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4.3Ecology & the Environment

Cycles within Ecosystems — Edexcel International GCSE Biology 4BI1 Revision Notes & Topical Questions

10 min read 8 parts Lesson 3 of 7 1 quiz

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Cycles within Ecosystems Edexcel IGCSE Biology 4BI1 revision notes.Plants cannot use nitrogen gas directly; they absorb nitrate ions.Nitrogen is needed to make amino acids, proteins and DNA.Leguminous plants (peas, beans, clover) have nitrogen-fixing bacteria in root nodules.Plou…

  • Nitrifying bacteria are needed
  • Explain why farmers often grow clover or beans in a field before growing wheat
  • Describe how carbon in a dead leaf can end up in the atmosphere
  • Cycles within Ecosystems Edexcel IGCSE Biology 4BI1: what to do next
  • Cycles within Ecosystems Edexcel IGCSE Biology 4BI1: revision checklist
  • Carbon cycle

Part 1 of 7Key ideas

1
Plants cannot use nitrogen gas directly; they absorb nitrate ions.
2
Nitrogen is needed to make amino acids, proteins and DNA.
3
Leguminous plants (peas, beans, clover) have nitrogen-fixing bacteria in root nodules.
4
Ploughing aerates soil, helping nitrifying bacteria and inhibiting denitrifying bacteria.
5
Fossil fuels store carbon from organisms that died millions of years ago.
6
Carbon enters producers only by photosynthesis.
7
Lightning provides energy to fix some atmospheric nitrogen.
8Four ways
Nitrogen can be fixed in four ways: free-living bacteria in soil, root nodule bacteria in legumes, lightning, and fertiliser production (chemical fixation).
9Mutualistic
Root nodule bacteria and leguminous plants have a mutualistic relationship.
10Anaerobic
Denitrifying bacteria work in anaerobic conditions — waterlogged or compacted soil.
11Nitrifying bacteria are needed
Nitrifying bacteria are needed because plants cannot absorb ammonium ions; ammonium must be converted to nitrites and then nitrates.
12Ammonia
Decomposers (bacteria and fungi) change dead organisms and waste into ammonia, which dissolves in soil water as ammonium ions.

Part 2 of 7Notes

    • Learning objectives
      • Describe the stages of the carbon cycle.
      • Describe the stages of the nitrogen cycle.
      • Explain the roles of different bacteria and decomposers.
      • Explain how human activities affect these cycles.
    • Essential knowledge
      • Carbon cycle overview carbon is found as CO2 in the air, in carbon compounds in organisms, dissolved in oceans, and in fossil fuels and limestone.
      • Removal of CO2 plants and algae take in CO2 for photosynthesis, making glucose which becomes starch, cellulose, proteins and fats.
        • Carbon then passes to animals when they feed.
      • Return of CO2 all organisms respire, releasing CO2.
        • When organisms die, decomposers (bacteria and fungi) feed on them and respire, releasing CO2.
        • If decay cannot happen, remains may form fossil fuels over millions of years;
          • burning (combustion) of these fuels releases the stored carbon.
      • Human impact on carbon cycle burning fossil fuels and deforestation increase CO2 in the atmosphere, contributing to the enhanced greenhouse effect.
      • Nitrogen cycle overview air is about 78% nitrogen but it is unreactive, so plants rely on nitrates in soil to make proteins.
        • Animals get nitrogen by eating plants or other animals.
      • Fixation nitrogen-fixing bacteria in soil and in the root nodules of legumes convert N2 into ammonium compounds.
        • Lightning and the industrial Haber process also fix nitrogen.
      • Decomposition and nitrification decomposers convert proteins in dead organisms, faeces and urea into ammonium ions.
        • Nitrifying bacteria oxidise ammonium to nitrite and then nitrate, which plants absorb.
        • These bacteria need oxygen.
      • Denitrification in waterlogged soils lacking oxygen, denitrifying bacteria convert nitrates into nitrogen gas, reducing soil fertility.
        • Farmers drain and plough soil to prevent this.
    • The four ways nitrogen can be fixed
      • Free-living bacteria
        • nitrogen fixation carried out in the soil by bacteria that live freely.
      • Root nodule bacteria
        • live in the nodules of leguminous plants; the relationship is mutualistic.
      • Lightningfixes N2 directly in the air.
      • Fertiliser production
        • chemical fixationnitrogen is fixed industrially and applied to soil as fertiliser.
    • Nitrogen in dead organisms returning to plants
      • Decomposers (bacteria and fungi) break down dead organisms and waste into ammonia.
      • Ammonia enters the soil water as ammonium ions.
      • Nitrifying bacteria convert the ammonium first to nitrites, then to nitrates.
      • Plants absorb the nitrates and use them to make proteins; animals get the nitrogen by feeding on the plants.

