The Carbon Cycle
Section: Biology | Syllabus: Cambridge Lower Secondary Checkpoint Science (0893)
What is the Carbon Cycle?
- The carbon cycle is the continuous movement of carbon atoms between the atmosphere, living organisms, and the Earth, through various processes
- this recycling process is essential for life
- Carbon is found in all living organisms (proteins, fats, carbohydrates, DNA)
- Carbon dioxide (CO₂) in the atmosphere is used by plants for photosynthesis
- Carbon is constantly recycled between organisms and the environment
- this keeps the carbon cycle in balance within ecosystems
Processes in the Carbon Cycle
- Five main processes control how carbon moves through the environment
| Process | What Happens to Carbon? | Organisms Involved |
|---|---|---|
| Photosynthesis | CO₂ removed from atmosphere → converted to glucose | Plants, algae |
| Respiration | Glucose broken down → CO₂ released to atmosphere | All living organisms |
| Feeding | Carbon compounds passed through food chains | Animals eating plants/other animals |
| Decomposition | Dead organisms broken down → CO₂ released | Bacteria, fungi (decomposers) |
| Combustion | Fossil fuels burned → CO₂ released | Human activity |
The carbon cycle: how carbon moves between the atmosphere, living organisms and fossil fuels
- A sixth process, fossilisation, is also a valid process in the carbon cycle
- fossilisation is the very slow burial and compression of dead organisms over millions of years, which locks their carbon away underground as fossil fuels (coal, oil, natural gas) instead of it being released back to the atmosphere
Key Processes Explained
Photosynthesis
- Word equation:
- CO₂ + H₂O → glucose + O₂
- Plants and algae remove CO₂ from the atmosphere
- carbon is stored in plant tissues as carbohydrates
- This is the only process that removes CO₂ from the air
Respiration
- Word equation:
- glucose + O₂ → CO₂ + H₂O + energy
- All living organisms respire continuously
- carbon in glucose is released as CO₂ back to the atmosphere
- Plants respire too, but photosynthesise more during the day
Feeding
- Animals eat plants or other animals
- carbon compounds move through food chains
- carbon stays in organic form, in animal bodies
Decomposition
- Decomposers (bacteria, fungi) break down dead organisms
- decomposers respire, releasing CO₂ to the atmosphere
- this returns carbon from dead matter back to the cycle
Combustion
- Burning fossil fuels (coal, oil, natural gas) or wood
- releases large amounts of CO₂ to the atmosphere
- increases atmospheric CO₂ levels, contributing to climate change
Carbon Stores
- Carbon exists in different forms in different locations
- these are called carbon stores, or reservoirs
| Carbon Store | Form of Carbon | How Carbon Enters | How Carbon Leaves |
|---|---|---|---|
| Atmosphere | Carbon dioxide (CO₂) | Respiration, decomposition, combustion | Photosynthesis |
| Living organisms | Organic compounds (glucose, proteins, fats) | Photosynthesis (plants), feeding (animals) | Respiration, death |
| Dead organic matter | Organic compounds in dead organisms | Death of organisms | Decomposition |
| Fossil fuels | Coal, oil, natural gas | Ancient dead organisms compressed over millions of years | Combustion |
- Fossil fuels contain carbon that was locked away millions of years ago
- burning them releases this ancient carbon into the atmosphere
The Role of Decomposers
- Decomposers are microorganisms (bacteria and fungi) that break down dead organic matter, releasing nutrients and carbon back into the environment
- they play a critical role in the carbon cycle
- Without decomposers, dead organisms would accumulate
- carbon would be "locked up" in dead matter indefinitely
- Decomposers release CO₂ back to the atmosphere through respiration
- this keeps carbon cycling through ecosystems
Human Impact on the Carbon Cycle
- Human activities have disrupted the natural balance of the carbon cycle
- this has led to increasing atmospheric CO₂ levels
Causes of Increased CO₂
- Burning fossil fuels — for electricity, transport, and heating
- Deforestation — cutting down forests reduces photosynthesis, so less CO₂ is removed
- Burning forests — releases stored carbon as CO₂
Effects of Increased CO₂
- Enhanced greenhouse effect
- Global warming and climate change
- Melting ice caps and rising sea levels
- Changes in weather patterns, including more extreme weather (storms, floods, droughts)
- Habitat loss and species extinction, as environments change faster than some species can adapt
- for the full list of climate change effects, see the Climate Change topic
Solutions to Reduce CO₂
- Use renewable energy (solar, wind, hydroelectric)
- Reduce fossil fuel use, by improving energy efficiency
- Plant more trees (reforestation)
- Protect existing forests
- Develop carbon capture technologies
- The natural carbon cycle is balanced
- human activities have disrupted this balance by adding more CO₂ than can be removed by natural processes
Balanced carbon cycle compared with an unbalanced carbon cycle disrupted by human activity
How a Sunlight-Blocking Event Disrupts the Carbon Cycle
- A sudden event that blocks sunlight from reaching Earth's surface — such as a large asteroid collision or a huge volcanic eruption — can disrupt the carbon cycle so severely that it causes mass extinction
Worked Example: The Asteroid and the Extinction of the Dinosaurs
Interactive revision notes, videos and practice questions load below.