Carbon Cycle – Grade 9 Understanding for IGCSE Biology 4.10


The Carbon cycle should really be much simpler to understand than the Nitrogen cycle I posted about yesterday.  This is because the processes involved in moving carbon atoms from one compartment to the next in an ecosystem are more straightforward.  There are four processes mentioned in the specification and you need to make sure you understand each.

  • Photosynthesis:  only happens in producers, takes CO2 from the air to produce complex molecules (carbohydrates/proteins/fats) that can be passed up food chain.
  • Respiration:  happens in all organisms (producers, consumers, decomposers) and turns carbohydrates into carbon dioxide
  • Combustion: fossil fuels and plants can be burnt for fuel releasing carbon dioxide into the atmosphere
  • Decomposition:  two types of decomposition – in aerobic conditions decomposer organisms (bacteria/fungi) convert complex molecules in faeces/dead organisms into carbon dioxide:  in anaerobic conditions, dead organisms can be turned into fossil fuels.

If you want to draw a carbon cycle from scratch to check you understand it, follow the procedure below.

1) Draw the following boxes showing where carbon atoms are found in an ecosystem – CO2 in air, carbon compounds in plants, carbon compounds in animals, fossil fuels and limestone, detritus in soil

2) Draw arrows linking the boxes with the following labels:  photosynthesis, respiration, feeding, combustion, death and decay, death and no decay

That’s about as complicated as it gets.

Warning:  Do not under any circumstances draw an arrow from the detritus in the soil directly to plants.  Plants do not absorb any carbon containing molecules from the soil into their roots.  Honestly, please believe me they don’t however much you want them to….  It would make the cycle more straightforward but they don’t – sorry.  The only carbon-containing molecule plants absorb from their environment is CO2 and that as you all know is absorbed from the air in leaves in the process of photosynthesis.

Zondle quiz on carbon cycle to follow in due course:  keep working hard!  (I am jealous of those of you in Portugal at the moment although weather has been fine today in Northants!)


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