What are the main cycles within ecosystems covered in the Edexcel IGCSE Biology curriculum?

The main cycles within ecosystems covered in the Edexcel IGCSE Biology curriculum include the carbon cycle, the nitrogen cycle, and the water cycle. These cycles are crucial for maintaining the balance of ecosystems by recycling essential elements and compounds.

How does the carbon cycle function in an ecosystem?

In an ecosystem, the carbon cycle involves the movement of carbon among the atmosphere, organisms, and the earth. Plants absorb carbon dioxide during photosynthesis to produce glucose. Animals consume plants, incorporating carbon into their bodies. Respiration by plants and animals releases carbon dioxide back into the atmosphere. Decomposition of dead organisms also returns carbon to the soil and atmosphere, completing the cycle.

Why is the nitrogen cycle important for ecosystems?

The nitrogen cycle is vital for ecosystems because it converts nitrogen from the atmosphere into forms that plants and animals can use to build proteins and nucleic acids. Nitrogen-fixing bacteria convert atmospheric nitrogen into ammonia, which is then transformed into nitrates by nitrifying bacteria. Plants absorb these nitrates, and animals obtain nitrogen by consuming plants. Decomposers return nitrogen to the soil, and denitrifying bacteria release it back into the atmosphere.

What role does the water cycle play in ecosystems?

The water cycle is essential for ecosystems as it distributes water, a critical resource for all living organisms. It involves the processes of evaporation, condensation, precipitation, and infiltration. Water evaporates from surfaces, forms clouds through condensation, and falls as precipitation. This cycle ensures the continuous supply of fresh water to ecosystems, supporting plant growth and providing habitats for aquatic life.

How do human activities impact cycles within ecosystems?

Human activities can significantly impact cycles within ecosystems. For example, burning fossil fuels increases carbon dioxide levels, affecting the carbon cycle and contributing to climate change. Excessive use of fertilizers can lead to nitrogen pollution, disrupting the nitrogen cycle and causing issues like eutrophication. Deforestation and urbanization alter the water cycle by changing evaporation and precipitation patterns, impacting water availability.

Why is "Saying plants only photosynthesise, not respire" a common mistake in Cycles within Ecosystems ?

Why it happens: Photosynthesis dominates during the day, so students think plants only photosynthesise. How to avoid it: Plants respire 24 hours a day, like all living organisms. During the day, photosynthesis EXCEEDS respiration → net O₂ release and CO₂ uptake. At night, only respiration happens → net CO₂ release. Source: 4BI1 Examiner Reports 2024

Why is "Confusing Nitrosomonas and Nitrobacter" a common mistake in Cycles within Ecosystems ?

Why it happens: Similar-sounding names; both are 'nitrifying bacteria'. How to avoid it: Mnemonic: **Nitroso**Monas does aMMonia first (NH₃ → NO₂⁻). **Nitro**Bacter does nitrIte to nitrAte (NO₂⁻ → NO₃⁻). Both need oxygen. Source: 4BI1 Examiner Reports 2023-2024

Why is "Confusing Rhizobium and Azotobacter, or saying both live in nodules" a common mistake in Cycles within Ecosystems ?

Why it happens: Both are 'nitrogen-fixing bacteria'. How to avoid it: **Rhizobium** (rhiza = root) lives INSIDE root nodules of legumes (mutualism). **Azotobacter** (azoto = nitrogen) is FREE-LIVING in soil — does not need a plant host. Source: 4BI1 Examiner Reports 2024

Why is "Saying denitrification is part of healthy nitrogen cycling" a common mistake in Cycles within Ecosystems ?

Why it happens: It's part of the cycle, so students assume it's beneficial. How to avoid it: Denitrification REMOVES nitrate (the form plants can use) → returns it to the atmosphere as inert N₂. For ECOSYSTEMS it is essential to balance fixation, but for FARMERS it is UNDESIRABLE — drained soils prevent it. Source: 4BI1 Examiner Reports 2024

Why is "Saying plants 'fix nitrogen' directly from the air" a common mistake in Cycles within Ecosystems ?

Why it happens: Confusion between plants and bacteria. How to avoid it: PLANTS cannot fix N₂. Only BACTERIA (Rhizobium, Azotobacter, blue-green algae) can fix atmospheric nitrogen. Plants only absorb the NITRATE produced by the bacterial chain (or applied as fertiliser). Source: 4BI1 Examiner Reports 2023

Why is "Describing the water cycle without mentioning transpiration" a common mistake in Cycles within Ecosystems ?

Why it happens: Most textbook diagrams emphasise evaporation/condensation/precipitation. How to avoid it: Transpiration from plant leaves is a HUGE contributor to atmospheric water — especially in forests. Always include transpiration AND evaporation in answers about the water cycle.

Ecology and the EnvironmentEdexcel IGCSE Biology (4BI1)

Cycles within Ecosystems

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Cycles within Ecosystems — Pearson Edexcel International GCSE Biology 4BI1 Study Notes (2026 onwards, Spec Issue 4)

How carbon, nitrogen and water are recycled through ecosystems. Carbon: photosynthesis (in), respiration / decomposition / combustion (out). Nitrogen (Paper 2): fixation (Rhizobium, Azotobacter, lightning), ammonification (decomposers), nitrification (Nitrosomonas → Nitrobacter), plant uptake of nitrate, denitrification (anaerobic). Water: evaporation, transpiration, condensation, precipitation. Covers spec 4.12-4.16P.

What you’ll learn

Mapped to the Cambridge IGCSE 4BI1 syllabus (2026 onwards).

4.12 — Describe the stages in the carbon cycle including photosynthesis, respiration, feeding, decomposition and combustion.
4.13P — (Paper 2) Describe the stages in the nitrogen cycle including the roles of nitrogen-fixing bacteria, nitrifying bacteria, denitrifying bacteria and decomposers (Higher Tier).
4.14 — Describe the water cycle including evaporation, transpiration, condensation, precipitation and surface runoff / infiltration.
4.15 — Understand the role of decomposers in the recycling of nutrients in ecosystems.
4.16P — (Paper 2) Understand how nitrate is taken up by plants from the soil and converted into amino acids and proteins.

The carbon cycle (spec 4.12)

Photosynthesis takes CO₂ out; respiration, decomposition, combustion put it back.

