CB9.3 — Material cycles (Edexcel 1SC0)

Water cycle

Evaporation from oceans/land → transpiration from plants → condensation (clouds) → precipitation (rain/snow) → runoff → rivers → oceans.

Carbon cycle

• Photosynthesis removes CO₂ from atmosphere → glucose stored in plants.
• Respiration (by all organisms) releases CO₂ back.
• Decomposition: microorganisms break down dead organisms → release CO₂ (and nutrients).
• Combustion of fossil fuels releases CO₂ stored for millions of years → increases atmospheric CO₂.
• Feeding: carbon passes through food chains.
• Fossilisation: dead organisms compressed over millions of years → coal, oil, gas.

Nitrogen cycle

• 78% of atmosphere is N₂ — cannot be used directly by most organisms.
• Nitrogen-fixing bacteria (Rhizobium in root nodules, free-living in soil) → convert N₂ → ammonium ions (NH₄⁺).
• Nitrifying bacteria in soil → convert NH₄⁺ → nitrites → nitrates (NO₃⁻).
• Plants absorb NO₃⁻ → used to synthesise proteins.
• Animals eat plants → nitrogen transferred.
• Decomposers break down dead organisms → release ammonium ions back into soil.
• Denitrifying bacteria → convert nitrates → N₂ back to atmosphere.