The chemicals in ecosystems are continually cycling through the natural…

The idea

Chemicals such as carbon, water and nitrogen are recycled between the living and non-living parts of ecosystems. Decomposers, including bacteria and fungi, break down dead organisms and waste, releasing mineral ions and carbon compounds back into the environment. Plants take up these minerals through their roots, and the elements are built into new biological molecules, so the same atoms are reused. The carbon cycle illustrates this: carbon moves between the atmosphere, producers, consumers, decomposers and fossil fuels through photosynthesis, feeding, respiration, combustion and decomposition.

What earns the marks

• Elements such as carbon and nitrogen, and compounds such as water, are recycled through biotic and abiotic components of ecosystems rather than being lost.
• Decomposers break down dead organisms and waste, releasing mineral ions and carbon compounds that producers can reuse.
• Photosynthesis removes carbon dioxide from the atmosphere and incorporates carbon into organic molecules in producers.
• Feeding transfers carbon compounds along food chains, and respiration returns carbon dioxide to the atmosphere.
• Combustion of fossil fuels releases carbon that was locked away for millions of years, adding carbon dioxide to the atmosphere.
• Microorganisms in the soil convert nitrogen compounds into forms that plants can absorb, so nitrogen also cycles through ecosystems.

Common mistakes and how to put them right

• Saying that chemicals disappear when organisms die.

  Put it right: Decomposers release the elements back into the environment so they can be reused.

• Confusing the carbon cycle with the nitrogen cycle.

  Put it right: Each cycle involves different compounds, organisms and processes, although both recycle materials.

• Thinking that energy is recycled in the same way as chemicals.

  Put it right: Chemicals are recycled, but energy flows through ecosystems and is eventually lost as heat.

Exam tips

• Practise drawing and labelling the carbon cycle with arrows showing photosynthesis, feeding, respiration, decomposition and combustion.
• When explaining recycling, name the decomposers and the specific chemicals or ions they release, such as carbon dioxide or nitrate ions.
• Use the phrase 'the same atoms are reused' to show understanding that matter is conserved in ecosystems.