The carbon cycle moves carbon atoms between the atmosphere, oceans, living organisms and fossil fuels. The atmosphere contains ~0.04 % CO₂ — small but vital.

Processes that REMOVE CO₂ from the atmosphere:

Photosynthesis (the ONLY major sink). $6CO_2 + 6H_2O \rightarrow C_6H_{12}O_6 + 6O_2$ Plants, algae and some bacteria absorb CO₂ → fix it into glucose using light energy. The carbon now enters the food chain.
Ocean dissolution. CO₂ dissolves in seawater forming carbonic acid (H₂CO₃) and bicarbonate ions (HCO₃⁻). Marine producers (phytoplankton) use this dissolved CO₂. Some carbon is locked into carbonate shells (forams, corals) → eventually limestone.

Processes that RETURN CO₂ to the atmosphere:

Respiration. All living organisms (plants, animals, decomposers) respire continuously: glucose + O₂ → CO₂ + H₂O + energy. This is the main natural source of atmospheric CO₂.
Decomposition. When organisms die, saprobiotic bacteria and fungi break down the dead matter. As they respire the absorbed nutrients, they release CO₂.
Combustion (burning). Burning wood OR fossil fuels (coal, oil, natural gas) releases CO₂. Fossil fuels are the compressed remains of plants and animals that died millions of years ago in anaerobic conditions — burning them releases carbon that has been locked away geologically.

Carbon flow up the food chain (feeding):

Plants fix CO₂ → glucose, then convert glucose to starch, cellulose, proteins, lipids, DNA — all organic carbon compounds.
Herbivores eat plants → digest carbon compounds → use for their own tissues + respiration.
Carnivores eat herbivores → same process.
All organisms eventually die → decomposers break them down → CO₂ returned to atmosphere.

Long-term lock-up:

Fossil-fuel formation: dead organisms buried in anaerobic conditions (peat bogs, ocean sediments) over millions of years → coal, oil, natural gas. Carbon locked out of the active cycle.
Limestone: carbonate shells of marine organisms accumulate on the seabed → become sedimentary rock. Released only by weathering or volcanism over geological timescales.

Human disruption. In the natural cycle, photosynthesis and respiration roughly balance. Burning fossil fuels and deforestation add CO₂ faster than photosynthesis can remove it → atmospheric CO₂ has risen from 280 ppm (pre-industrial) to >420 ppm today → ENHANCED greenhouse effect → global warming.

Photosynthesis = ONLY major process removing CO₂.
Respiration + decomposition + combustion return CO₂.
Fossil fuels = ancient carbon locked away; burning returns it.
Carbon flows up food chain via feeding; returns via respiration / decomposition.
Human disruption: combustion + deforestation outpace photosynthesis → CO₂ rise.

See the full worked example for cycles within ecosystems →

The nitrogen cycle — Paper 2 only (spec 4.13P, 4.16P)

Five named groups of bacteria. Get the names exact for full marks.

Atmospheric nitrogen (N₂) makes up 78 % of air — but it is INERT. Plants and animals cannot use N₂ directly. It must be CONVERTED (fixed) into ammonia, then nitrate, before plants can absorb it.

Step 1 — Nitrogen fixation (N₂ → NH₃ / NH₄⁺).

Three routes:

Rhizobium (a bacterium) lives in MUTUALISTIC root nodules of LEGUMINOUS plants (peas, beans, clover, lentils). The plant supplies sugars; Rhizobium fixes N₂ → ammonia → used by the plant. After harvest, the nitrogen-rich roots are ploughed back into the soil — natural fertiliser.
Azotobacter (a free-living soil bacterium) fixes atmospheric N₂ → ammonia in the soil itself, without a plant host. Some species live in water.
Lightning. The huge energy in a lightning strike combines atmospheric N₂ with O₂ → nitrogen oxides → dissolved in rain → reach the soil as nitric acid → nitrate.
(Industrial: the Haber process N₂ + 3H₂ → 2NH₃ makes ammonia fertiliser — not biological, but a major source of soil nitrogen on farmland.)

Step 2 — Ammonification (proteins / urea → ammonia).

Saprobiotic decomposers (bacteria and fungi) break down:

DEAD ORGANISMS (proteins, nucleic acids).
ANIMAL WASTE (urea in urine).

They release AMMONIA (NH₃) / AMMONIUM ions (NH₄⁺) into the soil.

Step 3 — Nitrification (NH₃ → NO₂⁻ → NO₃⁻).

Nitrifying bacteria in well-AERATED (aerobic) soil carry out a TWO-STEP oxidation:

Nitrosomonas: NH₃ / NH₄⁺ → NO₂⁻ (nitrite). Requires O₂.
Nitrobacter: NO₂⁻ → NO₃⁻ (nitrate). Requires O₂.

Both reactions release energy that the bacteria use for their own metabolism.

Step 4 — Plant uptake of nitrate (spec 4.16P).

Plants absorb NITRATE (NO₃⁻) from the soil through their ROOT HAIR CELLS by ACTIVE TRANSPORT (nitrate concentration is higher inside the cell than outside). The nitrate is transported in the xylem to leaves.

In the leaves, plants use nitrate to make AMINO ACIDS, which they then link together into PROTEINS. Plants also need nitrogen for:

DNA and RNA (nitrogenous bases).
Chlorophyll (contains N).
ATP.

Step 5 — Consumers — nitrogen passes UP the food chain.

Animals eat plants (or other animals). They DIGEST plant proteins → amino acids → use these to build their own proteins.

Step 6 — Excretion.

When animals break down excess amino acids in the LIVER (deamination), they form UREA, which they excrete in urine. Decomposers then convert this back to ammonia (Step 2 again).

Step 7 — Denitrification (NO₃⁻ → N₂).

Denitrifying bacteria (e.g. Pseudomonas) are anaerobic — they thrive in WATERLOGGED soils where there is no O₂.

They convert NO₃⁻ → N₂ gas, which escapes to the atmosphere. This step is UNDESIRABLE for farmers because it REMOVES nitrate from the soil → plants are short of nitrogen → poor growth.

Why farmers care.

Drain and plough fields → keep soil aerated → favours nitrifying bacteria over denitrifying.
Plant legumes (clover, peas) in rotation → Rhizobium adds nitrogen.
Apply nitrate fertiliser → direct route to plant nitrogen.
Avoid over-applying (causes EUTROPHICATION in streams).

