Edexcel IGCSE Biology · Spec 4.10
The Carbon Cycle
How carbon moves through the environment via photosynthesis, respiration, decay and combustion.
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The Carbon Cycle
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Explained
The carbon cycle, and the one process that runs the other way
Four processes move carbon around, and three of them do the same thing. Respiration, combustion and decomposition all put carbon dioxide into the air. Only photosynthesis takes it out. If you remember nothing else from this topic, remember that asymmetry, because almost every question about rising carbon dioxide depends on it.
Where the carbon actually goes
Carbon does not disappear and it is not created. It moves between stores: the air, living things, dead material, the oceans and fossil fuels. A cycle diagram is really a map of those stores with arrows showing what moves carbon from one to another.
- Photosynthesis takes carbon dioxide from the air and fixes the carbon into glucose in plants.
- Respiration releases energy from glucose and returns carbon dioxide to the air.
- Combustion burns fuels, including fossil fuels, and releases carbon dioxide.
- Decomposition breaks down dead material, and the decomposers release carbon dioxide through their own respiration.
That last one catches people out. Decomposition does not release carbon dioxide by some separate route. It releases it because the decomposers are respiring, exactly as you do.
Why fossil fuels change the balance
Coal, oil and gas formed over millions of years from the remains of organisms. While that carbon sits underground it is out of circulation, which is what makes it a store. Burning it puts carbon back into the air far faster than photosynthesis and the oceans can take it out.
A carbon sink is any store that absorbs more carbon than it releases. Forests and oceans are the two usually named. Cutting forests down does two things at once: it removes carbon dioxide absorption, and burning the timber releases what was already stored.
What examiners say about this topic
Principal examiner reports for Edexcel International GCSE Biology show that the marks in this area often sit in experimental work rather than in the diagram. One question asks candidates to give the energy conversion during photosynthesis, where the credited answer names sunlight and chemical energy.
Another asks learners to predict the result when carbon dioxide is unavailable. The reports note that many correctly explained that no starch would be present, so iodine would stay orange, because there was no carbon dioxide for photosynthesis. The reasoning chain is what earns the marks: no carbon dioxide, no photosynthesis, no starch, no colour change.
Drawing and reading the cycle
When you are asked to complete a cycle diagram, work out which direction each arrow must point before you write anything. An arrow into the air is respiration, combustion or decomposition. An arrow out of the air is photosynthesis, and there is only one of those.
If you are asked how a change affects carbon dioxide levels, say which process it speeds up or slows down, then say which direction that moves carbon. Answers that name a process without saying which way the carbon travels rarely get full credit.
Spec 4.10
What you need to know
- Describe how carbon moves through the cycle
- Name the key processes
- Explain how humans raise CO2 levels
Active recall
Quick check
Answer each question before opening the answer.
Which processes add carbon dioxide to the atmosphere?
Respiration, combustion (burning) and decomposition.
Which process removes carbon dioxide from the atmosphere?
Photosynthesis.
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