Edexcel IGCSE Biology: Double Science 复习笔记 4.3.1 Water Cycle & Carbon Cycle

Edexcel IGCSE Biology: Double Science 复习笔记 4.3.1 Water Cycle & 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




  • 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.

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



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.