FIVE bacteria types: Rhizobium, Azotobacter, Nitrosomonas, Nitrobacter, denitrifying bacteria (Pseudomonas).
Fixation: N₂ → NH₃ (Rhizobium in root nodules of legumes; Azotobacter free in soil; lightning).
Ammonification: decomposers break down dead matter + urea → NH₃.
Nitrification (aerobic): Nitrosomonas (NH₃ → NO₂⁻) → Nitrobacter (NO₂⁻ → NO₃⁻).
Plants absorb NITRATE (NO₃⁻) by active transport.
Denitrification (anaerobic, waterlogged): NO₃⁻ → N₂ (BAD for farmers).

See the full worked example for cycles within ecosystems →

The water cycle (spec 4.14)

Evaporation + transpiration → condensation → precipitation → runoff / infiltration.

The water cycle (hydrological cycle) moves water between oceans, atmosphere, land and living organisms — continuously, powered by solar energy.

Stages:

Evaporation. Heat from the sun causes liquid water in oceans, rivers, lakes, soil, etc. to evaporate → water vapour rises into the atmosphere. The largest source is tropical oceans.
Transpiration. Water inside plant leaves evaporates from the mesophyll cell surfaces and diffuses out through the STOMATA. In rainforests, transpiration can account for >50 % of atmospheric moisture.
Condensation. As water vapour rises, it cools (temperature decreases with altitude). At a low enough temperature, the vapour condenses around tiny dust / aerosol particles → forms droplets → forms clouds.
Precipitation. Droplets merge until heavy enough to fall as RAIN (or snow, hail, sleet). Returns water to land or ocean.
Surface runoff. On land, water flows over the surface → into streams → rivers → back to the sea.
Infiltration / percolation. Water seeps through soil into groundwater (aquifers) → eventually feeds rivers and springs.
Plant uptake. Roots absorb soil water by osmosis through ROOT HAIR CELLS. The water rises through the xylem to leaves — the transpiration stream — driven by the suction of evaporation from leaves. Delivers dissolved mineral ions to all tissues.

Why plants matter to the water cycle:

Transpiration cools the local climate — like sweat cools the body, transpired water carries latent heat away.
Tree cover slows runoff → roots hold soil; canopy intercepts rain → more water enters groundwater (recharges aquifers) rather than washing away.
Deforestation disrupts this — without trees, less transpiration → less local rainfall (drier climate); more runoff → flash floods; soil erosion downstream.

Drinking-water resources. Most accessible fresh water is in groundwater (aquifers); a small fraction is in rivers and lakes. Over-extraction faster than recharge (e.g. for irrigation) depletes aquifers — a major sustainability concern in agriculture.

Evaporation + transpiration → atmosphere.
Condensation → clouds.
Precipitation (rain, snow) → land/sea.
Runoff or infiltration → rivers/groundwater.
Plants pull water from soil + release as transpiration — significant in forests.

See the full worked example for cycles within ecosystems →

Role of decomposers (spec 4.15)

Recycle nutrients — without them, cycles would stop and dead matter would accumulate.

Decomposers are saprobiotic bacteria and fungi that break down DEAD organic matter and waste. They are essential for recycling nutrients.

How they work (saprobiotic nutrition):

Secrete digestive ENZYMES onto the dead material.
Enzymes break large molecules down into smaller, soluble nutrients (proteins → amino acids; carbohydrates → sugars; lipids → fatty acids + glycerol).
Absorb the soluble nutrients across their cell wall.
Use the nutrients for respiration (release CO₂) and growth.

Role in the carbon cycle:

They RESPIRE the carbon they absorb → release CO₂ back to the atmosphere.
Without decomposers, carbon would stay locked in dead bodies → atmosphere starved of CO₂ → plants couldn't photosynthesise → ecosystem collapses.

Role in the nitrogen cycle:

They break down PROTEINS (in dead organisms) and UREA (in urine) → release AMMONIA (ammonification).
This ammonia is the starting material for nitrifying bacteria (Nitrosomonas → Nitrobacter) → eventually plant nitrate.
Without decomposers, plant nitrogen would have no way back into the soil → crops would fail.

Factors affecting rate of decomposition:

Temperature — higher = faster (enzymes work better). Compost heaps heat themselves up.
Moisture / water — decomposers need water for enzyme activity. Dry conditions slow decay.
Oxygen — most decomposers are aerobic. Waterlogged anaerobic conditions slow decomposition and produce methane.
pH — most decomposers prefer near-neutral pH; acidic soils slow decomposition.
Material — soft tissues decay faster than woody/lignin-rich material; bones and shells last longest.

Practical relevance:

Composting: mix garden waste with food scraps; keep moist + warm + aerated → fast decomposition → nutrient-rich compost for plants.
Sewage treatment: uses aerobic bacteria (decomposers) to break down sewage in trickling filters.
Bioremediation: specific bacteria are used to clean up oil spills, heavy metals, etc.

Without decomposers: within a few years, dead bodies would pile up; soil nutrients would deplete; plant growth would crash; the entire ecosystem would fail.

Decomposers = saprobiotic bacteria + fungi.
Carbon cycle: respire dead matter → release CO₂.
Nitrogen cycle: ammonification of proteins / urea → NH₃.
Rate depends on: temperature, moisture, O₂, pH, material type.
Without decomposers → nutrient lock-up → ecosystem collapse.

See the full worked example for cycles within ecosystems →

Cycles compared — atoms recycle, energy doesn't

Carbon, nitrogen, water all cycle through living + non-living parts.

Common features of biogeochemical cycles:

Feature	Carbon	Nitrogen	Water
Atmospheric reservoir	CO₂ (~0.04 %)	N₂ (78 %)	Water vapour (variable)
Living entry process	Photosynthesis	Fixation (Rhizobium, Azotobacter)	Transpiration / root uptake
Decomposer role	Respiration → CO₂	Ammonification → NH₃	(Releases dissolved water as vapour)
Returns to atmosphere	Respiration, combustion	Denitrification	Evaporation, transpiration
Long-term storage	Fossil fuels, limestone	Soil nitrate	Glaciers, oceans, groundwater

Key contrasts with energy flow:

Atoms (C, N, water molecules) are RECYCLED through the cycle — they pass between organisms and environment endlessly.
Energy is NOT recycled — it flows in ONE direction (sun → producers → consumers → heat lost to space). The Sun must continuously resupply energy via photosynthesis.

