By the end of this lesson
- Describe the stages of the carbon cycle.
- Explain the roles of respiration, photosynthesis, decomposition and combustion.
- Explain how human activity changes the balance of the cycle.
BIG QUESTION
Where does the carbon in a log go when it rots or burns?
Decomposition and combustion return stored carbon to the air as carbon dioxide by different routes.
Carbon enters living organisms
photosynthesisThe endothermic process in which green plants use light energy, absorbed by chlorophyll, to make glucose from carbon dioxide and water, releasing oxygen. by plants, algae and some bacteria removes carbon dioxide from the air or water and converts it into glucose and other carbon compounds.
Feeding passes these carbon compounds along food chains to consumers.
Carbon returns to the atmosphere
respirationA series of enzyme-controlled chemical reactions in cells that release energy from nutrient molecules, such as glucose; it is not the same as breathing. by plants, animals and microorganisms releases carbon dioxide. Plants respire all the time, including in the light.
decompositionThe breakdown of dead organic matter by decomposers, which releases carbon dioxide and mineral ions.: when organisms die or produce waste, decomposers digest the carbon compounds and release carbon dioxide by their own respiration.
Fossil fuels and combustion
If dead organisms do not decompose fully, for example in waterlogged or oxygen-poor conditions, their carbon can be stored for millions of years as fossil fuelA fuel such as coal, oil or natural gas, formed over millions of years from the remains of dead organisms; burning it releases carbon dioxide. such as coal, oil and natural gas.
combustionBurning a fuel in oxygen, releasing energy and carbon dioxide. of fossil fuels, wood and other biomass releases carbon dioxide rapidly. Burning fossil fuels returns carbon that has been out of the cycle for a very long time.
Balance in the cycle
If photosynthesis removes carbon dioxide as fast as respiration, decomposition and combustion release it, atmospheric concentration stays fairly stable.
Large-scale burning of fossil fuels and deforestation have increased atmospheric carbon dioxide because release now exceeds removal.
Processes in the carbon cycle
State whether each process removes or releases carbon dioxide.
| Process | Effect on atmospheric CO₂ | Organisms or source involved |
|---|---|---|
| Photosynthesis | Removes CO₂. | Plants, algae, some bacteria. |
| Respiration | Releases CO₂. | All living organisms. |
| Decomposition | Releases CO₂ (by decomposer respiration). | Bacteria and fungi. |
| Combustion | Releases CO₂ rapidly. | Burning fossil fuels and biomass. |
| Fossilisation | Locks carbon away for millions of years. | Incompletely decomposed remains. |
Language that earns precision
- carbon cycle
- The continuous movement of carbon between the atmosphere, organisms, oceans and rocks.
- combustion
- Burning a fuel in oxygen, releasing carbon dioxide and energy.
- fossil fuel
- A fuel formed over millions of years from the remains of organisms.
- decomposition
- The breakdown of dead organic matter by decomposers.
Common misconceptions
Claim: Plants only photosynthesise and do not respire.
Correction: Plants respire continuously; in bright light photosynthesis is usually faster than respiration.
Claim: Decomposers release CO₂ by breaking molecules apart.
Correction: They digest the material and release CO₂ through their own respiration.
Claim: Combustion and respiration are the same process.
Correction: Both release CO₂, but combustion is burning; respiration is enzyme-controlled reactions in cells.
Match the answer to the command word
Use the named processes: photosynthesis, feeding, respiration, decomposition, combustion.
Label arrows with the process and show direction of carbon movement.
Recall, apply, then justify
Describe how carbon in a dead leaf is returned to the atmosphere.
Hint
Who feeds on it, and what process releases CO₂?
Suggested marking points
- Decomposers such as bacteria and fungi feed on the leaf.
- They digest the carbon compounds and absorb the products.
- They respire, releasing carbon dioxide into the air.
Explain how burning fossil fuels changes the carbon cycle.
Hint
Stored carbon and rate.
Suggested marking points
- Combustion releases carbon that was stored for millions of years.
- This adds carbon dioxide to the atmosphere faster than photosynthesis removes it.
Peat forms in waterlogged bogs. Explain why carbon accumulates in peat and what would happen if the bog were drained.
Planning hint
Oxygen, decomposers and respiration.
What a strong design includes
- Waterlogged soil has little oxygen.
- Decomposers cannot respire aerobically or work quickly.
- So dead plant material decomposes very slowly.
- Carbon compounds build up as peat.
- Draining lets oxygen in, so decomposition increases.
- Decomposer respiration releases the stored carbon as CO₂.