Biology - The Carbon Cycle (AQA Syllabus A)

In GCSE Science, students will look at energy and biomass in food chains. This is the last of three quizzes on that topic and it looks in particular at the carbon cycle.

It is quite possible that your body contains carbon atoms that were once breathed by a dinosaur. Or perhaps eaten by a king or queen or even excreted in dog poo. Yeuch! But how is this possible? The answer is one of the most important cycles for life - the carbon cycle. Our planet has only a limited stock of chemical elements and they are constantly being recycled in different ways. If they weren't, they would all have been used up long ago and the Earth would be an unchanging dead planet.

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When looking at the carbon cycle, it doesn't really matter where you start. A good place to start though is with the carbon dioxide in the air. This is absorbed by plants and used by them to produce biomass. The carbon in the plants then enters into food chains. When a plant is eaten by an animal, the carbon is passed on. The animal may use the carbon from the plant to make its own biomass, in which case it will be locked up until the animal dies. It could be used in respiration to generate energy, in which case, the carbon ends up back as carbon dioxide in the air. If the carbon is excreted as waste material, the decomposers will either release it back into the air as carbon dioxide straight away or lock it up inside their bodies as biomass.

And so it goes on, constantly being exchanged between plants, animals and the air. But food chains are not the only places where carbon is stored. It can even be locked away in rocks like limestone and chalk, only to be released millions of years later when molten magma touches them. Or it can be fossilised, as coal or oil, and released when it is burnt to generate energy.

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  1. What process takes carbon dioxide from the air and makes it available to land living animals?
    Plants use CO2 from the air in the process of photosynthesis. This same is done in the sea by phytoplankton
  2. Which two natural processes return carbon dioxide to the air?
    Other natural processes include volcanic action and wildfires started by lightning strikes
  3. Which human activity returns carbon dioxide to the air?
    Anything we do that involves the use of burning fossil fuels puts carbon dioxide back into the air
  4. Why do fossil fuels have a high carbon content?
    Life as we know it is based on carbon
  5. Which of the following is a problem caused by humans burning fossil fuels?
    Some scientists believe that global warming is not caused by human activities
  6. How might burning fossil fuels be creating global warming?
    The natural carbon cycle is always balanced with lots of carbon locked away in plants, animals, rocks and fossil fuels. Only a certain amount is found in the air. Burning fossil fuels artificially increases the CO2 in the air, putting things out of balance. If we put too much CO2 into the air we could get a runaway greenhouse effect, in which case, most life on Earth would perish
  7. How is carbon in plants passed on to the rest of the cycle?
    When dead plants form coal, carbon is locked out of the cycle for millions of years
  8. Carbonate rocks lock carbon out of the cycle for millions of years. Which of the following does not put CO2 from these rocks back into the current carbon cycle?
    All three ways involve a lot of heating. This decomposes the carbonate rocks, releasing carbon dioxide. Grinding is a physical change only
  9. Why do waste materials and dead plants or animals decay?
    They are also broken down as they are eaten by detritus feeders e.g. woodlice eating dead leaves
  10. How is the CO2 from the air introduced into a food web?
    When the plant is eaten, these are passed to the herbivores in the food web

Author: Kev Woodward

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