Why this matters. When humans interfere with a cycle:

Carbon cycle disruption (excess combustion of fossil fuels + deforestation) → atmospheric CO₂ rises → enhanced greenhouse effect → climate change.
Nitrogen cycle disruption (excess fertiliser / sewage) → eutrophication of waterways.
Water cycle disruption (deforestation, over-extraction of groundwater) → drought, flooding, water shortages.

Sustainable management seeks to keep cycles in balance — reforest, reduce fossil-fuel use, treat sewage, manage fertiliser carefully.

Atoms cycle (carbon, nitrogen, water); energy flows one way.
Each cycle has an atmospheric reservoir + a living reservoir + storage forms.
Decomposers are central to all biogeochemical cycles.
Human disruption: CO₂ rise (carbon), eutrophication (N), water shortage (water).

See the full worked example for cycles within ecosystems →

How it’s examined

Paper 1 (4BI1/1B): identify processes in the carbon cycle (photosynthesis, respiration, combustion, decomposition) — short recall questions (1-4 marks). Paper 2 (4BI1/2B): EXTENDED nitrogen cycle questions — name bacteria + their roles (4-6 marks); explain why farmers drain soil; describe a full cycle with diagrams (8-10 marks). Water cycle questions usually focus on the role of transpiration. Examiner reports note that Higher Tier candidates MUST use the specific bacteria names (Rhizobium, Nitrosomonas, Nitrobacter) — generic 'nitrifying bacteria' loses marks.

Sources: Pearson Edexcel International GCSE Biology 4BI1 specification — Issue 4, September 2024 (valid for 2026 onwards); Pearson Edexcel 4BI1 Examiner Reports 2022-2024; 4BI1/1B May/Jun 2024 question paper and mark scheme; 4BI1/1B January 2024 question paper and mark scheme; 4BI1/2B May/Jun 2024 question paper and mark scheme; 4BI1/2B January 2024 question paper and mark scheme. Last reviewed 2026-05-12.

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Step-by-step worked examples — Cycles within Ecosystems

Step-by-step solutions to past-paper-style questions on cycles within ecosystems , written exactly the way a tutor would explain them at the board.