Part 3 of 7Process & sequence

    • Nitrogen gas in the air is fixed by bacteria in root nodules or by lightning.
    • Plants absorb nitrates from soil by active transport and make amino acids and proteins.
    • Animals eat plants, gaining nitrogen as protein.
    • Death and excretion return nitrogen compounds to soil; decomposers make ammonium.
    • Nitrifying bacteria convert ammonium to nitrites then nitrates.
    • Denitrifying bacteria in waterlogged soil return nitrogen gas to the air.

    How carbon is cycled through an ecosystemthe five links.

    • Photosynthesisplants remove CO2 from the air.
    • Biomass
      • carbon is stored as carbohydrates, fats and proteins.
    • Feeding
      • herbivores, then carnivores, pass the carbon along.
    • Respirationplants and animals release CO2.
    • Decomposition
      • microorganisms respire dead material, releasing CO2.
    • Combustionfossil fuels burned → CO2.

    The nitrogen cyclewrite these steps in this order; each step is a marking point.

    1. N2 fixed by bacteria/lightning
    2. Nitrates absorbed by plants
    3. Proteins passed to animals
    4. Decomposers make ammonium
    5. Nitrifying bacteria make nitrate
    6. Denitrifying bacteria release N2

    How carbon is cycled through an ecosystemin order

    1. Photosynthesisplants remove CO2 from the air.
    2. Biomass
      • carbon stored as carbohydrates, fats and proteins.
    3. Feedingherbivores, then carnivores.
    4. Respirationplants and animals release CO2.
    5. Decompositionmicroorganisms respire dead material.
    6. Combustionfossil fuels burned → CO2.

Part 4 of 7Worked examples

1Explain why farmers often grow clover or beans in a field before growing wheat.
  • Legumes have root nodules containing nitrogen-fixing bacteria that convert nitrogen gas into ammonium compounds.
    • When the crop is ploughed in, decomposers and nitrifying bacteria convert these into nitrates, increasing soil nitrate for the following wheat crop to make proteins.
2Describe how carbon in a dead leaf can end up in the atmosphere.
  • Decomposers such as fungi and bacteria digest the carbon compounds in the leaf and absorb the products.
    • They use them in respiration, which releases carbon dioxide into the atmosphere.
  • Why are nitrifying bacteria needed in a field, when decomposers have already made ammonium compounds?
    • Because plants can't absorb ammonium ions.
    • Nitrifying bacteria convert the ammonium compounds into nitrites and then nitrates, which the plants can absorb and use to make proteins.
  • Field soil has become compacted and waterlogged. Explain the effect on the nitrogen cycle.
    • Compacted, waterlogged soil has little oxygen — the conditions are anaerobic.
    • Denitrifying bacteria become active in these conditions and convert soil nitrates into nitrogen gas (N2).
    • Nitrate is removed from the soil, so plants take up less nitrogen and make less protein.

Part 5 of 7Summary

    • Carbon is removed from air by photosynthesis.
    • Carbon returns by respiration, decomposition and combustion.
    • Plants take up nitrogen as nitrates.
    • Nitrogen-fixing, nitrifying and denitrifying bacteria drive the nitrogen cycle.
    • Decomposers recycle nutrients from dead matter.

