What you'll learn
The carbon cycle is a fundamental biological process that recycles carbon atoms through ecosystems. This topic appears regularly in AQA GCSE Biology papers, particularly in questions about decay, photosynthesis, and environmental change. You'll learn how carbon moves between the atmosphere, living organisms, and the environment through key biological and chemical processes.
Key terms and definitions
Carbon cycle — the continuous movement of carbon atoms through living organisms and the environment (air, water, soil)
Photosynthesis — the process by which plants and algae absorb carbon dioxide from the atmosphere and convert it into glucose using light energy
Respiration — the process that releases energy from glucose in all living organisms, returning carbon dioxide to the atmosphere
Decomposition — the breakdown of dead organic material by decomposers (bacteria and fungi), releasing carbon dioxide through respiration
Combustion — the burning of materials containing carbon (fossil fuels, wood), which releases carbon dioxide into the atmosphere
Fossil fuels — coal, oil and natural gas formed from the remains of dead organisms over millions of years, containing stored carbon
Detritus feeders — organisms such as earthworms and woodlice that feed on dead organic matter, breaking it down into smaller pieces
Carbon sink — anything that absorbs more carbon than it releases, such as oceans, forests, and peat bogs
Core concepts
The carbon cycle overview
Carbon is constantly recycled in nature through various processes. The main carbon reservoir in the atmosphere is carbon dioxide (CO₂), which makes up approximately 0.04% of air. This seemingly small percentage represents billions of tonnes of carbon that cycles through ecosystems.
The carbon cycle operates through four main processes:
- Photosynthesis removes CO₂ from the atmosphere
- Respiration returns CO₂ to the atmosphere
- Decomposition releases CO₂ from dead organisms
- Combustion releases CO₂ from burning carbon-containing materials
Carbon atoms can move through the cycle in days (eaten and respired) or remain locked away for millions of years (in fossil fuels).
Photosynthesis and carbon fixation
Plants, algae, and some bacteria remove carbon dioxide from the atmosphere through photosynthesis. This process converts carbon from an inorganic form (CO₂) into organic compounds (glucose and other carbohydrates).
The word equation for photosynthesis: carbon dioxide + water → glucose + oxygen
The carbon from CO₂ becomes incorporated into:
- Glucose molecules for respiration
- Starch for storage
- Cellulose for cell walls
- Proteins and lipids (with other elements)
These carbon compounds form the plant biomass. When animals eat plants, the carbon compounds are transferred along food chains. The carbon becomes part of animal tissues including proteins, fats, and carbohydrates.
On a global scale, photosynthesis by rainforests and ocean phytoplankton removes enormous quantities of CO₂ from the atmosphere daily. This makes these ecosystems vital carbon sinks.
Respiration returns carbon to the atmosphere
All living organisms carry out respiration to release energy from glucose. This process occurs continuously in plant cells, animal cells, and microbial cells.
The word equation for aerobic respiration: glucose + oxygen → carbon dioxide + water (+ energy)
Every living organism respires and therefore returns CO₂ to the atmosphere:
- Plants respire 24 hours a day (though they photosynthesise only in light)
- Animals respire continuously
- Decomposers (bacteria and fungi) respire as they break down dead matter
- Detritus feeders respire as they consume decaying material
The carbon that was fixed during photosynthesis is released back into the atmosphere, completing one cycle. In a balanced ecosystem, the rate of photosynthesis roughly equals the rate of respiration over time.
Decomposition and the role of decomposers
When organisms die, their bodies contain carbon compounds in tissues such as proteins, fats, and carbohydrates. Decomposers (primarily bacteria and fungi) break down this dead organic matter in a process called decomposition.
The decomposition process involves:
- Detritus feeders (earthworms, woodlice, maggots) physically break down dead material into smaller pieces, increasing surface area
- Decomposer microorganisms secrete enzymes onto the dead material
- Enzymes break down large molecules into smaller, soluble substances
- Decomposers absorb these substances and use them for growth and respiration
- Carbon dioxide is released into the atmosphere through decomposer respiration
Factors affecting the rate of decomposition:
- Temperature — higher temperatures increase enzyme activity (up to the optimum)
- Oxygen availability — aerobic decomposers need oxygen for respiration
- Water availability — decomposers need water for enzyme function and to prevent desiccation
- pH — decomposers work best at specific pH ranges
In waterlogged or acidic conditions (such as peat bogs), decomposition is very slow. Dead plant material accumulates over thousands of years, storing carbon instead of releasing it. This is why peatlands are important carbon sinks.
Combustion and fossil fuels
Millions of years ago, dead plants and animals were buried under layers of sediment. Heat and pressure converted their remains into fossil fuels: coal (from plants), crude oil, and natural gas (from marine organisms).
The carbon in these organisms became locked underground for millions of years, removed from the active carbon cycle. When humans burn fossil fuels for energy, this ancient carbon is released as CO₂ in a process called combustion.
The word equation for combustion of fossil fuels: fossil fuel (hydrocarbon) + oxygen → carbon dioxide + water (+ energy)
Wood and other plant materials can also be burned as fuels, releasing CO₂. However, there's a key difference:
- Burning wood releases carbon that was recently absorbed from the atmosphere (decades ago)
- Burning fossil fuels releases carbon that was removed from the atmosphere millions of years ago
The large-scale combustion of fossil fuels since the Industrial Revolution has increased atmospheric CO₂ levels from approximately 0.028% to 0.042%. This is a significant disruption to the natural carbon cycle and contributes to climate change.
