Carbon, water and decomposition

The carbon cycle

CO₂ → photosynthesis → plant biomass → eaten by animals → respiration → CO₂ back to atmosphere. Carbon also returns when organisms die and decomposers break down the remains, or when fossil fuels (locked-up carbon) are burned (combustion).

Key processes to name: photosynthesis, respiration, feeding, decomposition, combustion, fossilisation.

The water cycle

Evaporation (mostly oceans) → condensation (cooling air, clouds) → precipitation (rain, snow) → run-off and infiltration → returns to seas, rivers and groundwater. Transpiration from plants is a major flux on land. Water cycle drives climate, weather and freshwater supply.

Decomposition

Decomposers (bacteria and fungi) break down dead organisms and waste, returning carbon, nitrogen and minerals to the soil/atmosphere.

Rate of decomposition depends on:

• Temperature — too cold → enzymes slow; too hot → enzymes denature.
• Water — needed for enzymes and for microbes to live.
• Oxygen — aerobic decomposers respire faster.

OCR PAG B5 — investigating decomposition

Set up milk + lipase at different temperatures, time pH change with universal indicator. Interpret rate vs T graph.

Human impact on biodiversity

• Deforestation → habitat loss, reduced biodiversity, more atmospheric CO₂.
• Pollution (water, air, land) → kills sensitive species, eutrophication.
• Climate change from CO₂ build-up → species cannot migrate fast enough → extinctions.
• Peat destruction for fuel/compost → releases stored CO₂.

Maintaining biodiversity

Conservation tools include captive breeding, reforestation, protected areas, fishing quotas, education and reduction of carbon emissions.

OCR exam tip

In a "describe the carbon cycle" 4-mark question, name four named processes in order (photosynthesis → respiration → decomposition → combustion). One missing process loses one mark.