1Identifying processes in the carbon cycle (4 marks, Paper 1)
Core• Adapted from 4BI1/1B January 2024 Q7(a)• carbon cycle, spec-4.12
Question
State the name of the process that: (a) removes CO₂ from the atmosphere; (b) returns CO₂ to the atmosphere from living organisms; (c) returns CO₂ to the atmosphere from fossil fuels; (d) returns CO₂ to the atmosphere from dead organisms. (4 marks)
Step-by-step solution
1. Step 1
  (a) Removing CO₂ from atmosphere (B1). PHOTOSYNTHESIS — carried out by plants, algae and some bacteria: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂. This is the ONLY major process that removes CO₂ from the atmosphere.
2. Step 2
  (b) CO₂ returned from living organisms (B1). RESPIRATION — plants, animals AND decomposers all respire: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy. Respiration happens 24/7 in every living cell.
3. Step 3
  (c) CO₂ returned from fossil fuels (B1). COMBUSTION (burning) — burning fossil fuels (coal, oil, natural gas) and wood releases CO₂ that has been LOCKED AWAY for millions of years. This is the main human contribution to atmospheric CO₂ rise.
4. Step 4
  (d) CO₂ returned from dead organisms (B1). DECOMPOSITION / decay — by saprobionts (decomposer bacteria and fungi). Decomposers respire as they break down dead matter → release CO₂.
Answer
(a) Photosynthesis. (b) Respiration. (c) Combustion / burning. (d) Decomposition / decay.
Examiner tip
Mark scheme requires PROCESS NAMES, not equations. Examiner reports flag candidates who say 'photosynthesis and respiration' for one part — be precise about which process answers which question. 'Eating' / 'breathing' do not score for respiration — must use 'respiration'.
2Naming bacteria in the nitrogen cycle (5 marks, Paper 2)
Extended• Adapted from 4BI1/2B May/Jun 2024 Q5(a)• nitrogen cycle, bacteria, Paper 2, spec-4.13P
Question
Name the type of bacteria responsible for each of the following steps in the nitrogen cycle, and state where they live. (a) Nitrogen fixation in plant roots. (b) Nitrogen fixation in free soil. (c) Nitrification of ammonia to nitrite. (d) Nitrification of nitrite to nitrate. (e) Denitrification. (5 marks)
Step-by-step solution
1. Step 1
  (a) Plant-root N-fixation (B1). Rhizobium — lives in mutualistic ROOT NODULES of leguminous plants (peas, beans, clover). The plant supplies sugars; Rhizobium supplies fixed nitrogen.
2. Step 2
  (b) Free-living N-fixation (B1). Azotobacter — free-living in the SOIL (and some species in water). Fixes atmospheric N₂ into ammonia without needing a plant host.
3. Step 3
  (c) Ammonia → nitrite (B1). Nitrosomonas — nitrifying bacteria in well-aerated soil. They obtain energy by oxidising NH₃/NH₄⁺ → NO₂⁻. Requires O₂.
4. Step 4
  (d) Nitrite → nitrate (B1). Nitrobacter — also nitrifying bacteria in aerated soil. Oxidise NO₂⁻ → NO₃⁻. Both Nitrosomonas and Nitrobacter are inhibited in waterlogged anaerobic soils (their reactions need O₂).
5. Step 5
  (e) Denitrification (B1). Denitrifying bacteria (often Pseudomonas) — live in WATERLOGGED, anaerobic soils. They convert NITRATE → NITROGEN GAS (N₂), which escapes to the atmosphere. UNDESIRABLE for farmers because it removes available nitrogen from the soil.
Answer
(a) Rhizobium — in root nodules of legumes. (b) Azotobacter — free-living in soil. (c) Nitrosomonas — nitrifying bacteria in aerated soil. (d) Nitrobacter — nitrifying bacteria in aerated soil. (e) Denitrifying bacteria (Pseudomonas) — in waterlogged anaerobic soils.
Examiner tip
Mark scheme awards B1 per correctly named bacterium AND location. Edexcel exam scripts MUST use the specific names — 'Rhizobium', 'Nitrosomonas', etc. Generic terms like 'nitrifying bacteria' score only partial marks unless paired with the specific role. Examiner reports note that candidates frequently confuse Nitrosomonas and Nitrobacter (alphabetical mnemonic: SoMonas = aMMonia first; Bacter = nitrite to nitrate).
3Why farmers drain waterlogged soil (4 marks, Paper 2)
Extended• Adapted from 4BI1/2B January 2024 Q7(b)• denitrification, agriculture, Paper 2, spec-4.13P
Question
A farmer in a low-lying field has noticed that the crops grow poorly after periods of heavy rain. Explain why this happens, using your knowledge of the nitrogen cycle. (4 marks)
Step-by-step solution
1. Step 1
  Heavy rain → waterlogged soil (B1). Soil pores fill with water → soil becomes ANAEROBIC (low oxygen).
2. Step 2
  Anaerobic conditions favour DENITRIFYING bacteria (B1). Denitrifying bacteria are anaerobic and thrive in waterlogged soils. They convert NO₃⁻ → N₂ gas, which escapes to the atmosphere.
3. Step 3
  Anaerobic conditions INHIBIT nitrifying bacteria (B1). Nitrosomonas and Nitrobacter need O₂ — they are inhibited / killed in waterlogged soil. So ammonia from decomposition cannot be converted to nitrate.
4. Step 4
  Plants are short of nitrate (B1). Without nitrate, plants cannot make amino acids → cannot make proteins → poor growth (yellow leaves / stunted growth, also known as nitrogen deficiency). Solution: drain the field, plough to add air to the soil, or apply nitrate fertiliser.
Answer
Waterlogged soil = anaerobic. Denitrifying bacteria flourish → convert NO₃⁻ → N₂ (lost to atmosphere). Nitrifying bacteria (Nitrosomonas, Nitrobacter) need O₂ → inhibited. Plants cannot make proteins without nitrate → poor growth. Solution: drain / aerate / add fertiliser.
Examiner tip
Mark scheme awards B1 each for: anaerobic; denitrification of NO₃⁻ → N₂; nitrification inhibited; plants cannot make protein. Examiner reports flag candidates who only mention 'less nitrate' without explaining WHY (anaerobic conditions).
4Carbon cycle from atmosphere to consumer (5 marks, Paper 2)
Extended• Adapted from 4BI1/2B May/Jun 2024 Q5(b)• carbon cycle, Paper 2, spec-4.12
Question
Describe how carbon atoms in atmospheric CO₂ can end up as part of the body of a tertiary consumer (e.g. an owl). Include the names of the processes involved. (5 marks)
Step-by-step solution
1. Step 1
  Step 1 — photosynthesis (B1). A plant absorbs CO₂ from the atmosphere through its stomata. CO₂ is reduced and combined with water during photosynthesis: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂. The carbon atoms are now in glucose.
2. Step 2
  Step 2 — biosynthesis in the plant (B1). Glucose is converted into other carbon-containing molecules: starch (storage), cellulose (cell walls), proteins (with N), lipids, DNA. The plant grows; the carbon is incorporated into all its tissues.
3. Step 3
  Step 3 — feeding (B1). A primary consumer (herbivore) eats the plant. Plant carbon compounds are digested and absorbed; some are used in respiration (carbon back to CO₂); some are converted into the herbivore's own tissues (carbon becomes part of the herbivore).
4. Step 4
  Step 4 — feeding again (B1). A secondary consumer eats the herbivore. Some carbon is respired (CO₂ to atmosphere); some is incorporated into the carnivore's tissues.
5. Step 5
  Step 5 — tertiary consumer (B1). The owl (tertiary consumer) eats the secondary consumer (e.g. a small mammal). Carbon is now part of the owl's tissues. Eventually, when the owl dies, decomposers will break it down and respiration / combustion will return the carbon back to the atmosphere.
Answer
(1) Photosynthesis: plant takes CO₂ from atmosphere and fixes it as glucose. (2) Glucose → other carbon molecules in the plant. (3) Primary consumer eats plant; carbon transferred up. (4) Secondary consumer eats primary; carbon transferred up. (5) Tertiary consumer (owl) eats secondary; carbon now in owl tissues. (Carbon returned by respiration / death + decomposition.)
Examiner tip
Mark scheme: B1 photosynthesis; B1 plant biosynthesis; B1 feeding (any consumer); B1 second feeding step (energy transfer); B1 final step + recognition that carbon flows up the food chain. Examiner reports note candidates often forget to name PHOTOSYNTHESIS — it is the entry point for CO₂ into the food chain.
5Factors affecting decomposition rate (4 marks, Paper 2)
Extended• Adapted from 4BI1/2B January 2024 Q7(c)• decomposition, Paper 2, spec-4.12
Question
Decomposition is faster in a hot, moist compost heap than in a cold dry desert. Explain THREE factors that affect the rate of decomposition by saprobiotic bacteria and fungi. (4 marks)
Step-by-step solution
1. Step 1
  Factor 1 — temperature (B1). Higher temperatures (up to the optimum of ~30-40 °C for most decomposers) increase the rate of enzyme-controlled reactions inside the decomposers → faster digestion of dead matter. Cold deserts/Arctic = slow decomposition; tropical = fast.
2. Step 2
  Factor 2 — moisture / water (B1). Saprobiotic bacteria and fungi need water — to grow, to make enzymes work, to dissolve digested products for absorption. Dry conditions slow decomposition. Compost heaps benefit from being damp (but not waterlogged).
3. Step 3
  Factor 3 — oxygen availability (B1). Most decomposers respire aerobically and need O₂. Waterlogged, anaerobic conditions slow decomposition (and produce methane instead of CO₂ — important in landfills).
4. Step 4
  Additional factor (B1, max 4 across all). Type of dead material — woody/lignin-rich plant matter decomposes much more slowly than soft tissues. Also pH (most decomposers prefer near-neutral pH), and presence of the decomposers themselves (a piece of meat in sterile conditions does not decay).
Answer
(1) Temperature — higher = faster (more enzyme activity, up to optimum). (2) Moisture — decomposers need water; dry conditions slow them. (3) Oxygen — aerobic decomposers need O₂; waterlogged = slow. (4) Material type / pH — soft tissue decays faster than woody tissue.
Examiner tip
Mark scheme: B1 per linked factor + explanation. Max 4. Examiner reports flag candidates who say 'temperature' alone (without saying higher = faster, up to the enzyme optimum). Linking to enzymes (not 'germs') scores higher.

