Nutrient CyclesCambridge IGCSE Biology: Revision notes
Section 1
Why do nutrients need to be recycled?
Unlike energy, which flows through an ecosystem in one direction and is eventually lost as heat, chemical nutrients such as carbon and nitrogen are recycled repeatedly between living organisms and the environment.
Decomposers are essential to this recycling: they get their energy from dead or waste organic material, breaking it down and releasing the nutrients locked inside back into the soil, water and air so that producers can use them again.
Energy flows one way and is lost; nutrients cycle round and round. Don't mix the two up in an exam answer.
Section 2
The carbon cycle
The carbon cycle describes how carbon moves between the atmosphere, living organisms and fossil fuels:
- Photosynthesis — producers remove carbon dioxide from the atmosphere and convert it into organic compounds such as glucose.
- Feeding — carbon compounds pass from producers to consumers along food chains.
- Respiration — all living organisms release carbon dioxide back into the atmosphere as they respire.
- Decomposition — decomposers break down dead organisms and waste, releasing carbon dioxide through their own respiration.
- Formation of fossil fuels — under certain conditions, dead organic matter that does not fully decompose is compressed over millions of years into coal, oil and gas.
- Combustion — burning fossil fuels (and wood) releases carbon dioxide that had been locked away, back into the atmosphere.
Coal formed from swamp plants millions of years ago; burning it today releases carbon that was removed from the atmosphere long before humans existed.
Section 3
The nitrogen cycle
Nitrogen is needed by all living organisms to build amino acids and proteins, but most organisms cannot use nitrogen gas (N2) directly from the air. The nitrogen cycle moves nitrogen between the atmosphere, soil and living organisms:
- Decomposition — decomposers break down proteins in dead plant and animal material into ammonium ions.
- Nitrification — bacteria convert ammonium ions into nitrite ions, then into nitrate ions, which plants can absorb.
- Nitrogen fixation — nitrogen gas in the air is converted into nitrogen compounds by lightning (which provides the energy to combine nitrogen with oxygen) and by nitrogen-fixing bacteria (free-living in soil, or in root nodules of legumes).
- Absorption — plant roots absorb nitrate ions from the soil and combine them with sugars from photosynthesis to build amino acids and proteins.
- Feeding and digestion — animals eat plants (or other animals), digest the proteins, and use the amino acids to build their own proteins.
- Deamination — excess amino acids in animals are broken down in the liver, producing urea, which is excreted.
- Denitrification — denitrifying bacteria convert nitrates back into nitrogen gas, returning it to the atmosphere.
Don't confuse nitrification (ammonium to nitrate, useful to plants) with denitrification (nitrate back to nitrogen gas, removes nitrogen from soil) — they are opposite processes.
Section 4
The roles of microorganisms in the nitrogen cycle
Microorganisms (mainly bacteria) drive the nitrogen cycle through four key roles:
| Role | What it does |
|---|---|
| Decomposition | Breaks down protein in dead material into ammonium ions |
| Nitrification | Converts ammonium ions into nitrate ions |
| Nitrogen fixation | Converts nitrogen gas into nitrogen compounds usable by plants |
| Denitrification | Converts nitrates back into nitrogen gas, returning it to the air |
Must Know
- Nutrients (unlike energy) are recycled, not lost, thanks largely to decomposers.
- The carbon cycle: photosynthesis removes CO2; respiration and combustion release it; decomposition and fossil fuel formation are also key stages.
- The nitrogen cycle involves decomposition, nitrification, nitrogen fixation, absorption, feeding/digestion, deamination and denitrification.
- Nitrogen fixation can happen via lightning or nitrogen-fixing bacteria.
- Deamination (in the liver) breaks down excess amino acids into urea.
- Bacteria drive four key roles in the nitrogen cycle: decomposition, nitrification, nitrogen fixation and denitrification.
That's the notes covered.
Carry on to the next subtopic.