Inheritance and ecology · GCSE Biology
The carbon cycle
Track carbon for GCSE Biology: photosynthesis removes CO₂, respiration and combustion return it, decomposition recycles biomass, and fossil-fuel use plus deforestation shift the atmosphere’s balance.
Carbon is not used up; it is cycled. Photosynthesis locks CO₂ into glucose. Respiration, decay and combustion unlock it. Extra fossil-fuel burning plus fewer trees raises atmospheric CO₂.
The important bits
What you need to know
- 1
Photosynthesis removes carbon dioxide from the air: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂. Carbon is then in biomass (glucose, starch, cellulose, lipids, proteins).
- 2
Respiration returns carbon dioxide: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O, in plants, animals and decomposers. This happens all the time.
- 3
When organisms die, microorganisms (bacteria and fungi) decompose the remains, respiring and releasing CO₂ (and methane if anaerobic).
- 4
Combustion of wood, peat and fossil fuels (coal, oil, gas) oxidises stored carbon to CO₂, adding extra carbon that had been locked underground for millions of years.
- 5
Fossil fuels formed from plants and plankton that did not fully decay, under heat and pressure. Burning them is a one-way dump into the modern atmosphere on human timescales.
- 6
Deforestation cuts a carbon store (wood) and often adds CO₂ if the wood is burned. It also removes photosynthetic capacity that would have taken CO₂ out.
- 7
The water cycle (evaporation, transpiration, condensation, precipitation) is a different cycle; do not mix arrows. Carbon arrows must show CO₂ in air, biomass, fossils, and back.
- 8
Increasing atmospheric CO₂ is a greenhouse gas story: more thermal energy absorbed, contributing to global warming and climate change, which then hits biodiversity.
Quotations worth analysing
Short evidence. Real method.
“Photosynthesis removes carbon dioxide; respiration returns it.”
If you remember only two arrows, remember these. Combustion and decay are the extras that exams add.
“Microorganisms decay dead material and release carbon dioxide by respiration.”
Decay is not a mystery arrow. It is decomposer respiration (or methane in anaerobic conditions).
“Combustion of fossil fuels adds extra carbon dioxide to the atmosphere.”
The carbon was locked. Burning it unbalances the cycle on a human timescale.
Go deeper
How do I label a carbon-cycle diagram without inventing arrows?
Atmosphere CO₂ → plants (photosynthesis). Plants → animals (feeding; carbon in food). Plants, animals, decomposers → atmosphere (respiration). Dead organisms → decomposers (decay). Ancient biomass → fossil fuels (formation over millions of years). Fossil fuels and wood → atmosphere (combustion). Do not draw an arrow from animals to plants labelled photosynthesis. Do not draw oxygen arrows on a carbon-cycle question unless asked. If methane from cattle or rice paddies appears, that is another carbon compound from anaerobic respiration/decay; it is still a greenhouse gas. Check that every arrow names a process. “Goes into the ground” is not a process.
Go deeper
Why does cutting a forest change more than one arrow?
Fewer leaves means less photosynthesis, so less CO₂ removed. If trees are burned, combustion adds a pulse of CO₂. If they rot, decomposers respire it out more slowly. Soil carbon can be lost when land is ploughed. Cattle on the cleared land add methane. The exam wants a mechanism, not “deforestation is bad”. Palm oil, soy and timber questions are the same biology: habitat plus carbon store plus cycle imbalance. Planting trees reverses the photosynthesis arrow if the trees survive, but a new sapling does not instantly replace an old woodland’s store. Peat bogs are a related store: draining and burning peat for compost or fuel releases carbon that had been locked in waterlogged, acidic, anaerobic conditions where decay was slow.
Go deeper
How do I connect the carbon cycle to a respiration or photosynthesis paper?
The equations are the cycle. If a pondweed practical produces oxygen, carbon is being locked into glucose at the same time. If compost heats up, decomposer respiration is transferring energy and releasing CO₂. If a germinating-seed respirometer uses oxygen, those seeds are returning carbon to CO₂. Global data showing CO₂ rising since the Industrial Revolution is combustion plus land-use change outrunning photosynthesis and dissolving in oceans. Oceans taking up CO₂ can acidify — Triple may mention it. For Combined, stay with atmosphere, biomass, fossils, photosynthesis, respiration, combustion, decay. That closed list is enough to write a full cycle in six sentences.
See the idea in action
A carbon atom in a starch grain of a leaf was CO₂ last week (photosynthesis). A rabbit eats the leaf (feeding). The rabbit respires some glucose, so the atom is CO₂ again. If the rabbit dies and a fungus decays the tissue, the fungus respires and the atom is CO₂ once more. If the leaf had been buried for millions of years as coal, a power station’s combustion would send the atom to the atmosphere in a pulse that photosynthesis cannot quickly match. The atom was never destroyed.
Exam technique
Turn knowledge into marks
On cycle diagrams, every arrow needs a process name. Photosynthesis in, respiration/decay/combustion out. Link extra fossil-fuel use to a rise in atmospheric CO₂. Do not say carbon is used up.
Common mistakes
Do not give these marks away
- 01
Saying carbon is used up, rather than cycled between atmosphere, organisms and fossil stores.
- 02
Drawing photosynthesis from animals to plants, or forgetting decomposer respiration.
- 03
Blaming climate change on “pollution” without naming carbon dioxide from combustion or reduced photosynthesis.
Which process removes carbon dioxide from the atmosphere in the carbon cycle?
ARespiration
BCombustion
CPhotosynthesis
DDecomposition
Show the answer
Photosynthesis. Producers absorb carbon dioxide to make glucose. Respiration, combustion and decomposition all return carbon dioxide to the air.
Quick questions
If this is the bit you searched
How does carbon dioxide leave the atmosphere in the carbon cycle?
Photosynthesis in plants and algae locks carbon into glucose and other organic compounds.
How does carbon dioxide return to the atmosphere?
Respiration of plants, animals and decomposers, and combustion of wood, peat and fossil fuels.
How do fossil fuels fit into the carbon cycle?
They are ancient stores of carbon from organisms that did not fully decay. Burning them releases extra CO₂ that had been locked away.
How does deforestation affect the carbon cycle?
It reduces photosynthesis (less CO₂ removed) and often adds CO₂ if trees are burned or decay. A carbon store is lost.