The carbon cycle
Key idea: Photosynthesis removes carbon dioxide from air; respiration and combustion return it. The atmospheric amount rises when release exceeds removal.
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Guided lessons · coming soon
Guided lesson
Guided steps
- Follow carbon atoms from one store to another.
- Compare carbon dioxide entering and leaving the atmosphere.
- Correct the photosynthesis arrow.
- Add the releases, then subtract the removal.
- Identify the immediate release and the lost removal process.
- Explain how clearing and burning a forest affect both transfers.
- Decide which plant processes continue in darkness and sunlight.
- Correct the claim about plant and animal respiration.
- Follow carbon from garden waste into the air.
- Write the respiration equation and state the energy transfer.
- Name both processes and decide when each can continue.
- Balance the transfers and decide whether the cycle stops.
- Explain why partial uptake does not cancel fossil-fuel emissions.
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ContinueFollow carbon between stores
By the end of this lesson you can:
- describe how photosynthesis, respiration and combustion move carbon into and out of the atmosphere;
- tell a carbon store from a process that moves carbon between stores;
- explain why atmospheric carbon dioxide stays steady when removal and release balance, and how burning fossil fuels and cutting down forests upset that balance.
You need two ideas from earlier topics. Carbon dioxide is about 0.04% of dry air. When a carbon-containing fuel burns completely, its carbon becomes carbon dioxide.
Stores and the processes that move carbon
Carbon atoms are not used up. They move between stores, places where carbon is held:
- the atmosphere, as carbon dioxide;
- living things, as the carbon compounds in plants and animals;
- fuels, such as coal, crude oil and natural gas, formed from the remains of living things over millions of years.
A process moves carbon from one store to another. This simplified model focuses on three processes:
| Process | What happens | Carbon moves |
|---|---|---|
| photosynthesis | green plants use carbon dioxide and water, with energy from light, to make glucose | from the atmosphere into plants |
| respiration | plants, animals and decomposers break down glucose to release energy | from living things into the atmosphere |
| combustion | fuels and wood burn in oxygen | from fuels into the atmosphere |
When plants and animals die, decomposers feed on them and respire, so decomposition returns their carbon to the atmosphere as well.
Of the three processes shown, photosynthesis takes carbon dioxide out of the air. Respiration and combustion put it into the air. The diagram shows part of the carbon cycle: it leaves out other stores and transfers, such as carbon dioxide dissolving in the oceans.
Plants belong to the living-organism store too: they respire as well as photosynthesise. Carbon also passes from plants to animals when animals feed.
When removal and release balance
Think of the atmosphere as a store with carbon dioxide entering and leaving. If the same amount enters as leaves over a period, the amount in the store stays steady. If more enters than leaves, the amount rises. The processes can continue in either case: a steady amount does not mean that the cycle stops.
In the simplified land-based model, photosynthesis removes carbon dioxide, while respiration, decomposition and combustion return it. This balance helps regulate atmospheric carbon dioxide; the amount stays steady only when release equals removal. The real carbon cycle also includes exchanges with the oceans.
Two human activities upset the balance:
- Burning fossil fuels releases carbon that was locked away underground for millions of years. It adds an extra source of carbon dioxide. Plants and oceans take up some of this extra carbon, but not all of it, so the amount in the atmosphere rises.
- Cutting down forests (deforestation) leaves fewer trees to photosynthesise, so less carbon dioxide is removed. If the trees are burned, their carbon is also added to the air.
Both make the amount of carbon dioxide in the atmosphere rise. Burning a biofuel such as bioethanol is different: the crop took in carbon dioxide by photosynthesis as it grew, so the carbon it releases on burning was taken from the air only recently. (The Organic Chemistry topic compares biofuels and fossil fuels in detail.) Growing, processing and transporting a biofuel can also release greenhouse gases: recent uptake by a crop does not make the whole fuel-production process emission-free.
The NASA carbon-cycle explanation describes the wider cycle, including uptake by the oceans.
Work through the arrows and the balance
Worked example 1
Correct a reversed arrow
Problem
A carbon cycle diagram shows: plants → photosynthesis → atmospheric carbon dioxide. Find the error and correct the arrow.
Show solution step by step
Sort the labels
Method
Decide which labels are stores and which is a process.
Reason
An arrow runs from one store to another; the process labels the arrow.
Working
Answer
Stores: plants, atmospheric carbon dioxide. Process: photosynthesis.
Follow the carbon
Method
Use what photosynthesis does to the carbon.Reason
Plants use carbon dioxide to make glucose.Working
Answer
atmospheric CO₂ → photosynthesis → plants. The arrow must point from the air to the plants.
Quick check
Check each arrow by asking where the carbon atom starts and where it ends.
Worked example 2
Decide whether carbon dioxide in the air rises or falls
Problem
In a model of one year, photosynthesis removes 120 units of carbon dioxide from the air. Respiration and decomposition return 118 units, and burning fossil fuels adds 9 units. (The figures are illustrative.) Does the amount of carbon dioxide in the air rise or fall, and by how much?
Show solution step by step
Add everything that puts carbon dioxide in
Method
Total the processes that release carbon dioxide.
Reason
Respiration, decomposition and combustion all add carbon dioxide to the air.
Working
Answer
118 + 9 = 127 units addedCompare with what is taken out
Method
Subtract the carbon dioxide removed by photosynthesis.
Reason
Photosynthesis is the only process here that removes carbon dioxide.
Working
Answer
127 − 120 = 7 unitsAnswer
Working
Answer
The amount of carbon dioxide in the air rises by 7 units. Without the fossil fuel, 118 units would be added and 120 removed: the natural processes almost balance, and the fossil fuel tips the balance.
