How does carbon move between the air, living things and the ground, over and over again?
Describe the carbon cycle, including the roles of photosynthesis, respiration, decomposition and combustion
A scaffolded answer to the N(A)-Level Biology outcome on the carbon cycle. How carbon moves between the air, plants and animals through photosynthesis, respiration, decomposition and burning.
Reviewed by: AI editorial process; not yet individually human-reviewed
Have a quick question? Jump to the Q&A page
Jump to a section
What this dot point is asking
This outcome wants you to describe how carbon moves around in a cycle between the air, plants, animals and the ground. You should be able to explain the four key processes that move carbon: photosynthesis, respiration, decomposition and combustion (burning). The marks reward saying which way each process moves carbon (into or out of the air) and naming the organisms involved, especially the decomposers. Unlike energy, carbon is recycled.
The answer
The idea of a cycle
Carbon is an element found in all living things, in carbon dioxide gas in the air, and in fuels in the ground. The carbon cycle is the way carbon moves between these stores, over and over, so the same carbon atoms are used again and again. Carbon dioxide is taken out of the air, becomes part of living things, and is later returned to the air.
Taking carbon out of the air
Carbon dioxide is removed from the air by photosynthesis. Green plants take in carbon dioxide and use light energy to make food (glucose). The carbon now becomes part of the plant. When animals eat plants, the carbon passes into the animals, and along the food chain.
Returning carbon to the air
Carbon dioxide is returned to the air by three main processes:
- Respiration. Plants, animals and microbes all respire, releasing carbon dioxide.
- Decomposition. When plants and animals die, decomposers (bacteria and fungi) break down their bodies and waste. As they feed, they respire and release carbon dioxide.
- Combustion (burning). Burning fuels such as wood and fossil fuels (coal, oil, gas) releases the carbon stored in them as carbon dioxide.
The role of decomposers
Decomposers are vital. By breaking down dead matter and respiring, they return carbon (as carbon dioxide) to the air so plants can use it again. They also clear away dead material and recycle the nutrients in it. Without them, dead bodies and the carbon locked inside would pile up.
Examples in context
Example 1. Why a compost heap warms up. In a compost heap, decomposers break down dead plant matter and respire fast, releasing carbon dioxide and heat. This is decomposition in the carbon cycle happening in your garden, returning carbon to the air and nutrients to the soil.
Example 2. Why burning fossil fuels raises carbon dioxide. Fossil fuels store carbon that was locked away long ago. Burning them releases that carbon as carbon dioxide much faster than photosynthesis can remove it, so the level in the air rises. It shows combustion shifting the balance of the carbon cycle, the link to climate change.
Try this
Q1. Name the process that removes carbon dioxide from the air. [1 mark]
- Cue. Photosynthesis.
Q2. State two processes that return carbon dioxide to the air. [2 marks]
- Cue. Any two of: respiration, decomposition (decomposers), and combustion (burning).
Q3. Explain the role of decomposers in the carbon cycle. [2 marks]
- Cue. They break down dead plants and animals and respire, releasing carbon dioxide back into the air so it can be used again.
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.
Original4 marksExplain how carbon dioxide is removed from the air and how it is returned to the air in the carbon cycle.Show worked answer →
Carbon dioxide is removed from the air by photosynthesis. Green plants take in carbon dioxide and use it to make food (glucose), so the carbon becomes part of the plant.
Carbon dioxide is returned to the air mainly by respiration: plants, animals and microbes all respire and release carbon dioxide. It is also returned by combustion (burning fuels, including wood and fossil fuels) and by decomposition, when decomposers break down dead matter and respire.
What markers reward: photosynthesis as the way carbon dioxide is removed, and respiration (plus combustion and decomposition) as the ways it is returned. Saying plants only put carbon dioxide back is wrong; they remove it by photosynthesis and return some by respiration.
Original4 marksExplain the role of decomposers in the carbon cycle, and why this role is important.Show worked answer →
Decomposers, such as bacteria and fungi, break down the bodies and waste of dead plants and animals. As they feed, they respire and release carbon dioxide back into the air.
This is important because it returns carbon (as carbon dioxide) to the air so it can be used again by plants in photosynthesis, and it clears away dead material and recycles the nutrients locked inside it. Without decomposers, dead matter and the carbon in it would pile up.
What markers reward: decomposers breaking down dead matter and respiring to release carbon dioxide, and the importance of recycling carbon back to the air (and clearing dead material). Saying decomposers make food by photosynthesis is wrong; they are consumers/decomposers.
Related dot points
- Describe food chains and food webs, and explain how energy flows from the Sun through producers and consumers
A scaffolded answer to the N(A)-Level Biology outcome on food chains. Producers and consumers, how food chains link into food webs, and why energy is lost at each step so chains are short.
- Describe the effects of human activity on the environment, including pollution and habitat loss, and ways to reduce the harm
A scaffolded answer to the N(A)-Level Biology outcome on human impact. How pollution, deforestation and the greenhouse effect harm the environment, and the steps people can take to reduce the damage.
- State the word equation for photosynthesis, describe the raw materials and conditions needed, and explain how light, carbon dioxide and temperature affect the rate
A scaffolded answer to the N(A)-Level Biology outcome on photosynthesis. The word equation, the raw materials and conditions needed, what the products are used for, and the limiting factors that affect the rate, with a worked rate example.
- Define respiration, write word equations for aerobic and anaerobic respiration, and compare the energy they release
A scaffolded answer to the N(A)-Level Biology outcome on respiration. What respiration is, the word equations for aerobic and anaerobic respiration in humans and yeast, and how much energy each releases.
- Describe the parts of the human digestive system, the process of physical and chemical digestion, and the absorption of digested food
A scaffolded answer to the N(A)-Level Biology outcome on human digestion. The main parts of the digestive system, physical and chemical digestion by enzymes, and how digested food is absorbed in the small intestine.