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💡 Breathe out, breathe in
Respiration and burning use up oxygen and release carbon dioxide; photosynthesis does the reverse. Together they keep the air breathable.
What is the oxygen cycle?
- About 21% of the atmosphere is free oxygen (O₂), and oxygen is a key part of carbohydrates, proteins, nucleic acids and fats.
- Respiration and the combustion of fuels use oxygen and release CO₂.
- Photosynthesis restores oxygen using sunlight, water and CO₂ to make glucose and release O₂ — balancing consumption and production across all spheres.
⏸▶ Pause and Ponder
5. If the biogeochemical cycles stopped, nutrients would no longer be recycled — plants would run out of usable carbon and nitrogen, oxygen would not be replenished, and food chains would collapse, so most life could not survive.
Important Definitions
- Biogeochemical cycle — the cyclic movement of matter between the biotic and abiotic components of the Earth.
- Water cycle — the movement of water through evaporation, condensation, precipitation and infiltration.
- Carbon cycle — the circulation of carbon among the atmosphere, biosphere, geosphere and hydrosphere.
- Nitrogen fixation — the conversion of atmospheric nitrogen into ammonia by bacteria or lightning.
- Nitrification — the conversion of ammonia into nitrite and then nitrate by bacteria.
- Denitrification — the conversion of nitrates back into nitrogen gas by bacteria.
- Oxygen cycle — the circulation of oxygen through respiration, combustion and photosynthesis.
- Photosynthesis — the process by which plants make food and release oxygen using sunlight.