Model Answers — Cycles within Ecosystems

High-scoring sample answers for cycles within ecosystems on the Pearson Edexcel IGCSE 4BI1 paper, with examiner-style notes mapping each response to the mark scheme and assessment objectives.

Question
Adapted from 4BI1/2B May/Jun 2024 Q58 marks
Describe the carbon cycle. Include the main processes that move carbon between the atmosphere, living organisms, and fossil fuels. (8 marks)
Model answer
The carbon cycle is the flow of carbon atoms between the atmosphere, living organisms, the oceans and fossil fuels.

1. PHOTOSYNTHESIS — atmosphere → producers (B1). Plants, algae and some bacteria absorb CO₂ from the atmosphere through stomata (or dissolved CO₂ from water). Equation: $6CO_2 + 6H_2O \rightarrow C_6H_{12}O_6 + 6O_2$ . The carbon is now FIXED into glucose, which is converted into starch, cellulose, proteins, lipids and DNA — the bulk of plant biomass.

2. FEEDING — producer → consumer → consumer (B1). When animals eat plants (or other animals), they take in the plant's carbon compounds. The carbon now passes UP the food chain, becoming part of the consumer's body tissues.

3. RESPIRATION — living organisms → atmosphere (B1). ALL living organisms (plants, animals, fungi, bacteria) respire continuously. Equation: $C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + \text{energy}$ . Respiration is the MAIN way carbon returns to the atmosphere from living organisms — both during life and from decomposers after death.

4. EGESTION + EXCRETION (B1). Animals release carbon back into the environment through faeces and urine, which decomposers then break down.

5. DECOMPOSITION — dead matter → atmosphere (B1). When a plant or animal dies, saprobiotic bacteria and fungi (decomposers) break down the dead matter. They secrete enzymes, absorb the digested products, and RESPIRE — releasing CO₂ back to the atmosphere.

6. COMBUSTION (burning) — fossil fuels → atmosphere (B1). Fossil fuels (coal, oil, natural gas) were formed from the remains of plants and animals MILLIONS of years ago, under high pressure and without O₂. Burning these fuels releases the locked-up carbon back into the atmosphere as CO₂. This is the LARGEST human contribution to atmospheric CO₂.

7. OCEAN EXCHANGE (B1). Atmospheric CO₂ dissolves in seawater forming carbonic acid (H₂CO₃) and bicarbonate ions. Marine producers (phytoplankton, algae) use dissolved CO₂ for photosynthesis. Some carbon is locked in carbonate shells (forams, corals) which can become sedimentary rock (chalk, limestone).

8. FOSSILISATION — long-term lock-up (B1). Over millions of years, dead organisms in anaerobic conditions (peat bogs, ocean sediments) can form coal, oil, natural gas, or limestone — locking carbon out of the active cycle for very long timescales.

Balance. In the natural cycle, photosynthesis and respiration broadly balance — the same amount of CO₂ taken in as is released. Human activities (combustion of fossil fuels, deforestation) UNBALANCE the cycle by adding CO₂ faster than photosynthesis can remove it → leading to global warming.
Why this scores
Mark scheme (max 8): B1 photosynthesis (CO₂ → glucose); B1 feeding / food chains; B1 respiration; B1 decomposition by saprobionts; B1 combustion of fossil fuels; B1 fossil-fuel formation (millions of years, anaerobic); B1 ocean / dissolved CO₂; B1 imbalance / global warming link. Examiner reports note that the strongest answers structure each process with named participants AND a stated equation/effect.

Question

Adapted from 4BI1/2B January 2024 Q710 marks

Describe the nitrogen cycle, naming all the bacteria involved and the processes they carry out. (10 marks)

Model answer

The nitrogen cycle moves nitrogen between the atmosphere (78 % N₂), the soil, plants, animals and microorganisms. Atmospheric N₂ is INERT — most organisms can't use it directly — so it must be 'fixed' into a usable form (ammonia / nitrate).

1. NITROGEN FIXATION — atmosphere → ammonia (B1 + B1). Atmospheric N₂ is converted to ammonia (NH₃) / ammonium (NH₄⁺).

Biological fixation: Rhizobium lives in the ROOT NODULES of leguminous plants (peas, beans, clover) — mutualistic relationship: plant gives sugars, bacteria fix N. Azotobacter is a FREE-LIVING soil bacterium that fixes N independently.
Physical fixation: lightning provides energy to combine N₂ with O₂ → nitrogen oxides → dissolved in rain as nitrate.
Industrial fixation: the Haber process (N₂ + H₂ → NH₃) makes ammonia for fertiliser.

2. AMMONIFICATION — proteins / urea → ammonia (B1). When organisms die OR excrete urea, saprobiotic decomposers (bacteria, fungi) break down the nitrogen-containing molecules → NH₃ / NH₄⁺ enters the soil.

3. NITRIFICATION — ammonia → nitrite → nitrate (B1 + B1). TWO-step oxidation by NITRIFYING BACTERIA in well-aerated soil. Both steps require O₂.

Nitrosomonas: NH₃ / NH₄⁺ → NO₂⁻ (nitrite).
Nitrobacter: NO₂⁻ → NO₃⁻ (nitrate). Plants prefer to absorb NITRATE because it is soluble and can be taken up by active transport in root hair cells.

4. PLANT UPTAKE — soil → plants (B1). Plants absorb NO₃⁻ from soil water through their root hair cells (active transport). The nitrate is used to build AMINO ACIDS, which are then linked into PROTEINS. Plants also need N for DNA, RNA, chlorophyll and many other molecules.

5. CONSUMERS — plants → animals (B1). Animals eat plants (or other animals), digesting plant proteins → amino acids → used to build the animal's own proteins. Nitrogen now moves up the food chain.

6. EXCRETION (B1). Animals break down excess amino acids in the LIVER, forming urea (deamination). Urea is excreted in urine — saprobionts then convert it back to ammonia (step 2). The cycle is closed.

7. DENITRIFICATION — soil nitrate → atmosphere (B1). Denitrifying bacteria (e.g. Pseudomonas) live in WATERLOGGED, anaerobic soils. They convert NO₃⁻ → N₂ gas, which is released back to the atmosphere. This step is UNDESIRABLE for farmers — it removes available nitrogen from the soil.

