The carbon cycle diagram looks busy, but it is built from only a handful of moves. Carbon comes out of the air by photosynthesis, travels along food chains by feeding, and goes back to the air by respiration and combustion. Learn those four and everything else on the diagram is detail.
📘 What you need to know
Carbon is finite and constantly recycled, so the total in the biosphere is roughly constant.
Organic stores: carbon compounds in living things and fossil fuels (carbohydrates, hydrocarbons).
Inorganic stores: carbon in the atmosphere, soils and oceans (carbon dioxide, carbonates).
Transformations change the chemical form: photosynthesis, respiration, combustion.
Transfers move carbon without changing its form: feeding, defecation, death.
Carbon sequestration is capturing atmospheric carbon dioxide and locking it into solid or liquid form.
An ecosystem behaves as a store, sink or source depending on whether uptake or release is winning.
Where the carbon is kept
It helps to split the stores in two, because exam questions often use the words organic and inorganic without explaining them.
Type
Where it is
What form the carbon takes
Organic
Living organisms, crude oil, natural gas, coal
Carbohydrates in bodies, hydrocarbons in fossil fuels
A store sits in equilibrium when uptake is balanced by release. A mature forest is the standard example: the carbon locked away by photosynthesis is matched by the carbon put back through respiration and decomposition, so the total held stays fairly steady.
The carbon cycle
Green arrows remove carbon from the atmosphere, red arrows return it, black arrows just move it between living stores. If you can label the colour of an arrow you can usually name the process.
In an exam you will often be given an unlabelled carbon cycle and asked what arrow X shows. Look at the two boxes it joins. Air to plant is photosynthesis. Anything to air is respiration or combustion. Living thing to living thing is feeding.
Transfer or transformation?
ESS makes a point of separating these two words, and it is easy marks once you see the rule: if the chemical form of the carbon changes, it is a transformation. If the carbon just moves somewhere else in the same form, it is a transfer.
Process
Type
What happens
Photosynthesis
Transformation
Carbon dioxide becomes carbohydrate in plant tissue
Cellular respiration
Transformation
Carbohydrate becomes carbon dioxide gas
Combustion
Transformation
Hydrocarbons in fuel become carbon dioxide
Feeding
Transfer
Organic carbon moves from prey to predator
Defecation
Transfer
Organic carbon moves from an animal into the soil
Death and decomposition
Transfer
Organic carbon moves from a dead body into the soil
Fossilisation and sequestration
When plants and animals die where decomposers cannot reach them — waterlogged swamps, deep sea sediments — the carbon in their bodies is not released. Over millions of years, heat and pressure turn those remains into peat and coal on land, and into oil and gas from marine organisms on the seabed.
That is one route to carbon sequestration: taking carbon dioxide out of the air and locking it into a solid or liquid form. Trees do the same thing on a much shorter timescale by storing carbon in wood.
Sequestration is not the same as storage. Storage just means carbon is sitting somewhere. Sequestration is the active process of capturing atmospheric carbon dioxide and locking it away. Trees sequester; a coal seam stores.
Ecosystems as stores, sinks and sources
Whether a forest counts as a store, a sink or a source is not a fixed property. It depends on which way the balance is tipped at that moment.
Young forest — a sink. The trees are growing fast, so photosynthesis outpaces respiration and carbon accumulates.
Mature forest — a store. Growth has slowed, uptake and release roughly match, and a large amount of carbon is simply held.
Burning or cleared forest — a source. Stored carbon is released back to the atmosphere far faster than any is taken up.
WORKED EXAMPLE
On a carbon cycle diagram, an arrow runs from “dead leaves in soil” to “carbon dioxide in the atmosphere”. Name the process and state whether it is a transfer or a transformation.
Step 1: Look at the two endsOrganic matter at one end, carbon dioxide gas at the otherStep 2: Has the form changed?carbohydrate → carbon dioxide, so yesStep 3: Name the processDecomposers respire as they break the leaves downDecomposition (respiration) – a transformation
WORKED EXAMPLE
A woodland takes up 2 100 g of carbon per square metre each year and releases 2 050 g m−2 yr−1. After a wildfire, uptake falls to 400 and release rises to 5 600 g m−2 yr−1. Describe the change in the woodland’s role.
Step 1: Before the fire2 100 − 2 050 = 50 gained – close to balancedUptake and release are almost equal, so it acts as a store (a very weak sink)Step 2: After the fire5 600 − 400 = 5 200 released each yearIt changes from a store into a strong carbon sourceCombustion releases carbon that took decades to build up
💡 Exam tip
Learn the four core moves first: photosynthesis in, feeding along, respiration out, combustion out.
Check whether the chemical form changes to decide transfer versus transformation.
Name the store precisely. “The atmosphere” and “soil organic matter” score; “the ground” does not.
Use the young, mature, burning forest sequence whenever a question asks how one ecosystem can be all three.
Remember plants respire too, so an arrow from plants back to the atmosphere is correct, not a mistake.
⚠ Common mix-up
Thinking only animals return carbon to the air. Plants and decomposers respire as well.
Calling feeding a transformation. The carbon stays as organic compounds, so it is a transfer.
Treating fossil fuels as a separate cycle. They are just a very slow carbon store within the same cycle.
Muddling sequestration with storage. Sequestration is the capture step, not the holding.
Saying a forest “is” a sink. Say acts as a sink, and give the condition — role depends on the balance.
Up next: How Humans Alter the Carbon Cycle — what happens when we start moving carbon between stores far faster than nature ever did.
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