The Carbon and Water Cycles

All the elements that make up the bodies of living organisms are recycled. When organisms die, their bodies are broken down by decomposers and the elements are returned to the soil, water and atmosphere, ready to be used again. Two of the most important cycles are the carbon cycle and the water cycle. This lesson covers both in detail for AQA GCSE Biology.

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Why Are Nutrient Cycles Important?

Living organisms need a range of elements to build their bodies:

Element	What It Is Used For
Carbon	Found in all organic molecules: carbohydrates, lipids, proteins, DNA
Nitrogen	Essential for amino acids (proteins) and nucleic acids (DNA, RNA)
Hydrogen	Component of water and organic molecules
Oxygen	Used in respiration; component of water and organic molecules
Phosphorus	Found in DNA, RNA and ATP (adenosine triphosphate)

These elements are finite — there is a fixed amount on Earth. They must be constantly recycled between living organisms and the environment. Without this recycling, life would eventually run out of the raw materials it needs.

Exam Tip: Remember that materials are recycled but energy is not. Energy flows through an ecosystem in one direction (from the Sun through food chains) and is eventually lost as heat. Materials, however, are used again and again.

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The Carbon Cycle

The carbon cycle describes how carbon moves between the atmosphere, living organisms, the oceans and the Earth's crust. Carbon exists in the atmosphere mainly as carbon dioxide (CO2).

Key Processes in the Carbon Cycle

Process	What Happens	Effect on Atmospheric CO2
Photosynthesis	Plants and algae absorb CO2 from the atmosphere and use it to make glucose	Removes CO2 (decreases)
Respiration	All living organisms break down glucose, releasing CO2 back into the atmosphere	Adds CO2 (increases)
Combustion	Burning fossil fuels (coal, oil, gas) and wood releases stored carbon as CO2	Adds CO2 (increases)
Decomposition	Decomposers (bacteria and fungi) break down dead organisms, releasing CO2 through their respiration	Adds CO2 (increases)
Fossilisation	Dead organisms may become buried and form fossil fuels over millions of years, locking carbon away	Removes CO2 (decreases)
Dissolving in oceans	CO2 dissolves in ocean water and can be used by marine organisms or stored as carbonate sediments	Removes CO2 (decreases)

graph TD
    ATM["Atmosphere<br/>CO2"] -->|"Photosynthesis"| PLANTS["Plants and Algae"]
    PLANTS -->|"Respiration"| ATM
    PLANTS -->|"Eaten"| ANIMALS["Animals"]
    ANIMALS -->|"Respiration"| ATM
    PLANTS -->|"Death"| DEAD["Dead Organisms<br/>and Waste"]
    ANIMALS -->|"Death"| DEAD
    DEAD -->|"Decomposition<br/>(microbial respiration)"| ATM
    DEAD -->|"Fossilisation<br/>(millions of years)"| FOSSIL["Fossil Fuels<br/>(coal, oil, gas)"]
    FOSSIL -->|"Combustion"| ATM
    ATM -->|"Dissolving"| OCEAN["Oceans"]
    OCEAN -->|"Release"| ATM
    style ATM fill:#bbdefb,stroke:#1565c0
    style PLANTS fill:#c8e6c9,stroke:#2e7d32
    style ANIMALS fill:#ffccbc,stroke:#d84315
    style DEAD fill:#d7ccc8,stroke:#5d4037
    style FOSSIL fill:#424242,stroke:#212121,color:#fff
    style OCEAN fill:#90caf9,stroke:#1565c0

Photosynthesis and Respiration: The Balance

In a stable ecosystem, the rate of carbon being removed from the atmosphere (mainly by photosynthesis) roughly balances the rate of carbon being returned (mainly by respiration and decomposition). This keeps atmospheric CO2 levels relatively stable.

However, human activities — particularly burning fossil fuels and deforestation — have disrupted this balance, leading to rising CO2 levels and climate change.

