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4.3Ecology and the Environment

Cycles within Ecosystems — Edexcel Linear - OL Revision Notes & Topical Questions

5 min read 7 parts Lesson 9 of 10 1 quiz

📚 Edexcel Linear - OL Hub / Topical Revision
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Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
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Lesson summary

The Carbon Cycle and the Nitrogen Cycle — EDEXCEL LINEAR - OL notes: Describe how carbon and nitrogen are cycled through ecosystems, naming the processes and the organisms involved.Carbon is part of the carbohydrates, proteins and fats in cells and is cycled by photosynthesis, fe…

  • respiration
  • decomposition
  • combustion
  • Nitrogen fixation
  • Worked example 1
  • Worked example 2

Part 1 of 5The carbon cycle

Carbon is part of the carbohydrates, proteins and fats in cells and is cycled by photosynthesis, feeding, respiration, decomposition and combustion.

1Photosynthesis
Plants take carbon dioxide from the atmosphere by photosynthesis and use the carbon to make glucose,
  • which becomes carbohydrates, fats and proteins in plant biomass.
2
Carbon passes to animals when herbivores eat plants and when carnivores eat other animals (feeding).
3Respiration
Carbon returns to the air as carbon dioxide by respiration in plants and animals.
4Decomposition
Bacteria and fungi use dead and waste material in respiration, releasing carbon dioxide during decomposition.
5Combustion
If dead organisms are buried where decomposers are absent, they can form fossil fuels; burning them (combustion) releases carbon dioxide.

Part 2 of 5Nitrogen in the cycle: fixation and uptake

Nitrogen is 78% of the air as N2 gas, which most organisms cannot use, and it is present in proteins in living tissue. Nitrogen fixation makes it available: nitrogen-fixing bacteria convert N2 into ammonium compounds, which are converted into nitrates in the soil.

Nitrogen in the cycle: fixation and uptake
These bacteria are either free-living in the soil or live in root nodules of leguminous plants (peas, beans, clover) in a mutualistic relationship.
Lightning and the industrial production of fertilisers also fix nitrogen.
Plants absorb nitrates and use them to build proteins; animals get nitrogen by eating plants and then other animals.

Because of this, farmers who grow legumes in rotation, or plough them back into the soil, can raise soil nitrate naturally. Waterlogged or compacted soil with little oxygen favours denitrifying bacteria, so well-aerated soil keeps more nitrate available to crops.

Part 3 of 5Returning nitrogen: decomposition, nitrification, denitrification

ProcessOrganismsChange
DecompositionBacteria and fungiProteins and urea from dead organisms and waste become ammonia (then ammonium ions)
NitrificationNitrifying bacteriaAmmonia to nitrites to nitrates, which plants can absorb
DenitrificationDenitrifying bacteria (anaerobic conditions)Nitrates to nitrogen gas, returned to the atmosphere

Plants cannot absorb ammonium ions directly, which is why nitrification matters. Denitrifying bacteria are active in waterlogged or compacted soil with little oxygen; farmers reduce denitrification by ploughing, which aerates the soil.

Part 4 of 5Comparing the cycles

Both cycles keep essential elements available because decomposers return them from dead matter.

Comparing the cycles
Carbon moves mainly as carbon dioxide in the air and as organic compounds in organisms; nitrogen moves mainly as nitrates taken up by roots and as proteins in organisms.
In both, bacteria and fungi are essential.
Human activities alter them: burning fossil fuels adds carbon dioxide, and fertilisers add nitrate.

Examination linksthe use of carbon dioxide in photosynthesis and its release in respiration balance each other in a stable ecosystem.

1
Burning fossil fuels and deforestation upset this balance and raise carbon dioxide levels.
2
In the nitrogen cycle, mutualism is shown by legumes and nitrogen-fixing bacteria: the bacteria receive carbohydrates and the plant receives nitrates.
3
Farmers use crop rotation with legumes to restore soil nitrate without adding as much fertiliser.
4
A good answer to a cycle question names the process, the organisms responsible and the chemical form of the element before and after the change.

Summary of forms

  • carbon dioxide in the air;
    • glucose and other organic compounds in plants and animals;
    • fossil fuels underground.
  • Nitrogen gas in the air;
    • ammonium ions and nitrates in soil;
    • proteins in organisms;
    • urea in waste.

Part 5 of 5Worked examples

  1. Name the process that removes carbon dioxide from the air and the process that returns it. [2]2 marks

  2. Describe the role of bacteria in the root nodules of clover. [2]2 marks

  3. Explain why waterlogged soil may become short of nitrates. [3]3 marks

  4. Why can plants not use nitrogen gas directly? [1]1 mark

Key terms

TermMeaning
Nitrogen fixationConversion of nitrogen gas into ammonium compounds and nitrates.
NitrificationConversion of ammonia to nitrites and nitrates by bacteria.
DenitrificationConversion of nitrates to nitrogen gas.
DecompositionBreakdown of dead matter by bacteria and fungi.
CombustionBurning, releasing carbon dioxide.
LegumePlant with nitrogen-fixing root nodules.

Avoid these mistakes

1
Saying plants take in nitrogen gas — they absorb nitrates.
2
Confusing nitrification with nitrogen fixation.
3
Forgetting decomposers release carbon dioxide by respiration.
4
Saying denitrification helps plants — it removes nitrate.

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