(c) Cycles within ecosystems
- Syllabus
- 2024
- Topic
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- Level
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Learning objectives
4.10Carbon cycleDescribe the stages in the carbon cycle, including respiration, photosynthesis, decomposition and combustion.411B Nitrogen cycleDescribe the stages in the nitrogen cycle, including the roles of nitrogen fixing bacteria, decomposers, nitrifying bacteria and denitrifying bacteria (specific names of bacteria are not required).
Route carbon through an ecosystem
The carbon cycle transfers carbon among atmospheric carbon dioxide, living biomass, dead organic matter and fuels; each arrow must be labelled by a process.
| From → to | Process and mechanism |
|---|---|
| atmospheric CO₂ → producer biomass | photosynthesis fixes carbon into carbohydrates and other organic molecules |
| producer → consumer | feeding transfers carbon compounds along food chains |
| living organism → atmospheric CO₂ | respiration oxidises organic molecules and releases carbon dioxide |
| dead material and waste → decomposers | feeding and extracellular digestion transfer carbon to bacteria and fungi |
| decomposers → atmospheric CO₂ | decomposer respiration releases carbon dioxide |
| biomass or fossil fuel → atmospheric CO₂ | combustion releases carbon dioxide |
A carbon sink absorbs more carbon than it releases. Increased photosynthesis can strengthen a sink, while respiration, decomposition, combustion and reduced photosynthesis after deforestation can weaken it.
Photosynthesis removes atmospheric CO₂; respiration, decomposition through decomposer respiration, and combustion return it. Carbon matter cycles, but the energy driving these processes does not cycle.
Route nitrogen through bacteria, plants and animals
Nitrogen cycles between atmospheric nitrogen gas, ammonium compounds, nitrates and nitrogen-containing molecules in organisms. Bacteria make the key inorganic conversions.
| Process | Conversion and role |
|---|---|
| nitrogen fixation | nitrogen-fixing bacteria convert atmospheric N₂ into ammonia or ammonium compounds |
| decomposition / ammonification | decomposers digest dead organisms and waste; nitrogen in proteins and nucleic acids becomes ammonium compounds |
| nitrification | nitrifying bacteria oxidise ammonium to nitrite and then nitrite to nitrate |
| plant uptake | roots absorb nitrate ions and plants use nitrogen to make amino acids, proteins and nucleic acids |
| feeding | nitrogen-containing organic molecules pass from plants to animals |
| denitrification | denitrifying bacteria convert nitrate into nitrogen gas, returning it to the atmosphere |
Animal excretion and death return organic nitrogen to decomposers. Leached nitrate can leave soil for water, while denitrification lowers soil nitrate availability. Specific bacterial species names are not required.
Plants cannot use atmospheric N₂ directly in this cycle: nitrogen fixation must first create usable compounds. Nitrifying bacteria make nitrate; denitrifying bacteria remove nitrate by returning nitrogen to the atmosphere.