3.9.8Additional lessons
The Carbon Cycle — IB Biology HL Revision Notes & Topical Questions
5 min read 5 parts Lesson 26 of 101
Part 1 of 5Moving Carbon Into Organisms
Photosynthesis
- Producers fix carbon dioxide into organic compounds: 6CO₂(g) + 6H₂O(l) → C₆H₁₂O₆(aq) + 6O₂(g).
Feeding
- Carbon in organic compounds moves along food chains when consumers eat producers and other consumers.
Dissolving in water
- Carbon dioxide dissolves in oceans, where it is used by algae and by organisms that form carbonate shells.
Sinks
- Forests, soils and oceans take up more carbon than they release, and so act as carbon sinks.
Part 2 of 5Returning Carbon to the Atmosphere
Returning Carbon to the Atmosphere
Respiration
- Cells oxidise organic compounds, releasing carbon dioxide: C₆H₁₂O₆(aq) + 6O₂(g) → 6CO₂(g) + 6H₂O(l).
Decomposition
- Saprotrophs and detritivores break down dead matter and respire, which releases carbon dioxide to the air.
Combustion
- Burning wood, coal, oil and natural gas, for example CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(g), releases carbon dioxide rapidly.
Methanogens
- In waterlogged conditions with no oxygen, archaea produce methane: 4H₂(g) + CO₂(g) → CH₄(g) + 2H₂O(g).
Part 3 of 5Storage and Long-Term Processes
Carbon can stay in some stores for millions of years.
1
In waterlogged or acid conditions, dead plants are not fully decomposed and accumulate as peat.
2
Over millions of years, heat and pressure change buried organic matter into coal, oil and natural gas, which are fossil fuels.
3
In the sea, shells and skeletons made of calcium carbonate settle and form limestone, a very large store of carbon.
4
Burning fossil fuels and clearing forests now moves carbon from long-term stores into the atmosphere much faster than natural processes remove it.
Part 4 of 5Atmospheric Carbon and Evidence
Atmospheric Carbon and Evidence
Keeling curve
- Measurements of carbon dioxide at Mauna Loa show a yearly rise and fall caused by seasonal photosynthesis, on top of a steady rise.
Seasonal pattern
- Concentration falls in the northern summer when plants take up more carbon dioxide and rises in winter.
Enhanced greenhouse effect
- More atmospheric carbon dioxide and methane traps additional heat and contributes to global warming.
Ocean acidification
- Dissolved carbon dioxide forms carbonic acid, which lowers ocean pH and makes shell formation harder.
| Process | Effect on atmospheric CO₂ |
|---|---|
| Photosynthesis | Decreases |
| Respiration | Increases |
| Combustion | Increases |
| Fossilisation | Decreases (long-term store) |
Part 5 of 5Exam Checklist & Tips
3.9.8 – Key Points for the Exam
- Carbon sink A store that takes in more carbon than it releases.
- Carbon fixation Conversion of inorganic carbon dioxide into organic compounds.
- Combustion Burning of organic fuels that releases carbon dioxide.
- Methanogen An archaean that produces methane in anaerobic conditions.
- Fossil fuel Fuel formed from ancient organic remains.
Common Mistakes
- Treating respiration as something only animals do, when plants, fungi and bacteria also release carbon dioxide.
- Forgetting methane when listing carbon-containing gases.
- Stating that fossil fuels are renewable on human timescales.
- Writing that burning fossil fuels creates carbon; it releases carbon that was already stored.
Last updated:
\n