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4.3Ecology and the Environment

Cycles within ecosystems — Edexcel Modular - OL Revision Notes & Topical Questions

5 min read 7 parts Lesson 4 of 5 2 quizzes

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Lesson summary

Cycles within ecosystems Edexcel O Level Modular Biology notes: describe the stages in the carbon cycle, including respiration, photosynthesis, decomposition and combustion describe the stages in the nitrogen cycle, including the roles of nitrogen fixing bacteria, decomposers, ni…

  • Cycles within ecosystems Edexcel O Level Modular Biology: what to do next
  • Biogeochemical cycle
  • Nitrogen fixation
  • Nitrification
  • Water cycle
  • Phosphorus cycle

Part 1 of 6Key ideas

Key ideas
Nitrogen is needed for amino acids and nucleic acids.
Phosphorus is needed for DNA and ATP.
Excess nutrients cause algal blooms.

Part 2 of 6Syllabus points

  1. describe the stages in the carbon cycle,
    • including respiration, photosynthesis, decomposition and combustion describe 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)

Part 3 of 6Process & sequence

  • N₂ in air
  • Fixation
  • NH₄⁺
  • Nitrification
  • NO₃⁻
  • Plants absorb
  • Denitrification → N₂
Carbon cycle
Removed by photosynthesis
Returned by respiration and decay
Combustion releases stored carbon

Part 4 of 6Summary

Summary
Bacteria convert N₂ to ammonia by nitrogen fixation.
The phosphorus cycle has no gas phase.

Part 5 of 6Keep going

Cycles within ecosystems Edexcel O Level Modular Biology: what to do next

These Cycles within ecosystems Edexcel O Level Modular Biology notes covered the key ideas, worked examples and exam technique for this topic. Check your Cycles within ecosystems Edexcel O Level Modular Biology understanding with the practice questions below, then reread any Cycles within ecosystems Edexcel O Level Modular Biology point you were unsure about before moving on.

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Practice: Cycles within ecosystems Edexcel O Level Modular Biology practice questions
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Previous lesson: 4.2 Feeding relationships — Notes
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Next lesson: 4.4 Human influences on the environment — Notes
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Course: Edexcel Modular - OL — all units
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Official syllabus: Pearson Edexcel International GCSEs

Cycles within ecosystems Edexcel O Level Modular Biology revision notes

Part 6 of 6Compare: Compare

Major cycles compared
CycleMain reservoirKey processesOrganisms involvedHuman impactGas phase?
CarbonAtmosphere, oceans, fossil fuelsPhotosynthesis, respiration, combustionPlants, decomposersBurning fuels → CO₂ riseYes
NitrogenAtmosphere (78% N₂)Fixation, nitrification, denitrificationRhizobium, NitrosomonasFertilisers → eutrophicationYes
WaterOceansEvaporation, transpiration, precipitationPlantsDams, deforestationYes
PhosphorusRocksWeathering, uptake, decompositionPlants, decomposersFertiliser run-offNo

Key terms

Command terms

Command terms
Give / State / Name
Recall a fact; one idea per mark.
Define
Give the exact meaning using the specification keywords.
Describe
Give an account of what happens / the pattern in data, quoting values; no reasons needed.
Explain
Give reasons with linked statements (cause → effect). Each linked step is usually a separate marking point.
Suggest
Use knowledge in a new context; more than one answer may be accepted.
Compare
Similarities and differences; each point must refer to both items.
Calculate
Show working; include units; check significant figures. Method marks are given even if the final answer is wrong.
Discuss
Explore the issue from different angles and link the points.
Evaluate / Assess
Weigh strengths and weaknesses and reach a supported judgement.
Deduce
Draw a conclusion from the information given.
Justify
Support a statement/choice with evidence.

Exam technique

Keywords the mark scheme looks for in this lesson

Keywords the mark scheme looks for in this lesson
Biogeochemical cycle · Carbon cycle · Nitrogen fixation · Nitrification · Denitrification · Water cycle · Phosphorus cycle · Reservoir
Learn each definition in the Key words tab exactly; a missing keyword in a definition usually loses the mark.

How to get full marks (examiner guidance)

How to get full marks (examiner guidance)
Use specification keywords exactly (e.g. eukaryotic/prokaryotic, enzyme–substrate complex, active site, complementary).
Link ideas
Link ideas for “explain” each marking point is usually a cause → consequence step; unlinked facts often score nothing.
Use the information in the question
“discuss/evaluate” answers must use the data provided and match the number of marks.
Convert units carefully (mg → g, cm³ → dm³) and show working for calculations.
Avoid vague words
“it”, “stuff”, “things”, “goes up”, “energy is made”. Name the substance and the process.
Do not extrapolate a graph line beyond the data or to zero unless asked.
Six-mark extended responses are marked on content AND a logical, linked line of reasoning.

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