A level Biology B · Edexcel 9BI0 · Topic 10C
Nutrient cycles
Key points
- Carbon cycle: CO is taken in by photosynthesis and released by respiration, decomposition and combustion of fossil fuels; carbon is also stored in oceans, rocks and peat.
- Decomposers (bacteria and fungi) break down dead matter by saprobiotic nutrition, releasing CO and ammonium.
- Nitrogen fixation: Rhizobium (in root nodules) and free-living bacteria turn N into ammonia.
- Nitrification: nitrifying bacteria oxidise ammonium to nitrite and then nitrate, which plants absorb; this needs aerobic conditions.
- Denitrification: anaerobic bacteria turn nitrate back into N, common in waterlogged soils.
- Excess fertiliser can run off into water causing eutrophication: algal bloom, light blocked, plants die, decomposers use up oxygen and fish die.
Topic notesPremium
Unlike energy, the chemical elements that make up living things are recycled over and over. Microorganisms are central to these cycles: without decomposers and specialised bacteria, the carbon and nitrogen locked in dead organisms would…
Unlock the full notes with Premium
Premium includes full topic notes, the paper generator, self-quizzes and progress tracking.
Practise this topicPremium
Premium members can generate an exam-style paper with a mark scheme, or take a quick quiz, on Nutrient cycles.