Summary diagram:

Step	Process	Bacteria	Substrate → product
1	Fixation	Rhizobium (in nodules)	N₂ → NH₃
1	Fixation	Azotobacter (free-living)	N₂ → NH₃
2	Ammonification	saprobionts	proteins / urea → NH₃
3	Nitrification	Nitrosomonas	NH₃ → NO₂⁻
3	Nitrification	Nitrobacter	NO₂⁻ → NO₃⁻
7	Denitrification	Pseudomonas	NO₃⁻ → N₂

Farming implication. A good farmer encourages nitrifying bacteria (drain and plough soil to keep it well aerated) and discourages denitrifying bacteria (avoid waterlogging). Crop rotation with legumes uses Rhizobium to enrich the soil naturally.

Why this scores

Mark scheme (max 10): B1 fixation by Rhizobium (in nodules); B1 fixation by Azotobacter (free-living); B1 lightning / Haber as alternatives; B1 saprobiont ammonification of proteins/urea; B1 nitrification by Nitrosomonas (NH₃ → NO₂⁻); B1 nitrification by Nitrobacter (NO₂⁻ → NO₃⁻); B1 plant uptake of nitrate by active transport; B1 nitrogen passes up food chain; B1 denitrification by anaerobic bacteria (NO₃⁻ → N₂); B1 farming application (drainage / ploughing). Examiner reports stress that BACTERIA NAMES are required for full marks — generic 'nitrifying bacteria' loses precision marks.

Question
Adapted from 4BI1/2B May/Jun 2024 Q5(c)6 marks
Describe the water cycle and explain how plants play a role in it. (6 marks)
Model answer
The water cycle moves water between oceans, atmosphere, land and living organisms — continuously, driven by solar energy.

Stages of the water cycle:

Evaporation (B1). Heat from the sun causes liquid water in oceans, rivers, lakes and soil to evaporate → water vapour rises into the atmosphere. Largest source: tropical oceans.

Transpiration (B1). Water inside plant leaves evaporates from the mesophyll cell surfaces and diffuses out through the STOMATA into the atmosphere. This is the plant's contribution to the cycle — pulling water up from soil and releasing it as vapour. In some habitats (e.g. tropical rainforest), transpiration accounts for a significant fraction of total atmospheric moisture.

Condensation (B1). As water vapour rises and cools (lower temperature at altitude), it condenses around dust / aerosol particles into tiny water droplets, forming CLOUDS.

Precipitation (B1). Droplets join together until heavy enough to fall as RAIN (or snow, hail). This delivers water back to the Earth's surface.

Runoff and infiltration (B1). On land, water either:

Flows over the surface as runoff → into streams, rivers → back to the sea.

Infiltrates / percolates through soil into groundwater → eventually feeds rivers / springs.

Plant uptake (B1). Roots absorb water from the soil by osmosis (root hair cells). Water moves up the xylem to leaves, replacing water lost by transpiration → creating the transpiration stream. This delivers water and dissolved mineral ions to all plant tissues.

Plants' wider role:

Transpiration cools the local climate — like the body's sweat, transpired water removes latent heat from the leaves.

Tree cover reduces runoff — roots hold soil; canopy intercepts rain → more water enters groundwater rather than washing away.

Deforestation disrupts the water cycle — without trees, less transpiration, less local rainfall, more runoff, soil erosion, and downstream flooding.
Why this scores
Mark scheme: B1 evaporation; B1 transpiration; B1 condensation; B1 precipitation; B1 runoff / infiltration; B1 plant root uptake / xylem. Max 6. Examiner reports flag candidates who omit TRANSPIRATION — this is the plant-specific step and the question explicitly asks about the role of plants.
Question
Adapted from 4BI1/2B January 2024 Q6(b)6 marks
Explain the role of decomposers in BOTH the carbon cycle AND the nitrogen cycle. (6 marks)
Model answer
Decomposers are saprobiotic bacteria and fungi that break down dead organic matter and animal waste. They are essential for recycling nutrients — without them, dead matter would accumulate and the cycles would stop.

Role in the CARBON cycle (B1 + B1 + B1):

Break down dead organisms — decomposers digest the carbohydrates, proteins, lipids and other carbon-containing molecules in dead plants and animals using extracellular enzymes.

Respiration releases CO₂ — decomposers respire the absorbed nutrients: $C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + \text{energy}$ . CO₂ returns to the atmosphere.

Net effect: carbon locked in dead biomass is released back into the atmosphere as CO₂, where it can be re-absorbed by plants via photosynthesis. Without decomposers, all the carbon would stay trapped in dead matter and there would be no recycling.

Role in the NITROGEN cycle (B1 + B1 + B1):

Ammonification — saprobiotic decomposers break down the PROTEINS in dead organisms and the UREA in animal urine → releasing AMMONIA (NH₃) / ammonium ions (NH₄⁺) into the soil.

Provide substrate for nitrification — the ammonia released is then oxidised by Nitrosomonas → NO₂⁻ → Nitrobacter → NO₃⁻. So decomposers PROVIDE the starting material that nitrifying bacteria use.

Plants get nitrate from this chain — plants absorb NO₃⁻ (formed via the decomposer-nitrifier chain) and use it to build amino acids → proteins. Animals eat the plants, and the cycle continues. Without decomposers, plant proteins from dead leaves and animal proteins from corpses would not be made available for the next generation.

Conclusion. In BOTH cycles, decomposers are the BRIDGE that releases nutrients locked in DEAD biomass back into a form usable by producers. They are essential for ecosystem sustainability — an ecosystem without decomposers would suffocate in dead bodies and starve of nutrients within a few years.
Why this scores
Mark scheme: B1 decomposers respire and release CO₂; B1 link to carbon cycle (recycle to atmosphere); B1 plants reabsorb via photosynthesis; B1 ammonification of proteins / urea; B1 link to nitrification (provide substrate for Nitrosomonas); B1 enable plant nitrogen uptake. Max 6. Examiner reports flag candidates who only mention ONE cycle — the question asks about BOTH.

Key Definitions and Keywords — Cycles within Ecosystems

Definitions to memorise and the exact keywords mark schemes credit for cycles within ecosystems answers — sharpened from recent examiner reports for the 2026 Pearson Edexcel IGCSE 4BI1 sitting.

