Cycling materials (AQA GCSE Biology): Revision Notes
Cycling materials
The water cycle
The water cycle keeps fresh water moving around our planet. This provides land plants and animals with the water they need to survive.
The water cycle is a continuous process that has been operating for billions of years, ensuring that water is constantly cleaned and redistributed across Earth's surface.
Key processes in the water cycle
Evaporation happens when water from oceans, rivers and lakes turns into water vapour. Heat from the sun causes this change.
Transpiration is when plants release water vapour through their leaves. Plants take up water through their roots and lose it through tiny pores.
Precipitation occurs when water vapour in clouds cools down and falls as rain or snow.
Respiration from plants and animals also adds water vapour to the air.
The water then flows through rocks and soil back to rivers and oceans, and the cycle starts again.
Worked Example: Water Cycle in Action
- Morning: Sun heats ocean water → evaporation occurs
- Afternoon: Water vapour rises and cools → clouds form
- Evening: Clouds become heavy → precipitation falls as rain
- Night: Rain soaks into soil → flows back to rivers and oceans
- Next day: The cycle begins again
The carbon cycle
Carbon moves around ecosystems in a continuous cycle. This cycling is essential for life on Earth.
Without the carbon cycle, carbon would become locked up in one place and living organisms would run out of this vital element needed for growth and energy.
How carbon moves through ecosystems
Photosynthesis removes carbon dioxide from the air. Plants absorb this gas and convert it into carbon compounds like sugars. This is how carbon enters food chains.
Respiration releases carbon dioxide back into the air. All living things respire, including plants, animals and microorganisms like bacteria and fungi.
Combustion of fossil fuels also releases carbon dioxide. When we burn coal, oil or gas, we add extra carbon to the atmosphere.
Decomposition happens when dead organisms are broken down by decomposers. This process releases carbon dioxide into the air.
Movement through food chains
Carbon compounds pass from plants to animals when animals eat plants. The carbon then moves from animal to animal along food chains when predators eat their prey.
Worked Example: Carbon Movement Through a Food Chain
- Plant absorbs CO₂ through photosynthesis → carbon stored in plant tissues
- Rabbit eats plant → carbon transfers to rabbit's body
- Fox eats rabbit → carbon transfers to fox's body
- Decomposers break down dead fox → carbon returns to atmosphere as CO₂
Role of microorganisms
Decomposers play a crucial role in cycling materials back into ecosystems.
Without decomposers, Earth would be covered in dead plant and animal material, and essential nutrients would remain locked up and unavailable for new life.
What decomposers do
Bacteria and fungi are the main decomposers. They feed on dead plants and animals, breaking down this waste material.
As decomposers digest dead material, they use nutrients to grow and reproduce. They release carbon dioxide into the atmosphere through respiration.
Returning nutrients to soil is another important job. Decomposers release mineral ions like nitrates back into the soil. Plants need these nitrates to make amino acids for growth.
Detritus feeders like worms and beetles help start the decay process. They eat dead organisms, making it easier for bacteria and fungi to break down the remaining material.
Without decomposers, dead material would pile up and nutrients would not be recycled. This would eventually stop new growth in ecosystems.
Key Points to Remember:
- Water cycles continuously through evaporation, transpiration, precipitation and flow back to oceans
- Carbon cycles through ecosystems via photosynthesis (removes CO₂), respiration (releases CO₂), and decomposition
- Decomposers like bacteria and fungi break down dead material and return nutrients to soil
- Both cycles are essential - they provide fresh water and recycle carbon and other nutrients
- Human activities like burning fossil fuels add extra carbon dioxide to the atmosphere