Human impact on the carbon cycle
Human activities have significantly altered the natural carbon cycle in several ways:
Deforestation:
- Removes trees that absorb CO₂ through photosynthesis
- Burning cleared vegetation releases stored carbon
- Reduces the size of carbon sinks
- Particularly significant in tropical rainforests such as the Amazon
Fossil fuel combustion:
- Releases stored carbon that was locked away for millions of years
- Occurs in power stations, vehicles, industry, and heating
- Has increased atmospheric CO₂ by approximately 50% since pre-industrial times
Agricultural practices:
- Rice paddies produce methane (another greenhouse gas containing carbon)
- Livestock digestion releases methane
- Ploughing releases CO₂ from soil
Peat harvesting:
- Removes peat (a carbon sink) for use as compost or fuel
- Draining peatlands allows decomposition to occur, releasing stored carbon
- Destroys ecosystems that took thousands of years to form
These disruptions mean more carbon enters the atmosphere than is removed, leading to a net increase in atmospheric CO₂. This enhanced greenhouse effect contributes to global warming and climate change.
Worked examples
Example 1: Describing the carbon cycle (4 marks)
Question: Describe how carbon is cycled between the atmosphere and living organisms.
Mark scheme answer:
Carbon dioxide is removed from the atmosphere by photosynthesis (1 mark) in plants/algae/photosynthetic bacteria (1 mark). Carbon compounds pass through food chains when animals eat plants/other animals (1 mark). Respiration in plants, animals and microorganisms releases carbon dioxide back into the atmosphere (1 mark).
Examiner note: This question requires you to identify processes and organisms. Don't just list processes — explain what happens to carbon at each stage.
Example 2: The role of decomposers (3 marks)
Question: Explain the role of decomposers in the carbon cycle.
Mark scheme answer:
Decomposers (bacteria and fungi) break down dead organisms/waste products (1 mark). They respire, which releases carbon dioxide (1 mark) back into the atmosphere (1 mark).
Examiner note: "Explain" requires you to give reasons or say how/why. Make sure you mention that decomposers respire — simply saying they "break down" material isn't sufficient for full marks.
Example 3: Effects of deforestation (6 marks)
Question: Large areas of rainforest are cut down and burned. Explain how this affects the carbon cycle and atmospheric carbon dioxide levels.
Mark scheme answer:
Fewer trees means less photosynthesis occurs (1 mark), so less carbon dioxide is removed from the atmosphere (1 mark). Burning the trees releases carbon dioxide through combustion (1 mark). The carbon stored in the trees is released (1 mark). Microorganisms decompose dead wood left behind (1 mark) and release carbon dioxide through respiration (1 mark).
Examiner note: This is a longer answer requiring multiple effects. Structure your answer clearly — consider immediate effects (burning) and longer-term effects (reduced photosynthesis). Use correct scientific terminology.
Common mistakes and how to avoid them
Confusing respiration with breathing — Respiration is a chemical process in cells that releases energy; breathing is the mechanical process of moving air in and out of lungs. In the carbon cycle, it's cellular respiration that releases CO₂.
Forgetting that plants respire too — Students often remember that plants photosynthesise but forget they also respire continuously. Plants both remove CO₂ (photosynthesis) and release CO₂ (respiration), though they are net absorbers during daylight.
Thinking decomposers "produce" CO₂ differently — Decomposers release CO₂ through normal aerobic respiration, just like any other organism. They don't have a special process.
Incomplete combustion answers — When discussing fossil fuels, state that combustion releases CO₂ that was locked away for millions of years, disrupting the natural balance. Don't just say "it releases CO₂."
Vague language instead of precise processes — Use specific terms: photosynthesis, respiration, combustion, decomposition. Avoid vague phrases like "plants use CO₂" or "animals breathe out CO₂."
Not explaining why peat bogs store carbon — Peat bogs are waterlogged and acidic, which slows decomposition. This means dead plant material accumulates rather than being broken down, storing carbon for thousands of years.
Exam technique for "The carbon cycle"
Command word awareness — "Describe" requires you to state what happens; "Explain" requires you to say why or how it happens. For carbon cycle questions, explaining usually means naming the process AND what it does to carbon/CO₂.
Six-mark questions often require multiple effects — If asked about human impacts on the carbon cycle, you need to discuss multiple activities (deforestation AND fossil fuels) or multiple consequences of one activity. Plan your answer to ensure you cover enough points.
Use the word equation if relevant — For processes like photosynthesis, respiration, or combustion, writing the word equation can help you gain marks and structure your thinking, especially in longer questions.
Link carbon movement to specific processes — Don't just say "carbon moves from plants to animals" — specify that it moves through feeding/consumption when animals eat plants, transferring carbon compounds through the food chain.
Quick revision summary
The carbon cycle continuously recycles carbon through ecosystems. Photosynthesis removes CO₂ from the atmosphere, incorporating carbon into plant biomass. Feeding transfers carbon through food chains. Respiration in all living organisms returns CO₂ to the atmosphere. Decomposers break down dead material, releasing CO₂ through respiration. Combustion of fossil fuels and wood also releases CO₂. Human activities (deforestation, burning fossil fuels) have increased atmospheric CO₂ levels, disrupting the natural cycle. Understanding these processes and their interactions is essential for GCSE exam questions.