Cycles within EcosystemsEdexcel GCSE Biology: Revision notes
Section 1
The Carbon Cycle
Photosynthesis removes CO2; respiration releases it; feeding passes carbon along; decomposers release CO2 from dead matter; combustion releases stored CO2 from fossil fuels.
Decomposers link biotic and abiotic carbon cycle stages.
Section 2
The Water Cycle & Potable Water
Evaporation → transpiration → condensation → precipitation. Potable water (safe to drink, not pure) can be produced via desalination: distillation or reverse osmosis (both energy-intensive).
Potable water is NOT pure water.
Section 3
The Nitrogen Cycle
Nitrogen-fixing bacteria convert N2 to usable compounds. Decomposers release ammonium. Nitrifying bacteria convert to nitrites/nitrates. Denitrifying bacteria return N2 to air. Fertilisers/crop rotation boost soil nitrates.
Legumes have root nodules with nitrogen-fixing bacteria, used in crop rotation.
Section 4
Indicator Species
| Pollution | Present | Clean |
|---|---|---|
| Water | Bloodworm, sludgeworm | Shrimp, stonefly |
| Air | (absence of) blackspot fungus | Lichen variety |
Low oxygen tolerant = polluted; high oxygen need = clean.
Section 5
Rate of Decomposition
Temperature, water content, oxygen availability affect decay rate. Preservation slows decay (cold/dry/vacuum); composting speeds it (warm/moist/aerated).
Rate = change in quantity ÷ time.
200g→120g over 8 days: rate=80÷8=10g/day.
Must Know
- Carbon cycle: photosynthesis removes CO2; respiration/decomposition/combustion release it.
- Water cycle stages; desalination for potable water.
- Nitrogen cycle: fixation, nitrification, denitrification; fertiliser/crop rotation boost nitrates.
- Indicator species reveal pollution levels.
- Decay rate affected by temp/water/oxygen; rate=change÷time.
That's the notes covered.
Carry on to the next subtopic.