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Biology · Cycles in Ecosystems

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Biology · OCR Paper 2

Cycles in Ecosystems

Cycling of Materials and Decomposers

Materials such as carbon, nitrogen and water cycle between the abiotic and biotic parts of an ecosystem.

  • Ecosystem All the living things in an area and their non-living surroundings.
  • Abiotic Non-living, such as air, water and soil.
  • Biotic Living, such as plants, animals and microorganisms.
A cycle in which plants take materials from the air, soil and water, animals eat plants, and decomposers return the materials from dead plants, dead animals and waste to the air, soil and water.
Decomposers return the materials in dead organisms and waste to the environment.

Producers, consumers and decomposers

  • Producers and consumers Take materials from the environment or from food; Build materials into their own bodies; Plants take in carbon dioxide, water and mineral ions
  • Decomposers Break down dead plants and animals and waste; Release enzymes to digest the material outside the cell; Return carbon dioxide and mineral ions to the environment

The Carbon Cycle

Carbon is cycled between the air, producers, consumers, decomposers and fossil fuels: photosynthesis removes carbon dioxide from the air, feeding passes carbon along food chains.

  • Carbon cycle The movement of carbon between the air, living things and fossil fuels.
  • Photosynthesis The process in which plants take carbon dioxide from the air and make glucose.
  • Respiration The release of energy from glucose, which releases carbon dioxide.
The carbon cycle, showing photosynthesis taking carbon dioxide from the air into plants, feeding passing carbon to animals, respiration, death and decay and combustion returning carbon dioxide to the air.
Photosynthesis takes carbon dioxide out of the air, and respiration, decay and combustion put it back.

Taking carbon in and putting it back

  • Carbon dioxide removed from the air Photosynthesis in plants and algae; Carbon is built into glucose and biomass; Carbon is stored in plants, in wood and in fossil fuels
  • Carbon dioxide returned to the air Respiration by plants, animals and decomposers; Decay of dead material by decomposers; Combustion of wood and fossil fuels
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Biology · OCR Paper 2

Cycles in Ecosystems continued

The Water Cycle

In the water cycle, the Sun's heat makes water evaporate and plants transpire, the water vapour condenses into clouds and falls as precipitation.

  • Water cycle The movement of water between the sea, the air, the land and living things.
  • Evaporation The change from liquid water to water vapour.
  • Transpiration The loss of water vapour from the leaves of plants.
The water cycle with evaporation from the sea, transpiration from plants, condensation into clouds, precipitation, run-off to the sea, and water soaking into the ground.
The Sun drives the water cycle, and the water moves between the sea, the air and the land.

Evaporation and transpiration

  • Evaporation From the sea, lakes, rivers and the soil; Caused by heat from the Sun; Liquid water becomes water vapour
  • Transpiration From the leaves of plants; Water is taken up by roots first; Water vapour leaves through the stomata

The Rate of Decomposition (Separate only)

Decomposers work fastest when it is warm, moist and there is plenty of oxygen.

  • Rate of decay How quickly dead material is broken down.
  • Aerobic With oxygen.
  • Anaerobic Without oxygen.
Three graphs showing that the rate of decay is fastest at an optimum temperature, is highest in moist but not waterlogged conditions, and is faster with oxygen than without it.
Decay is fastest when it is warm, moist and there is plenty of oxygen.

Aerobic and anaerobic decay

  • Aerobic decay Oxygen is available; Aerobic respiration releases more energy; Faster and more complete
  • Anaerobic decay No oxygen, for example in waterlogged soil; Anaerobic respiration releases less energy; Slower and less complete

Investigating the best conditions for composting

  • Set up Put the same mass of chopped plant material, such as grass…
  • Change one factor Change only one condition, for example the temperature, the…
  • Measure Measure how quickly the material decays.
  • Repeat Repeat each condition several times and calculate a mean.