How Materials are Cycled (AQA GCSE Biology): Revision Note

Recycling in an ecosystem

• Many different materials cycle through the abiotic and biotic components of an ecosystem

• All materials in the living world are recycled to provide the building blocks for future organisms

Carbon cycle

• Elements such as carbon are not endless resources

• There is a finite amount of each element on the planet

• Elements need to be recycled in order to allow new organisms to be made and grow

Carbon cycle processes

• Carbon is taken out of the atmosphere in the form of carbon dioxide by plants and algae during photosynthesis

  • The carbon is used to make glucose, which can be turned into carbohydrates, fats and proteins – biomass – within plants and algae

• This carbon is passed on to animals (and microorganisms) when they feed on plants and algae

• It is returned to the atmosphere in the form of carbon dioxide during respiration by plants, animals and microorganisms (that release CO₂ during decomposition)

• If animals and plants die in conditions where decomposing microorganisms are not present, the carbon in their bodies can be converted, over millions of years and significant pressure, into fossil fuels

• When wood or fossil fuels are burned (a process known as combustion), the carbon within them combines with oxygen, and carbon dioxide is released into the atmosphere

Diagram of the carbon cycle

Use this image

Water cycle

• The water cycle provides fresh water for plants and animals on land before draining into the seas

• Water is continuously evaporated and precipitated

• Water molecules move between bodies of water—such as rivers and oceans—and the atmosphere via multiple stages

Stages of the water cycle

1. Water enters the atmosphere as water vapour

   • Energy from the Sun heats the Earth’s surface, and water evaporates from oceans, rivers and lakes

   • Transpiration from plants releases water vapour into the air

2. The warmer air of the lower atmosphere rises, taking the water vapour with it

   • The moist air cools down as it rises

   • Water vapour condenses back into liquid water, forming clouds

3. Water returns to Earth in the form of precipitation:

   • As the water droplets in the cloud get bigger and heavier, they begin to fall as rain, snow and sleet

   • This is called precipitation

Diagram of the water cycle

Use this image

Role of microorganisms

• Microorganisms play a vital role in cycling material through an ecosystem; they return carbon to the atmosphere as carbon dioxide and mineral ions to the soil

  • When living organisms produce waste products or organisms die, the waste products and dead organisms are broken down (digested) by microorganisms known as decomposers

  • Bacteria and fungi are the main groups of decomposers

• Decomposition (also known as decay) is the process by which waste products or dead organisms are broken down and the materials they contain (such as carbon and mineral ions such as iron, magnesium and nitrates) are returned to the environment

• When decomposing microorganisms respire, carbon is returned to the atmosphere in the form of carbon dioxide