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Learn5090Paper 2 · TheoryRelationships of organisms with one another and with the environmentNutrient cycles
Relationships of organisms with one another and with the environment · Subtopic 3 of 5

Nutrient Cycles

Elements are recycled in ecosystems. Carbon cycle: CO₂ is removed from air by photosynthesis; returned by respiration (plants, animals, decomposers) and combustion of fuels; passed along food chains by feeding; trapped in fossil fuels if decomposition doesn't happen. Nitrogen cycle: nitrogen-fixing bacteria (in soil and root nodules of legumes) turn N₂ into nitrogen compounds; decomposers turn proteins and urea into ammonium; nitrifying bacteria convert ammonium → nitrite → nitrate, absorbed by plants; denitrifying bacteria return nitrate to N₂ in waterlogged soil. Water cycle: evaporation, transpiration, condensation, precipitation.

Cambridge 5090 Paper 1Cambridge 5090 Paper 2Cambridge 0610 Papers 2, 4Cambridge 0610 Papers 1, 3Edexcel 4BI1 Paper 1BEdexcel 4BI1 Paper 2B≈ 40 minRevision 1 · updated 2026-10-30
Section 1 of 14

Learning objectives

  • Describe the carbon cycle.
  • Describe the nitrogen cycle and the roles of bacteria.
  • Describe the water cycle.
Pause and think

The carbon atoms in your body may once have been in a dinosaur. How is that possible?

Section 2 of 14

Core idea

Start hereRecycling atoms

Energy flows through ecosystems and is lost, but atoms of carbon and nitrogen go round and round for billions of years.

The big idea

Photosynthesis, respiration, feeding and decomposition cycle carbon. Bacteria drive the nitrogen cycle. Water cycles by evaporation, transpiration and rain.

1. The carbon cycle

Carbon cycle

One process removes CO₂; several return it.

2. The nitrogen cycle

Worked exampleFrom air to protein

Trace a nitrogen atom from the air into a cow's protein.

Goal
Steps and organisms.
Strategy
Follow the bacteria.
  1. N2→ammonium\mathrm{N_2}\rightarrow\text{ammonium}
    Nitrogen-fixing bacteria in clover root nodules.
  2. ammonium→nitrate\text{ammonium}\rightarrow\text{nitrate}
    Nitrifying bacteria in soil.
  3. nitrate→plant protein→cow protein\text{nitrate}\rightarrow\text{plant protein}\rightarrow\text{cow protein}
    Absorbed by roots; eaten.
AnswerFixation, nitrification, absorption, feeding.
✓ CheckReturned via decomposition of dung and dead cows ✓.
Key takeawayDenitrification In waterlogged soils, denitrifying bacteria turn nitrate back into N₂, reducing soil fertility.
Pause and think

Why does composting need air?

Summary

  • Carbon: photosynthesis vs respiration and combustion.
  • Nitrogen: four types of bacteria.
  • Water: evaporate, transpire, condense, precipitate.
Key words
Nitrogen fixation
Converting nitrogen gas into nitrogen compounds.
Nitrification
Ammonium converted to nitrate by bacteria.
Decomposer
Organism that breaks down dead material.
HistoryRoot nodules. In 1888 Hermann Hellriegel and Hermann Wilfarth showed that legumes get nitrogen from the air through bacteria in their root nodules, explaining why crop rotation with clover improves soil.
Fun factLightning fertiliser. Lightning bolts provide enough energy to combine nitrogen and oxygen; it's estimated to fix millions of tonnes of nitrogen each year.
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