This topic summary covers Knowledge Organiser within Carbon Cycle for GCSE Biology. Topic 3: Carbon Cycle It is section 10 of 11 in this topic. Use this topic summary to connect the idea to the wider topic before moving on to questions and flashcards.
Topic position
Section 10 of 11
Practice
15 questions
Recall
20 flashcards
Knowledge Organiser
Key Terms
- Carbon cycle: Continuous movement of carbon through living and non-living systems
- Carbon sink: Reservoir that absorbs more carbon than it releases
- Carbon source: Releases more carbon than it absorbs
- Fossil fuels: Long-term carbon stores (coal, oil, gas)
- Combustion: Burning — releases CO2 rapidly
- Decomposition: Breakdown by bacteria/fungi — releases CO2
Processes Summary
- CO₂ removed from atmosphere: photosynthesis only (by producers)
- CO₂ returned to atmosphere: respiration (all living organisms), combustion (burning), decomposition (bacteria and fungi)
- Plants both absorb CO₂ (photosynthesis) AND release CO₂ (respiration) — they are net absorbers overall
- Fossil fuels take millions of years to form but seconds to burn — releasing ancient stored carbon that was removed from the active cycle long ago
- Peat bogs: waterlogged and oxygen-poor (anaerobic) → most decomposers cannot respire aerobically → decomposition very slow → carbon accumulates
- Grade 7+ separator: Burning fossil fuels is different from respiration — it adds carbon to the active cycle that has been locked away for millions of years, increasing the total amount of CO₂ in the atmosphere. Respiration is part of a balanced cycle; fossil fuel combustion is a net addition.
Human Impacts
- Burning fossil fuels: releases ancient stored carbon as CO2
- Deforestation: reduces photosynthesis + releases carbon via combustion/decomposition
- Draining peat bogs: enables aerobic decomposition, releasing stored CO2
- All increase atmospheric CO2, enhancing greenhouse effect
Common Mistakes
- Saying plants only absorb CO2: Plants carry out both photosynthesis (absorbs CO2) and respiration (releases CO2). They are net absorbers of CO2 overall, but they do release CO2 — especially at night when photosynthesis stops.
- Giving only one reason why deforestation raises CO2: Deforestation increases atmospheric CO2 for two reasons: (1) less photosynthesis removing CO2, and (2) burning or decomposing felled trees releases stored carbon. Both reasons are required for full marks.
- Confusing combustion with respiration in the cycle: Combustion of fossil fuels releases carbon that was locked away millions of years ago — this is a one-way addition to the active carbon cycle. Respiration by living organisms is part of the natural cycle and is balanced by photosynthesis.
- Forgetting decomposers in the carbon cycle: Decomposers (bacteria and fungi) break down dead organic matter, releasing CO2 through respiration. Without naming decomposers, answers about how carbon returns to the atmosphere from dead organisms are incomplete.
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Practice Questions for Carbon Cycle
Which process removes CO₂ from the atmosphere?
Explain how decomposers return carbon to the atmosphere.
Quick Recall Flashcards
How does decomposition return carbon to the atmosphere?
Decomposers (bacteria and fungi) break down dead organisms.
They respire, releasing CO₂ back into the atmosphere.
Without decomposers, carbon would be locked in dead material forever.
What does the carbon cycle do?
The carbon cycle continuously moves carbon between the atmosphere (as CO₂), living organisms, the soil, and fossil fuels.
Carbon is never created or destroyed — it is recycled.
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