KS3 · Chemistry

The carbon cycle

Some of the carbon in your breakfast was floating in the air not long ago — and some of it will soon be back there, breathed out by you.

Follow one carbon atom

Rebuild the loop, then break it

4 stages are missing. Use the rest of the loop to work out which goes where.

  1. Stage 1: Carbon in the air

    Our atom starts here, inside a molecule of carbon dioxide, CO₂: one carbon atom joined to two oxygen atoms. Most of the carbon in the air is in this form.

  3. Stage 3: Carbon in plants

    The carbon from the glucose is now part of the plant's biomass. Plants respire too — glucose + oxygen → carbon dioxide + water — so some carbon goes straight back to the air from here.

  5. Stage 5: Carbon in animals

    The carbon is now part of the animal's body. Animals respire as well, breathing some carbon back out as carbon dioxide.

  7. Stage 7: Carbon in dead matter and soil

    Our atom is now locked inside dead leaves, dead animals or waste, waiting to be broken down.

…and stage 8 leads back to stage 1.

Up or down?

What does each process do to the air?

Does this process add carbon dioxide to the air, or take it out?

Still to sort

Adds CO₂ to the air (0)

Carbon leaves a store and goes back into the atmosphere.

Where the line is: Follow the carbon: if it finishes in the air, the process adds carbon dioxide.

Takes CO₂ out of the air (0)

Carbon leaves the atmosphere and goes into a living thing or the ocean.

Where the line is: Follow the carbon: if it finishes in a plant, a microscopic living thing or sea water, the process takes carbon dioxide out.

8 of 8 still to sort.

Some processes put carbon dioxide into the atmosphere; others take it out. Sort each one, then read why.

Watch out: Burning wood is still combustion, so it adds carbon dioxide even though wood comes from plants. The tree took that carbon out of the air while it grew; burning puts it straight back.

Two slow stores of carbon

Carbon in sedimentary rockvsCarbon in fossil fuels

Some carbon drops out of the fast loop and gets buried for millions of years. Here are the two ways it happens.

Focus

Made from

Carbon in sedimentary rock

The shells and bones of sea animals

Carbon in fossil fuels

The soft tissue of dead plants and animals

The insight

This is the easy one to mix up: shells and bones become rock; soft tissue becomes coal, oil and gas.

How the carbon got there

Carbon in sedimentary rock

Carbon dioxide dissolved in the ocean formed carbonate ions, which sea animals used to build their shells and bones.

Carbon in fossil fuels

Plants took in carbon dioxide by photosynthesis, and the carbon passed along food chains into animals' bodies.

How it forms

Carbon in sedimentary rock

After the animals die, their shells and bones settle on the ocean floor and become cemented together.

Carbon in fossil fuels

Dead plants and animals are trapped in sediment and, over millions of years, form coal, oil and gas.

How long the carbon stays there

Carbon in sedimentary rock

Millions of years, because the rock cycle is very slow.

Carbon in fossil fuels

Millions of years — unless people dig the fuels up and burn them.

What do you think?

Where does the carbon go when a log burns?

A log burns on a bonfire until only a little ash is left. It looks as if most of the log has simply disappeared.

What has happened to the carbon that was in the log? Pick the idea closest to what you think right now.
How sure are you?

Guess first

How much of the air is carbon dioxide?

Carbon dioxide is in the news all the time. What percentage of the air around you do you think is carbon dioxide?

Your estimate

15%

0%30%

From a power station to the weather

?

Reason it through

Carbon dioxide is only a tiny part of the air. So how can burning fossil fuels change the climate?

Link 1 of 4

First link · your turn

Coal, oil and gas hold carbon that has been stored for millions of years. What does burning them in power stations, vehicles and homes do with that carbon?

2
Locked — reveal the link above first
3
Locked — reveal the link above first
4
Locked — reveal the link above first

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

Carbon never gets used up. The same atoms keep moving between the air, living things, the oceans and rocks — and what we burn changes where they end up.

What you need to know

  • The carbon cycle describes how carbon moves continuously between the atmosphere, living organisms, the oceans, and the Earth's surface and rocks. It is a closed system: carbon atoms are not created or destroyed, so the total stays the same and only its location changes.
  • Photosynthesis (by plants on land, and phytoplankton and some bacteria in the oceans) and dissolving in the oceans take carbon dioxide out of the air. Respiration, decomposition and combustion put it back.
  • Carbon passes between organisms by eating, as carbon compounds such as carbohydrates and proteins, and reaches the soil through waste and dead organisms, where decomposers break it down.
  • Burning fossil fuels in power stations, vehicles and homes quickly adds carbon dioxide to the air. Carbon dioxide is a greenhouse gas, so more of it causes global warming, which leads to climate change.

