Describe how different materials cycle through the abiotic and biotic…

The idea

Materials such as carbon, nitrogen and water move repeatedly between the non-living (abiotic) parts of an ecosystem — air, soil, water and rock — and the living (biotic) parts — producers, consumers and decomposers. In the carbon cycle, producers take in carbon dioxide from the air during photosynthesis and build it into carbohydrates; consumers gain carbon by eating producers or other consumers; respiration in all living organisms returns carbon dioxide to the air; decomposers break down dead material and release carbon dioxide; and combustion of fossil fuels adds carbon dioxide. Nitrogen passes from soil to plants as nitrates, into animals through feeding, and back to soil when decomposers break down proteins and urea, with nitrogen-fixing bacteria and nitrifying bacteria replenishing soil nitrates. Water evaporates from surfaces, condenses, falls as rain and is taken up by roots.

What earns the marks

• Carbon enters the biotic component when producers absorb carbon dioxide from the air and use it in photosynthesis.
• Carbon returns to the abiotic component when organisms respire and release carbon dioxide.
• Decomposers break down dead organisms and waste, releasing carbon dioxide and returning mineral ions such as nitrates to the soil.
• Nitrogen is taken up by plants as nitrates from the soil and passed to consumers through feeding.
• Water cycles by evaporation, condensation and precipitation, and is absorbed by plant roots.
• Combustion of fossil fuels releases stored carbon back to the atmosphere as carbon dioxide.

Common mistakes and how to put them right

• Thinking carbon is 'used up' by organisms.

  Put it right by stressing that carbon atoms are recycled between stores, not destroyed.

• Believing plants only release oxygen.

  Put it right by noting that plants also respire and release carbon dioxide.

• Confusing decomposers with producers.

  Put it right by explaining that decomposers break down dead material and release minerals, whereas producers make food by photosynthesis.

Exam tips

• Name the store (air, soil, living organism) and the process (photosynthesis, respiration, decomposition) for each step you describe.
• Use the words abiotic and biotic correctly to show which component a material is moving into or out of.
• Link each process to a named organism group, such as producers, consumers or decomposers, to gain credit.