Notesale: Turn your study into money

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Carbon cycling

Topic 4
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3

Biology SL

Methane
➢ Some organisms are capable of living in anaerobic conditions, such as
methanogenic archaeans
➢ When these methanogenic archaeans metabolise food, they produce methane
instead of carbon dioxide, as a waste gas
➢ They’re also common in wetlands where they produce marsh gas, which can
sometimes glow at night
➢ Methane forms slowly and large quantities can be trapped in the ground
➢ Methane is a very potent greenhouse gas
➢ It can be burnt in oxygen to form carbon dioxide
CH4 + 2O2 ⇒ 2H2O + CO2
➢ Methane is the main ingredient in the fossil fuel we call natural gas
➢ The carbon found in methane was borrowed from a carbon dioxide molecule that
was removed from the atmosphere millions of years ago during photosynthesis
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Methane formed and accumulated underground

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Through burning, the carbon is returned to the atmosphere as carbon
dioxide
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The problem is that the burning is very fast, but the formation very
slow

Peat

➢ Peat is an organic substance that consists of partially decomposed plant material
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It’s a heterogeneous mixture of many things

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At least 30% of its dry mass has to be composed of dead organic material

➢ Used as fossil fuel
➢ It’s a kind of waterlogged soil found in certain types of wetlands
➢ The low pH hinders decomposition
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Anaerobic conditions are created

○

Certain types of microorganisms grow, but microorganisms that would help
in the decomposition of plant material are prevented from growing

➢ It takes a long time to form and it isn’t considered to be a renewable source of
energy
➢ Wetlands are an important part of the ecosystem and a habitat for unique species
➢ Pollen trapped in deep layers of the bogs can provide evidence of what the climate
was like in the past

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Carbon cycling

Topic 4
...
3

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Hydrogen carbonate ions

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Calcium ions

Biology SL

➢ When combined, molecules of calcium carbonate are formed
2+

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Ca + 2HCO3 ⇒ CaCO3 + CO2 + H2O
➢ As these organisms die, their shells are deposited on the bottom of the sea and they
accumulate in sediments
○

When the sediments go through the process of lithification, they form
limestone

➢ The process of taking carbon out of the environment and “locking it up” in a
substance for an extended period of time is called carbon sequestration
○

When it happens naturally, it is called biosequestration
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This is one way in which balance is maintained in the carbon cycle

➢ When limestone is used, some of the carbon is released back into the atmosphere
as carbon dioxide cancelling out the biosequestration

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