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Materials are recycled, energy is not

  1. All the materials in living things, such as carbon, water and mineral ions, are recycled to build future organisms.
  2. Energy, in contrast, flows one way through an ecosystem and is eventually lost as heat.
  3. So materials are used again and again, but energy must be constantly resupplied by the Sun.

Energy from the Sun flows one way through producers, consumers and decomposers and is lost as heat, while nutrients are recycled back to the producers.

Key Idea

Matter goes round in cycles, but energy passes straight through and is not recycled.

The water cycle

Definition

Water cycle

The continuous evaporation and precipitation of water that provides fresh water for organisms on land before it drains back to the seas.

  1. The water cycle keeps fresh water available for plants and animals on land.
  2. The Sun's heat evaporates water from the seas, turning it into water vapour.
  3. The vapour rises, cools and condenses into clouds.
  4. The water falls as precipitation, such as rain or snow, providing fresh water on land.
  5. This water is used by organisms and drains through rivers back into the seas, where the cycle starts again.

The carbon cycle

Definition

Carbon cycle

The constant recycling of carbon between the air, living organisms and the soil through photosynthesis, respiration, decomposition and combustion.

  1. Carbon is recycled between the air, living organisms and the soil.
  2. Producers remove carbon dioxide from the air by photosynthesis, locking the carbon into glucose and biomass: 6CO2+6H2O→C6H12O6+6O26CO_2 + 6H_2O \rightarrow C_6H_{12}O_6 + 6O_26CO2​+6H2​O→C6​H12​O6​+6O2​.
  3. The carbon then passes along food chains as animals eat producers and one another.
  4. All organisms return carbon dioxide to the air by respiration: C6H12O6+6O2→6CO2+6H2OC_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2OC6​H12​O6​+6O2​→6CO2​+6H2​O.
  5. Burning, or combustion, of wood and fossil fuels also releases carbon dioxide back into the air.

carbon-cycle-24c14283-genie.png

Common Mistake
  • Do not confuse the carbon cycle with the water cycle.
  • Photosynthesis takes carbon dioxide out of the air, while respiration and combustion put it back.

The role of microorganisms

Definition

Decomposer

A microorganism, such as a bacterium or fungus, that breaks down dead organisms and waste, returning carbon dioxide to the air and mineral ions to the soil.

  1. Microorganisms, especially decomposers, are essential for recycling carbon and mineral ions.
  2. Decomposers, such as bacteria and fungi, feed on dead organisms and waste.
  3. As they respire, they return carbon to the atmosphere as carbon dioxide.
  4. They also release mineral ions, such as nitrates, back into the soil for plants to absorb.
Self review
  • Why are materials recycled but energy is not?
  • Describe the two main stages of the water cycle.
  • Which process removes carbon dioxide from the air, and which two return it?
  • How do decomposers return carbon and mineral ions to an ecosystem?
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7.2.2 How materials are cycled Revision Guide

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