The big picture

The carbon cycle is how carbon moves continuously between the atmosphere, living things, the oceans and the Earth's rocks. Photosynthesis and dissolving in the oceans take carbon dioxide out of the air; respiration, decomposition and combustion put it back. Carbon is never created or destroyed — it only changes place. Burning fossil fuels moves long-stored carbon into the air quickly, and because carbon dioxide is a greenhouse gas, this causes global warming and climate change.

Key points

1Most carbon in the air is in carbon dioxide, CO₂ (one carbon atom and two oxygen atoms). It makes up only about 0.04 % of the atmosphere.
2Photosynthesis: carbon dioxide + water → glucose + oxygen, using energy from sunlight. The carbon is stored in plant biomass.
3Respiration: glucose + oxygen → carbon dioxide + water, releasing energy. It happens in plant and animal cells and is the reverse of photosynthesis.
4Decomposers (worms, beetles, fungi, bacteria) break down dead matter and waste. Fungi and some bacteria turn its carbon compounds into carbon dioxide, and decomposers respire too.
5The more carbon dioxide there is in the air, the more can dissolve in the oceans. Dissolved carbon dioxide can form carbonate ions, which sea animals use for shells and bones.
6Shells and bones on the ocean floor can be cemented into sedimentary rock, which stores carbon for millions of years. Almost all of Earth's carbon is in rocks.
7Fossil fuels (coal, oil and gas) form over millions of years from the soft tissue of dead plants and animals trapped in sediment — not from shells and bones.
8Combustion: fuel + oxygen → carbon dioxide + water. Greenhouse gases absorb and re-emit infrared from the Earth's surface, reducing heat lost to space; too much of them causes global warming and climate change.

Worked example

Problem

A large area of trees is cut down and the wood is burned. Explain what this does to the amount of carbon dioxide in the atmosphere.

⚠ Watch out

Saying that carbon is 'used up' when something burns or respires. It isn't: combustion and respiration move carbon into the air, usually as carbon dioxide. The total amount of carbon stays the same — only where it is changes.

🧠

Memory hook

Carbon is a traveller, not a fuel that runs out: it only ever changes address. Photosynthesis and the oceans take it out of the air; respiration, rot and fire put it back.

✓

Check yourself

Close the page and draw the loop from memory, naming the process on every arrow. Then add an arrow for burning fossil fuels: which way does it push the carbon dioxide in the air?

Flashcards

(14)
What does the carbon cycle describe?
How carbon moves continuously between the atmosphere, living organisms, the oceans, and the Earth's surface and rocks.
Why is the carbon cycle called a closed system?
Carbon atoms are not created or destroyed, so the total amount of carbon stays the same. Only where the carbon is changes.
In what form is most of the carbon in the air?
Carbon dioxide, CO₂: each molecule has one carbon atom and two oxygen atoms.
About what percentage of the atmosphere is carbon dioxide?
About 0.04 % — roughly 4 parts in every 10,000.
Which living things carry out photosynthesis?
Plants on land, and microscopic phytoplankton and some bacteria in the oceans.
How does carbon pass from a plant to an animal?
By eating. The carbon travels in carbon compounds such as carbohydrates and proteins, not as a gas.
How does carbon get into the soil?
Through animal waste, and when dead plants and animals decay.
What do decomposers do in the carbon cycle?
Worms, beetles, fungi and bacteria break down dead matter and waste. Fungi and some bacteria turn its carbon compounds into carbon dioxide, and decomposers release more as they respire.
What are the two processes in the cycle that take carbon dioxide out of the air?
Photosynthesis, and dissolving in the oceans. The more carbon dioxide there is in the air, the more can dissolve.
How does carbon end up stored in sedimentary rock?
Dissolved carbon dioxide forms carbonate ions; sea animals use them for shells and bones; these settle on the ocean floor and are cemented into rock that stores carbon for millions of years.
What are fossil fuels formed from?
The soft tissue of dead plants and animals trapped in sediment over millions of years — not shells and bones.
How does human activity increase the carbon dioxide in the air?
By burning fossil fuels — natural gas, coal and oil — in power stations, vehicles and homes.
What does a greenhouse gas do?
It absorbs some of the infrared radiation given off by the Earth's surface and re-emits it, so less heat is lost to space.
What is the difference between global warming and climate change?
Global warming is the rise in average global temperature. Climate change is what it leads to: changing global weather patterns, more extreme weather and threats to biodiversity.

Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.

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