17.3 The Carbon Cycle

A free GCSE Biology lesson for AQA (8461) from OBA School: how carbon moves between the air, living things, dead material and fossil fuels. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

• Explain why carbon must be cycled
• Describe how photosynthesis removes carbon dioxide from the air
• Describe how respiration, decay and combustion return carbon dioxide to the air
• Explain how carbon passes along food chains and becomes locked in fossil fuels
• Interpret data about carbon dioxide levels

Key words

• Carbon cycle: The continual movement of carbon between the atmosphere, living things, dead material and the oceans and rocks.
• Photo- synthesis: The process plants and algae use to make glucose from carbon dioxide and water, using light energy.
• Respiration: The release of energy from glucose in living cells. Aerobic respiration releases carbon dioxide.
• Combustion: Burning a fuel in oxygen, which releases carbon dioxide and energy.
• Fossil fuels: Coal, oil and natural gas, formed over millions of years from the remains of organisms that did not decay.

Carbon on the move

Carbon is in every major molecule of life: carbohydrates, proteins, fats and DNA. Only about 0.04% of the air is carbon dioxide, yet it is the source of carbon for nearly all food chains. The carbon cycle returns carbon from organisms to the atmosphere as carbon dioxide, so it can be used by plants in photosynthesis again.

Taking carbon out of the air

Photosynthesis by green plants and algae removes carbon dioxide from the atmosphere:

carbon dioxide + water → glucose + oxygen (light energy, chlorophyll)

The carbon becomes part of glucose, then starch, cellulose, proteins and fats in the plant. When animals eat plants, the carbon compounds are digested and used to build the animals’ bodies. In this way feeding passes carbon along food chains.

Oceans also absorb carbon dioxide. Some sea organisms use carbon to make shells of calcium carbonate, which can form limestone rock over time.

Putting carbon back into the air

• Respiration by plants and animals; How carbon dioxide is released Glucose is broken down to release energy, producing carbon dioxide.
• Respiration by decomposers; How carbon dioxide is released Bacteria and fungi digest dead organisms and waste, then respire the products.
• Combustion; How carbon dioxide is released Burning wood and fossil fuels releases carbon dioxide (and energy).

Key fact: When dead organisms do not decay (for example in waterlogged, acidic or oxygen-free conditions), the carbon stays locked up. Over millions of years these remains can form fossil fuels. Burning them releases carbon that was stored long ago.

Carbon dioxide levels are rising

Worked example: percentage increase

Increase from 1960 to 2020 = 414 − 317 = 97 ppm.
Percentage increase = 97 ÷ 317 × 100 = 30.6% (3 s.f.).
Mean rate from 1990 to 2020 = (414 − 354) ÷ 30 = 2.0 ppm per year.

Answer: about 31% higher in 2020 than in 1960

The rise is mainly due to burning fossil fuels and cutting down and burning forests (deforestation), which also means less photosynthesis.

Common mistakes

• Saying plants only photosynthesise and animals only respire. Plants respire all the time, day and night.
• Forgetting decomposers. Their respiration returns a large amount of carbon dioxide to the air.
• Saying fossil fuels are made from decayed organisms. They form because the remains did not fully decay.
• Drawing arrows the wrong way: photosynthesis goes from the air to plants; respiration and combustion go to the air.

Revision questions

Try these, then check your answers below each one.

Question 1. Which process removes carbon dioxide from the atmosphere? (Choose from: Photosynthesis, Respiration, Combustion, Decay.)

Answer: Photosynthesis

Working: Photosynthesis takes in CO₂; the other three release it.

Question 2. True or false? Fossil fuels formed from organisms that decayed completely.

Answer: False

Working: False. Fossil fuels formed from remains that did not fully decay, so the carbon stayed locked up.

Question 3. Why is the carbon dioxide level over a forest lower in summer than in winter? (Choose from: There is more photosynthesis in summer, Trees stop respiring in summer, Decomposers die in summer, More fuel is burned in summer.)

Answer: There is more photosynthesis in summer

Working: More light and leaves in summer means more photosynthesis, which removes more CO₂.

Related lessons

• Previous lesson: 17.2 Materials Cycling
• Next lesson: 17.4 Rates of Decomposition
• In this chapter: 17.1 Feeding Relationships
• In this chapter: 17.5 Required Practicals: Organising an Ecosystem

Continue in the full course

This lesson is section 17.3 (AGCBH1703) of our free GCSE Biology course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Biology course.