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How does carbon move between the air, living things and the ground?

Describe the carbon cycle and the processes that add and remove carbon dioxide from the air

A focused answer to the O-Level Biology outcome on the carbon cycle. How photosynthesis, respiration, decomposition and combustion move carbon between the air, living things and fossil fuels.

Generated by Claude Opus 4.88 min answer

Reviewed by: AI editorial process; not yet individually human-reviewed

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  1. What this dot point is asking
  2. The answer
  3. Examples in context
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What this dot point is asking

SEAB wants you to describe the carbon cycle: how carbon moves between the air (as carbon dioxide), living organisms, and fossil fuels, through the processes of photosynthesis, respiration, feeding, decomposition and combustion. You should be able to trace a carbon atom around the cycle and name human activities that add extra carbon dioxide.

The answer

The store of carbon in the air

Carbon is present in the air as the gas carbon dioxide. The carbon cycle is the way this carbon moves between the air, living things, and the ground (including fossil fuels), being used and returned over and over.

Processes that remove carbon dioxide from the air

Photosynthesis is the main process that removes carbon dioxide. Green plants take in carbon dioxide and use it to make glucose. The carbon becomes part of the plant's molecules (carbohydrates, fats and proteins). When animals eat plants, this carbon passes into animals to build their bodies (feeding).

Processes that return carbon dioxide to the air

Respiration returns carbon dioxide. All living things (plants, animals and decomposers) respire, breaking down glucose and releasing carbon dioxide back into the air.

Decomposition. When organisms die, decomposers (bacteria and fungi) break down the dead matter. As they feed and respire, they release the carbon as carbon dioxide back to the air.

Combustion (burning). Burning fossil fuels (coal, oil and gas) and wood releases carbon dioxide that was stored in them.

Fossil fuels

Long ago, the remains of dead organisms that did not fully decompose were buried and, over millions of years, turned into fossil fuels. These store carbon. Burning them releases that carbon as carbon dioxide, which is why human use of fossil fuels adds extra carbon dioxide to the air.

A balanced cycle disturbed

Naturally, photosynthesis (removing carbon dioxide) and respiration plus decomposition (returning it) roughly balance. Human activities such as burning fossil fuels and deforestation (cutting down trees, which also reduces photosynthesis) add extra carbon dioxide faster than it is removed, raising its level in the air.

Examples in context

Example 1. Coal in a power station. Coal is a fossil fuel made from plants that lived millions of years ago, storing their carbon. Burning it in a power station releases that ancient carbon as carbon dioxide, adding to the air today.

Example 2. A forest as a carbon store. A growing forest takes in large amounts of carbon dioxide by photosynthesis, locking carbon into wood. Cutting down and burning the forest releases this carbon and removes the trees that would have absorbed more, a double effect on the cycle.

Try this

Q1. Name the process that removes carbon dioxide from the air. [1 mark]

  • Cue. Photosynthesis (carried out by green plants).

Q2. State two processes that return carbon dioxide to the air. [2 marks]

  • Cue. Any two of: respiration, decomposition (decomposers respiring), combustion (burning fuels).

Q3. Explain how human activity has increased the carbon dioxide in the air. [2 marks]

  • Cue. Burning fossil fuels releases stored carbon as carbon dioxide, and deforestation removes trees that would absorb it, so carbon dioxide is added faster than it is removed.

Exam-style practice questions

Practice questions written in the style of SEAB exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

Original6 marks(a) Name the process that removes carbon dioxide from the air and the process carried out by all living things that returns it. (b) Describe how carbon in a dead animal is returned to the air. (c) State one way human activity adds extra carbon dioxide to the air.
Show worked answer →

(a) Photosynthesis removes carbon dioxide from the air (carried out by green plants). Respiration, carried out by all living things, returns carbon dioxide to the air.

(b) When an animal dies, decomposers (bacteria and fungi) break down its body. The decomposers respire as they feed on the dead matter, releasing the carbon as carbon dioxide back into the air.

(c) One way: burning fossil fuels (combustion) in vehicles and power stations releases carbon dioxide; or deforestation (which also reduces the carbon dioxide removed by photosynthesis). Any one is accepted.

Markers reward photosynthesis removing and respiration returning carbon dioxide, decomposers respiring to release carbon from dead matter, and a valid human source such as burning fossil fuels.

Original4 marksExplain how carbon, which is in the air as carbon dioxide, becomes part of the body of an animal.
Show worked answer →

Green plants take in carbon dioxide from the air and use it in photosynthesis to make glucose, so the carbon becomes part of the plant's molecules (such as carbohydrates). When an animal eats the plant, the carbon-containing molecules pass into the animal and are used to build the animal's body (for example, as it makes its own proteins, fats and carbohydrates). So the carbon moves from the air, into the plant by photosynthesis, then into the animal by feeding.

Markers reward photosynthesis fixing carbon dioxide into glucose in the plant, and the carbon passing to the animal when it eats the plant and uses the molecules to build its body.

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