Human Activity	Effect on the Carbon Cycle
Burning fossil fuels	Releases carbon that was locked away millions of years ago, adding extra CO2 to the atmosphere
Deforestation	Removes trees that absorb CO2 through photosynthesis; burning or decomposing felled trees also releases CO2
Cement production	Heating limestone (CaCO3) releases CO2

Exam Tip: When drawing or labelling the carbon cycle in the exam, make sure you include ALL the key processes: photosynthesis, respiration (of plants, animals AND decomposers), combustion, decomposition and fossilisation. Missing any one of these will cost you marks.

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The Role of Decomposers in the Carbon Cycle

Decomposers play a critical role in the carbon cycle. When organisms die, decomposers (mainly bacteria and fungi) break down the complex carbon compounds in dead tissue. The decomposers use these compounds for their own respiration, releasing CO2 back into the atmosphere.

Without decomposers:

• Dead material would accumulate
• Carbon would remain locked in dead organisms
• Nutrients would not be returned to the soil
• Plants could not grow (no mineral ions available)
• The entire ecosystem would eventually collapse

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The Water Cycle

The water cycle (also called the hydrological cycle) describes the continuous movement of water between the Earth's surface and the atmosphere. Unlike the carbon and nitrogen cycles, the water cycle is primarily driven by physical processes (evaporation, condensation, precipitation) rather than biological ones, though living organisms do play a role.

Key Processes in the Water Cycle

Process	What Happens
Evaporation	Water from oceans, lakes, rivers and soil surfaces is heated by the Sun and turns into water vapour
Transpiration	Water evaporates from the leaves of plants through stomata (this is a biological contribution)
Condensation	Water vapour rises, cools and condenses to form tiny water droplets, creating clouds
Precipitation	Water falls from clouds as rain, snow, sleet or hail
Surface runoff	Water flows over the ground surface into rivers, lakes and eventually the sea
Infiltration	Water soaks into the soil and underlying rock
Groundwater flow	Water moves slowly through underground rock (aquifers) and eventually reaches rivers or the sea
Percolation	Water moves downward through soil and rock under the influence of gravity

graph TD
    SUN["Sun<br/>(energy source)"] -->|"Heats water"| EVAP["Evaporation<br/>from oceans and lakes"]
    PLANTS2["Plants"] -->|"Transpiration"| VAPOUR["Water Vapour<br/>in atmosphere"]
    EVAP --> VAPOUR
    VAPOUR -->|"Cooling and<br/>condensation"| CLOUDS["Clouds<br/>(condensation)"]
    CLOUDS -->|"Precipitation<br/>(rain, snow)"| LAND["Land Surface"]
    LAND -->|"Surface runoff"| RIVERS["Rivers and Lakes"]
    LAND -->|"Infiltration"| GROUND["Groundwater"]
    RIVERS -->|"Flow to sea"| OCEAN2["Oceans"]
    GROUND -->|"Groundwater flow"| OCEAN2
    OCEAN2 --> EVAP
    LAND -->|"Absorbed by roots"| PLANTS2
    style SUN fill:#fff9c4,stroke:#f9a825
    style VAPOUR fill:#e3f2fd,stroke:#1565c0
    style CLOUDS fill:#cfd8dc,stroke:#546e7a
    style OCEAN2 fill:#90caf9,stroke:#1565c0
    style PLANTS2 fill:#c8e6c9,stroke:#2e7d32
    style LAND fill:#d7ccc8,stroke:#5d4037
    style RIVERS fill:#81d4fa,stroke:#0277bd
    style GROUND fill:#bcaaa4,stroke:#5d4037

The Role of Living Organisms in the Water Cycle

Although the water cycle is primarily physical, living organisms contribute in important ways:

Organism	Contribution
Plants	Take up water through roots; release water vapour through transpiration from leaves
Animals	Release water vapour through respiration and excretion (urine, sweat)
Decomposers	Release water as a product of aerobic respiration when breaking down dead material