CIE IGCSE Biology: 复习笔记：19.1.5 Nutrient Cycles

• 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

• 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

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