Cycling materials — water and carbon cycles
Living things need a constant supply of water, carbon, oxygen and nitrogen. The Earth has only a finite stock of each, so these substances must be cycled — used, released and re-used. GCSE focuses on the water cycle and the carbon cycle.
The water cycle
Water moves between the atmosphere, land and sea via:
- Evaporation — heat from the Sun turns liquid water into vapour from oceans, lakes, soil. Plants also release water by transpiration.
- Condensation — vapour cools high in the atmosphere and forms clouds.
- Precipitation — clouds release water as rain, snow or hail.
- Run-off / infiltration — water returns to rivers and seas, or seeps into the soil and groundwater.
A learning sketch: Sun → evaporation → condensation (clouds) → precipitation (rain) → run-off / groundwater → back to the sea → repeat.
The water cycle delivers the fresh water that all land organisms depend on.
The carbon cycle
Carbon moves between the atmosphere, oceans, plants, animals and fossil fuels. Five processes you must know:
- Photosynthesis — plants take CO₂ from the air and convert it into glucose and other organic compounds. Plants act as the main carbon "pump" out of the atmosphere.
- Respiration — plants, animals and decomposers break glucose down to release energy, returning CO₂ to the atmosphere.
- Feeding — animals eat plants (or other animals), so carbon moves through the food chain.
- Decomposition — when organisms die, decomposers (bacteria and fungi) break down dead material, releasing CO₂ during their respiration.
- Combustion — burning fossil fuels (coal, oil, gas) releases stored carbon as CO₂. Modern human activity has greatly increased combustion (B7.6).
Diagram outline: arrows from atmosphere CO₂ → plant (photosynthesis); from plants/animals/decomposers → atmosphere (respiration); plants → animals (feeding); dead matter → fossil fuels (over millions of years); fossil fuels → atmosphere (combustion).
Decomposition — the engine that recycles carbon
Decomposers (mainly bacteria and fungi) break down dead organisms and waste, releasing nutrients (carbon, nitrogen, etc.) back into the soil and atmosphere. Without decomposers, dead material would accumulate and nutrient cycles would stop.
Factors affecting decomposition rate
The required practical investigates how decay rate depends on conditions:
- Temperature — warmer = faster (up to ~37 °C); too hot kills enzymes.
- Water (moisture) — needed for decomposers' enzymes to work.
- Oxygen — most decomposers are aerobic; oxygen accelerates decay.
- pH — extreme acidity slows decay (one reason peat bogs preserve bodies).
Compost and biogas
Gardeners encourage decomposition to produce compost, a natural fertiliser. Conditions: warm, moist and aerobic.
In biogas generators, decomposition is anaerobic (no oxygen). Bacteria break down waste to produce methane gas (biogas), used as a fuel.
⚠Common mistakes
- Saying plants only photosynthesise. They respire too (especially at night).
- Saying water vapour disappears. It just changes form / condenses.
- Saying decomposers eat everything. They release enzymes onto dead material and absorb the soluble products (extracellular digestion).
- Forgetting that combustion is now a major source of atmospheric CO₂.
Links
Forms the basis for B7.4 (HT — decay rate / biogas), B7.6 (greenhouse gases and global warming), and B7.7 / B7.8 (biodiversity, biomass).
AI-generated · claude-opus-4-7 · v3-deep-biology