Quick check
A small extra release every year adds up, because removal does not keep pace with the extra release.
Explain a forest being cleared
Guided practice 3
A forest is cleared and burned
Problem
A forest is cut down and the trees are burned to clear the land for buildings. First identify the immediate release and the process lost when the trees are gone. Explain both effects fully in the next activity.
Choose each part
Hints
Hint 1
release
What happens to the carbon in the wood when it burns?
Hint 2
removal
Which process took carbon dioxide out of the air while the trees were alive?
Show solution step by step
Carbon added
Method
Follow the carbon in the wood.Reason
Wood is a carbon store; combustion moves its carbon to the air.
Working
Answer
wood → combustion → atmospheric CO₂
Carbon no longer removed
Working
Answer
With the forest gone, there is less photosynthesis, so less carbon dioxide is taken out of the air each year. Both changes make atmospheric carbon dioxide rise.
Quick check
The second effect continues every year after the burning has stopped.
Check your understanding 1 Explain both changes to the atmospheric store
A forest is cleared and its trees burned for buildings. Explain both the immediate carbon release and the continuing reduction in carbon removal. Work on paper or in the optional notes.
Show answer
Wood stores carbon. Combustion transfers that carbon to the air as carbon dioxide. Removing the living trees also reduces photosynthesis, so less carbon dioxide is taken from the atmosphere each year. Both make the atmospheric store rise relative to retaining the forest; the reduction in removal continues after the burning stops.
Plants respire too
Check your understanding 2 Check the plant processes before correcting the claim
For living green plants, choose which process can continue in darkness and whether sunlight switches respiration off.
Show answer
Plants respire in darkness and sunlight. In sunlight they can also photosynthesise. Correct the complete claim and both carbon directions in the next activity.
Spot the mistake 4
“Plants take in carbon dioxide; animals give it out”
Learner claim
A learner writes: “Plants only photosynthesise and animals only respire, so plants remove carbon dioxide and animals add it.” Find the error and correct it on paper or in the optional notes. Compare both plant processes and the directions of carbon transfer.
Correct the statement
Show solution step by step
Find the error
Method
Check which living things respire.Reason
Every living cell needs energy from respiration.Working
Answer
Plants respire as well as photosynthesise.
Correct it
Working
Answer
Plants, animals and decomposers all respire and add carbon dioxide to the air. Plants also photosynthesise, which removes carbon dioxide. A growing plant removes more than it adds.
Common mistake
Green plants photosynthesise when they have light. They respire in both light and darkness: do not leave the respiration arrow out of a plant carbon-cycle diagram.
Drawing photosynthesis as an arrow into the air. Writing a list of stores and processes with no arrows. Saying the carbon is destroyed when fuel burns (it moves to the air as carbon dioxide). Forgetting that deforestation reduces photosynthesis even when no trees are burned.
Follow carbon from a compost heap
Check your understanding 3 From garden waste to the air
Plant cuttings are put on a compost heap with enough air for aerobic respiration. A few months later the heap is smaller. Explain how some of the carbon in the cuttings reaches the air, naming the process.
Work on paper or in the optional notes. Compare the decomposers, aerobic respiration, carbon dioxide and carbon that remains.
Show answer
Decomposers, such as bacteria and fungi, feed on the dead plant material. During aerobic respiration they release some of the carbon as carbon dioxide into the air. Other carbon remains in the compost and in the decomposers themselves. The carbon atoms are not destroyed: a smaller heap does not mean that all its carbon has reached the atmosphere.
Try these
Check your understanding 4 Construct the aerobic respiration equation
Write the word equation for aerobic respiration and state the direction of energy transfer.
Work on paper or in the optional notes. Compare both reactants, both products and the direction of energy transfer.
Show answer
glucose + oxygen → carbon dioxide + water. Energy is released. The carbon in glucose becomes carbon dioxide; it is transferred rather than destroyed.
Check your understanding 5 Name the arrows and the conditions
In this chain for living green plants, A transfers carbon from the air into plants and B transfers carbon back: atmospheric carbon dioxide → [A] → plants → [B] → atmospheric carbon dioxide. A plant is kept in darkness, then returned to sunlight. Name A and B and state which can continue in each condition.
Show answer
A is photosynthesis and B is respiration. In darkness, respiration can continue but photosynthesis cannot because it requires light. In sunlight, both processes can occur: photosynthesis takes carbon dioxide in, while respiration releases it. Sunlight does not switch respiration off.
Check your understanding 6 Balance the transfers without stopping them
In a model year, photosynthesis removes 120 units of carbon dioxide from the air, and respiration and decomposition return 115 units. How many units of combustion would keep the amount in the air steady? In that steady year, has the cycle stopped?
Show answer
5 units: the total added must equal the total removed, so 115 + 5 = 120. The cycle has not stopped: 120 units still leave the air and 120 units enter it. A steady store means balanced transfers, not stopped transfers.
Check your understanding 7 Explain why uptake does not cancel all release
Explain why burning fossil fuels increases atmospheric carbon dioxide even though plants and oceans take up some of it.
Work on paper or in the optional notes. Compare the long-held fuel store, extra release and removal that does not keep pace.
Show answer
Combustion releases carbon stored underground for millions of years as carbon dioxide. This adds to ongoing releases in the carbon cycle. Plants and oceans remove some, but removal does not keep pace with total release, so the atmospheric store grows.
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Syllabus and review details
- SEC G2 Science Chemistry component 2027 · 2027
Chemistry content structure and subject content, PDF pages 25–33
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