The Water, Carbon and Nitrogen Cycles

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The Water Cycle

  • Heat from the sun makes water evaporate from the land and sea, turning it into water vapour. Water also evaporates from plants - this is known as transpiration
  • The warm water vapour is carried upwards (as air rises). As it gets higher up, the water vapour cools and condenses to form clouds
  • Water falls from the clouds as precipitation (usually rain, but sometimes snow or hail) and is returned to the land and sea


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The Carbon Cycle

1)     Carbon is taken in by plants through photosynthesis to make carbohydrates, fats and proteins

2)     Eating passes the carbon compounds in the plant along to animals in a food chain or web

3)     Both plants and animals respire while the organisms are alive which releases CO2 back into the air

4)     When plants and animals die and are decomposed, they’re broken down by bacteria and fungi. These decomposers release CO2 back into the air by respiration as they too are alive

5)     Some useful plant and animal products, e.g. wood and fossil fuels, are burned (combustion). This also released CO2 back in the air

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The Carbon Cycle...(2)


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The Nitrogen Cycle

1) Nitrogen is needed for making proteins for growth

2) Plants get their nitrogen from the soil, so the nitrogen from the air has to be turned into nitrogen compounds (such as nitrates) before plants can use it. Animals can only get proteins by eating plants (or each other)

3) Nitrogen fixation is the process of turning N2 from the air into nitrogen compounds in the soil which the plants can use. There are two main ways this happens

  • LIGHTNING – there’s so much energy in a bolt of lightning that it’s enough to make nitrogen react with oxygen in the air to give nitrates
  • NITROGEN-FIXING BACTERIA in roots and soil

4) There are FOUR different types of bacteria involved in the nitrogen cycle:

  • DECOMPOSERS – break down proteins (in rotting plants and animals) and urea (in animal waste) and turn them into ammonia (a nitrogen compound)
  • NITRIFYING BACTERIA – turn ammonia in decaying matter into nitrates
  • NITROGEN-FIXING BACTERIA – turn atmospheric N2 into nitrogen compounds that plants can use
  • DENITRIFYING BACTERIA – turn nitrates back into N2 gas 


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The Nitrogen Cycle...(2)


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