Cycles within ecosystems
Edexcel International GCSE BiologyΒ· 4.10, 4.11BΒ· 15 min read
1. Overview of Nutrient Cycles in Ecosystemsβ βββββ± 3 min
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Nutrients such as carbon and nitrogen are finite resources. Ecosystems rely on continuous cycling of these elements between living organisms (the biotic community) and non-living parts of the environment (atmosphere, soil, water) to support new growth and sustain life.
Nutrient cycle
A natural process that constantly recycles essential elements between the biotic and abiotic components of an ecosystem, so elements can be reused repeatedly.
Why are nutrient cycles essential for ecosystems?
They create new elements to support growth
They recycle finite elements for repeated use
They remove toxic waste from the atmosphere
They increase the total mass of elements available
Reveal answer
1 βCorrect! Elements like carbon and nitrogen are not created out of nothing, so cycles allow them to be reused by new generations of organisms.
2. The Carbon Cycle (All Candidates, 4.10)β β ββββ± 5 min
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The carbon cycle describes the continuous movement of carbon atoms between the atmosphere (as ), living biomass, fossil fuels, and oceans. All mark scheme answers must include the four required processes listed in the specification.
Outline the main stages of the carbon cycle as required for Edexcel IGCSE Biology exam answers.
- 1
- Atmospheric is the main abiotic reservoir of carbon.
- 2
- Photosynthesis: Plants and algae absorb from the air to make glucose for growth, locking carbon into plant biomass. Carbon passes along food chains when animals eat plants or other animals.
- 3
- Respiration: All living organisms (plants, animals, decomposers) carry out respiration to release energy, which returns to the atmosphere.
- 4
- Decomposition: Decomposers (fungi, bacteria) break down dead organisms and waste products, releasing back to the air via their own respiration.
- 5
- Combustion: Burning of fossil fuels (formed from dead organisms compressed over millions of years) or wood releases stored carbon as into the atmosphere.
3. The Nitrogen Cycle (Biology Only, Paper 2, 4.11B)β β β βββ± 7 min
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Nitrogen is a key component of amino acids (the building blocks of proteins) and DNA, so it is essential for growth and reproduction in all living organisms. 78% of the atmosphere is gas, but it is unreactive and cannot be used directly by most plants or animals.
Describe the role of bacteria in the nitrogen cycle, as required by the Edexcel specification.
- 1
- Nitrogen-fixing bacteria: Convert unreactive atmospheric into usable nitrogen compounds (ammonium or nitrates) that plants can absorb. Some live in root nodules of legume plants (e.g., peas, beans), others are free-living in soil. Lightning also fixes small amounts of atmospheric into usable forms.
- 2
- Decomposers: Break down proteins in dead organisms and urea in animal waste into ammonium compounds in the soil.
- 3
- Nitrifying bacteria: Convert ammonium compounds in soil into nitrates, the form of nitrogen most easily absorbed by plant roots.
- 4
- Denitrifying bacteria: Found in waterlogged, low-oxygen soil, they convert excess soil nitrates back into gas, returning it to the atmosphere to complete the cycle.
- 5
- Additional stage: Plants absorb nitrates to make their own proteins; animals get the nitrogen they need by eating plants or other animals and digesting their proteins.
4. Common Pitfalls
Wrong move:
Forgetting to include decomposition as a process that returns to the atmosphere in the carbon cycle.
Why:
Many students only list respiration and combustion, missing that decomposers respire as they break down dead matter, which is a required mark point.
Correct move:
Always explicitly name photosynthesis, respiration, decomposition, and combustion as the four key carbon cycle processes.
Wrong move:
Stating that plants can absorb atmospheric nitrogen gas directly.
Why:
Nitrogen gas is unreactive, so plants cannot use it without first being converted into nitrates or ammonium by bacteria.
Correct move:
Specify that plants only absorb nitrogen in the form of nitrates from soil via their roots.
Wrong move:
Naming specific bacteria species (e.g., Rhizobium) in nitrogen cycle answers.
Why:
The Edexcel specification explicitly states specific bacterial names are not required, and including them wastes time and risks misspelling and losing marks.
Correct move:
Refer only to the functional groups: nitrogen-fixing, nitrifying, denitrifying, and decomposer bacteria.
Wrong move:
Confusing the roles of nitrifying and denitrifying bacteria.
Why:
Students often mix up which bacteria convert ammonium to nitrates and which convert nitrates back to nitrogen gas.
Correct move:
Use the mnemonic: nitrifying bacteria make nitrates, denitrifying bacteria remove nitrates from soil.
Wrong move:
Omitting feeding as a way carbon and nitrogen move along food chains.
Why:
Many students focus only on abiotic transfers, forgetting that both elements are passed between trophic levels when organisms eat other organisms.
Correct move:
Add a line stating carbon/nitrogen is passed up food chains via feeding in longer answer questions.
5. Quick Reference Cheatsheet
Cycle | Key Processes / Roles | Inputs to atmosphere | Outputs from atmosphere |
|---|---|---|---|
Carbon Cycle (All candidates) | Photosynthesis, respiration, decomposition, combustion, feeding | Respiration, decomposition, combustion | Photosynthesis ( uptake) |
Nitrogen Cycle (Biology only) | Nitrogen fixation, decomposition, nitrification, denitrification, feeding | Denitrification ( release) | Nitrogen fixation (bacteria + lightning) |
6. Frequently Asked
Do I need to memorise specific names of bacteria for the nitrogen cycle?
No, Edexcel explicitly states you only need to refer to functional bacterial groups (nitrogen-fixing, nitrifying, denitrifying, decomposer) not individual genus names like Rhizobium.
Is the nitrogen cycle tested on both Paper 1 and Paper 2?
No, the full nitrogen cycle (4.11B) is Biology-only content, tested exclusively on Paper 2 for 4BI1 single award Biology candidates. Double Award (4SD0) candidates do not need to learn the nitrogen cycle for this topic.
What processes return carbon dioxide to the atmosphere in the carbon cycle?
Three core processes return COβ to the air: respiration (all living organisms), decomposition (decomposers respiring as they break down dead matter), and combustion (burning of fossil fuels or wood).
What's Next
Now that you have mastered the core nutrient cycles in ecosystems, you can move on to learning about human impacts on these cycles, including how excess from combustion drives global warming and how excess nitrogen fertilisers cause eutrophication, which are covered in the next ecology sub-topic. You should also practice past paper questions on both cycles to familiarise yourself with mark scheme wording, and make sure you can label blank cycle diagrams from memory for 4-6 mark structured questions. Double Award candidates can skip nitrogen cycle revision and move directly to human impacts content.