Photosynthesis (in the carbon cycle)
Examiner keyword
The ONLY process that significantly removes CO₂ from the atmosphere. Plants, algae and some bacteria absorb CO₂ → fix it into glucose using light energy. $6CO_2 + 6H_2O \rightarrow C_6H_{12}O_6 + 6O_2$ .
Respiration (in the carbon cycle)
Examiner keyword
Returns CO₂ to the atmosphere from ALL living organisms (plants, animals, fungi, bacteria). Continuous: 24/7 in every cell. $C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + \text{energy}$ .
Combustion
Examiner keyword
Burning of fossil fuels (coal, oil, gas) and wood. Releases CO₂ that was locked underground for millions of years. The MAIN human contribution to atmospheric CO₂ rise.
Saprobiont / decomposer
Examiner keyword
An organism (mainly bacteria and fungi) that obtains nutrients by breaking down DEAD organic matter using EXTRACELLULAR digestion (releasing enzymes outside the cells). Essential for recycling carbon and nitrogen.
Rhizobium
Examiner keyword
Nitrogen-fixing bacteria that live in MUTUALISTIC associations in the root nodules of leguminous plants (peas, beans, clover). The plant supplies sugars; Rhizobium fixes atmospheric N₂ → NH₃, which the plant uses.
Azotobacter
Examiner keyword
FREE-LIVING (not host-dependent) nitrogen-fixing bacteria in the soil. Fixes atmospheric N₂ → NH₃ without needing a plant host.
Nitrosomonas
Examiner keyword
Nitrifying bacteria. Convert ammonia / ammonium (NH₃ / NH₄⁺) to NITRITE (NO₂⁻) — the FIRST step of nitrification. Requires oxygen (aerobic soils).
Nitrobacter
Examiner keyword
Nitrifying bacteria. Convert nitrite (NO₂⁻) to NITRATE (NO₃⁻) — the SECOND step of nitrification. Plants then absorb the nitrate. Requires oxygen.
Denitrifying bacteria
Examiner keyword
Anaerobic bacteria that convert nitrate (NO₃⁻) back to nitrogen gas (N₂), which escapes to the atmosphere. Flourish in WATERLOGGED soils. UNDESIRABLE for farmers — they REMOVE available nitrogen from the soil.
Transpiration (in the water cycle)
Examiner keyword
Evaporation of water from plant leaves via the stomata. The plant's contribution to the water cycle — moves water from soil → atmosphere. In rainforests, transpiration can equal evaporation as a source of atmospheric moisture.

Common Mistakes and Misconceptions — Cycles within Ecosystems

The traps other students keep falling into on cycles within ecosystems questions — taken from recent Pearson Edexcel IGCSE 4BI1 examiner reports and mark schemes — and how to avoid them.

✕Saying plants only photosynthesise, not respire
4BI1 Examiner Reports 2024
Why it happens
Photosynthesis dominates during the day, so students think plants only photosynthesise.
How to avoid it
Plants respire 24 hours a day, like all living organisms. During the day, photosynthesis EXCEEDS respiration → net O₂ release and CO₂ uptake. At night, only respiration happens → net CO₂ release.
✕Confusing Nitrosomonas and Nitrobacter
4BI1 Examiner Reports 2023-2024
Why it happens
Similar-sounding names; both are 'nitrifying bacteria'.
How to avoid it
Mnemonic: NitrosoMonas does aMMonia first (NH₃ → NO₂⁻). NitroBacter does nitrIte to nitrAte (NO₂⁻ → NO₃⁻). Both need oxygen.
✕Confusing Rhizobium and Azotobacter, or saying both live in nodules
4BI1 Examiner Reports 2024
Why it happens
Both are 'nitrogen-fixing bacteria'.
How to avoid it
Rhizobium (rhiza = root) lives INSIDE root nodules of legumes (mutualism). Azotobacter (azoto = nitrogen) is FREE-LIVING in soil — does not need a plant host.
✕Saying denitrification is part of healthy nitrogen cycling
4BI1 Examiner Reports 2024
Why it happens
It's part of the cycle, so students assume it's beneficial.
How to avoid it
Denitrification REMOVES nitrate (the form plants can use) → returns it to the atmosphere as inert N₂. For ECOSYSTEMS it is essential to balance fixation, but for FARMERS it is UNDESIRABLE — drained soils prevent it.
✕Saying plants 'fix nitrogen' directly from the air
4BI1 Examiner Reports 2023
Why it happens
Confusion between plants and bacteria.
How to avoid it
PLANTS cannot fix N₂. Only BACTERIA (Rhizobium, Azotobacter, blue-green algae) can fix atmospheric nitrogen. Plants only absorb the NITRATE produced by the bacterial chain (or applied as fertiliser).
✕Describing the water cycle without mentioning transpiration
Why it happens
Most textbook diagrams emphasise evaporation/condensation/precipitation.
How to avoid it
Transpiration from plant leaves is a HUGE contributor to atmospheric water — especially in forests. Always include transpiration AND evaporation in answers about the water cycle.

Practice questions

Exam-style questions with step-by-step worked solutions. Try one before checking the method.

Past paper style quiz

Get a report showing which sub-topics you've nailed and which ones still need work.

Cycles within Ecosystems — frequently asked questions

The things students keep getting wrong in this sub-topic, answered.

Cycles within Ecosystems

Short Study Notes in the form of Slides

Detailed Study Notes

What you’ll learn

How it’s examined

1Identifying processes in the carbon cycle (4 marks, Paper 1)

2Naming bacteria in the nitrogen cycle (5 marks, Paper 2)

3Why farmers drain waterlogged soil (4 marks, Paper 2)

4Carbon cycle from atmosphere to consumer (5 marks, Paper 2)

5Factors affecting decomposition rate (4 marks, Paper 2)

Photosynthesis (in the carbon cycle)

Respiration (in the carbon cycle)

Combustion

Saprobiont / decomposer

Rhizobium

Azotobacter

Nitrosomonas

Nitrobacter

Denitrifying bacteria

Transpiration (in the water cycle)

✕Saying plants only photosynthesise, not respire

✕Confusing Nitrosomonas and Nitrobacter

✕Confusing Rhizobium and Azotobacter, or saying both live in nodules

✕Saying denitrification is part of healthy nitrogen cycling

✕Saying plants 'fix nitrogen' directly from the air

✕Describing the water cycle without mentioning transpiration

Practice questions

Past paper style quiz

Assess your understanding

Cycles within Ecosystems — frequently asked questions