Revision Notes

19.4 Nutrient Cycles

The Carbon Cycle

  • Nutrients such as carbon and nitrogen are not endless resources
  • There is a finite amount of each element on the planet and as such, 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) by 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
  • In addition, mass deforestation is reducing the amount of producers available to take carbon dioxide out of the atmosphere by photosynthesis
  • This problem is exacerbated by the fact that in many areas of the world, deforestation is taking place for land rather than for the trees themselves, and as such they are burnt down, releasing yet more carbon dioxide into the atmosphere

The carbon cycle, IGCSE & GCSE Biology revision notesThe Carbon Cycle

Exam Tip

The carbon cycle is simple:

  • Carbon is taken out of the atmosphere by photosynthesis
  • It is passed on to animals and decomposers by feeding
  • It is returned by respiration; in plants, in animals and in decomposing
  • microorganisms
    In addition, it is returned (in increasing amounts) by combustion of fossil fuels

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

The Water Cycle

  • Water molecules move between various locations – such as rivers, oceans and the atmosphere – by specific processes
  • This is possible because water changes state at a relatively low temperature


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

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

Exam Tip

Make sure you can identify each of these water cycle processes on a diagram as this is a common multiple choice question.

Extended Only

The Nitrogen Cycle

  • Nitrogen as an element is required to make proteins
  • Neither plants nor animals can absorb it from the air as N2 gas is very stable and the bonds holding the nitrogen atoms together would need massive amounts of energy to break (the two nitrogen atoms in a nitrogen molecule are held together by a triple covalent bond)
  • However, there are two ways it can be taken out of the air and converted into something easier to absorb:
    • Nitrogen fixing bacteria found ‘free living’ in soil and also in the root nodules of certain plants (peas, beans, clover – we call them leguminous plants) take N2 gas and change it into nitrates in the soil
    • Lightning can ‘fix’ N2 gas, splitting the bond between the two atoms and turning them into nitrous oxides like N2O and NO2 that dissolve in rainwater and ‘leach’ into the soil
  • Plants absorb the nitrates they find in the soil and use the nitrogen in them to make proteins
  • Animals eat the plants (or other animals) and get the nitrogen they need from the proteins in the plant or animal
  • Waste (urine and faeces) from animals sends nitrogen back into the soil as ammonium compounds (the urea in urine contains nitrogen)
  • When the animals and plants die, they decay and all the proteins inside them are broken down into ammonium compounds and put back into the soil by decomposers
  • The plants can’t absorb ammonium compounds though, so a second type of soil bacteria, nitrifying bacteria, convert the ammonium compounds to nitrites and then to nitrates, which can then be absorbed by plants – and so the cycle goes on
  • Finally, there is a third, unhelpful type of (anaerobic) bacteria called denitrifying bacteria found in poorly aerated soil (ie not much oxygen)
  • These bacteria take the nitrates out of the soil and convert them back into N2 gas
  • Farmers can help reduce the amount of these unhelpful bacteria by ploughing and turning over soil

The nitrogen cycle, IGCSE & GCSE Biology revision notes The Nitrogen Cycle

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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