Part 6 of 7Keep going

  1. Cycles within Ecosystems Edexcel IGCSE Biology 4BI1: what to do next

    These Cycles within Ecosystems Edexcel IGCSE Biology 4BI1 notes covered the key ideas, worked examples and exam technique for this topic. Check your Cycles within Ecosystems Edexcel IGCSE Biology 4BI1 understanding with the practice questions below, then reread any Cycles within Ecosystems Edexcel IGCSE Biology 4BI1 point you were unsure about before moving on.

    Cycles within Ecosystems Edexcel IGCSE Biology 4BI1: revision checklist

    • Revise the main Cycles within Ecosystems Edexcel IGCSE Biology 4BI1 definitions until you can write them from memory.
    • Practise Cycles within Ecosystems Edexcel IGCSE Biology 4BI1 exam questions under timed conditions.
    • Link each Cycles within Ecosystems Edexcel IGCSE Biology 4BI1 idea to a diagram or a worked example.
    • List the common Cycles within Ecosystems Edexcel IGCSE Biology 4BI1 mistakes and how to avoid them.
    • Explain one Cycles within Ecosystems Edexcel IGCSE Biology 4BI1 idea out loud as if teaching a friend.
    • Summarise Cycles within Ecosystems Edexcel IGCSE Biology 4BI1 in five key points on a revision card.

Part 7 of 7Compare: Compare

Processes in the carbon cycle
ProcessEffect on atmospheric CO2Carried out by
PhotosynthesisRemovesPlants, algae
RespirationAddsAll living organisms
DecompositionAdds (via respiration)Bacteria and fungi
CombustionAddsBurning fossil fuels and wood
FeedingNo direct change – carbon passes along food chainsAnimals
Ways nitrogen can be fixed
Way of fixingWhere / howType
Free-living bacteriaIn soilBiological
Root nodule bacteriaIn leguminous plants — a mutualistic relationshipBiological
LightningN2 fixed in the airNatural
Fertiliser productionChemical fixation made industrially, added to soilHuman / chemical
Bacteria in the nitrogen cycle
BacteriaConversionConditions
Nitrogen-fixingN2 → ammonium compoundsRoot nodules of legumes, or free in soil
DecomposersProteins/urea → ammoniumSoil
NitrifyingAmmonium → nitrite → nitrateAerobic, well-aerated soil
DenitrifyingNitrate → N2Anaerobic, waterlogged soil

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
Carbon cycle · Nitrogen cycle · Decomposer · Nitrogen fixation · Nitrification · Denitrification · Ammonification · Root nodules · Combustion
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)

1
Use specification keywords exactly (e.g. eukaryotic/prokaryotic, enzyme–substrate complex, active site, complementary).
2Link ideas
Link ideas for “explain” each marking point is usually a cause → consequence step; unlinked facts often score nothing.
3Use the information in the question
“discuss/evaluate” answers must use the data provided and match the number of marks.
4
Convert units carefully (mg → g, cm3 → dm3) and show working for calculations.
5Avoid vague words
“it”, “stuff”, “things”, “goes up”, “energy is made”. Name the substance and the process.
6
Do not extrapolate a graph line beyond the data or to zero unless asked.
7
Six-mark extended responses are marked on content AND a logical, linked line of reasoning.
8Exam-practice wording
  • For denitrifying bacteria conditions give both ideas: anaerobic (no oxygen) and waterlogged or compacted soil — "compacted" is often left out.
  • When explaining why nitrifying bacteria are needed, write "plants cannot absorb ammonium ions" — not "plants need nitrogen" alone.
  • Farmers reduce denitrification by ploughing, which aerates the soil.
  • Use the word mutualistic for root nodule bacteria and leguminous plants;
    • name chemical fixation / fertiliser production as a way nitrogen is fixed — a common answer that omits it scores one mark fewer.

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