (and microorganisms) via feeding
  • It is returned to the atmosphere in the form of carbon dioxide by plants, animals and microorganisms as a result of respiration
  • 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 fossil fuels are burned (the process is known as combustion), the carbon combines with oxygen and carbon dioxide is released into the atmosphere
  • Increased use of fossil fuels is contributing to an increase in the carbon dioxide content of the atmosphere
    • This problem is exacerbated by the fact that in many areas of the world, deforestation is taking place for land (for livestock grazing) rather than for the trees themselves
    • As such, trees are burnt down, releasing yet more carbon dioxide into the atmosphere
    • In addition, mass deforestation is reducing the amount of producers available to take carbon dioxide out of the atmosphere by photosynthesis

The carbon cycle, IGCSE & GCSE Biology revision notes

The carbon cycle

The importance of the carbon cycle

  • Carbon is a component of all organic molecules, many of which are essential for life on earth, such as glucose
  • The carbon cycle is a key process which allows the movement of carbon between global reservoirs including the
    • Atmosphere
    • Biomass
    • Oceans
    • Soil
  • The balance of carbon between these reservoirs is fundamental due to the role that carbon plays in sustaining life
    • If the balance is disrupted, somewhere along the way organisms will suffer
    • Global warming is a large scale example of the effect which may be seen if the global carbon balance is disrupted

Exam Tip

The carbon cycle is quite straightforward if you think about the story of a single carbon atom since the beginning of the universe:

  • For billions of years the carbon atom would have been combined with oxygen as CO2
  • At the beginning of life, it might have found itself part of a sugar molecule by photosynthesis
  • An organism would have returned the carbon atom to the atmosphere as CO2 by respiration
  • This cycle would have repeated thousands of times 
  • The carbon may have become locked away as coal as a tree decomposed
  • Millions of years later, it is returned by the combustion of the coal
  • ....and the process repeats again...and again...

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

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Author: Phil

Phil has a BSc in Biochemistry from the University of Birmingham, followed by an MBA from Manchester Business School. He has 15 years of teaching and tutoring experience, teaching Biology in schools before becoming director of a growing tuition agency. He has also examined Biology for one of the leading UK exam boards. Phil has a particular passion for empowering students to overcome their fear of numbers in a scientific context.