Explain how materials cycle through ecosystems, the importance of the carbon, water and nitrogen cycles including the role of microorganisms, and the factors affecting decomposition.

What this dot point is asking

Edexcel statements 9.12 to 9.15 and 9.17B to 9.19B want you to explain how materials cycle through ecosystems, describe the carbon, water and nitrogen cycles (including the role of microorganisms), explain how potable water is produced, and explain the factors affecting the rate of decomposition in food preservation and composting (9.17B to 9.19B are Biology only).

Why materials cycle

The carbon cycle

Carbon moves between the atmosphere (as carbon dioxide) and living things:

• Photosynthesis removes carbon dioxide from the air and fixes the carbon into glucose and then into plant tissues.
• Feeding passes carbon along food chains to animals.
• Respiration by plants, animals and microorganisms returns carbon dioxide to the air.
• Decomposition: when organisms die, microorganisms (decomposers) break them down and release carbon dioxide as they respire.
• Combustion: burning fossil fuels and wood releases stored carbon as carbon dioxide.

So plants take carbon out of the air, and respiration, decomposition and combustion put it back, keeping the cycle going.

The water cycle

Water also cycles continuously. The Sun's energy evaporates water from oceans, lakes and plants (transpiration); the vapour rises and condenses into clouds; and it falls as precipitation (rain, snow), running back to the sea or soaking into the ground.

Where fresh water is scarce, potable water (safe to drink) is produced by treating fresh water (filtering and sterilising it) or, in areas of drought, by desalination of seawater (removing the salt, for example by distillation or reverse osmosis), which is effective but uses a lot of energy.

The nitrogen cycle

Plants need nitrogen to make proteins, but they cannot use nitrogen gas from the air directly. Bacteria make nitrogen available:

• Nitrogen-fixing bacteria (in the soil and in legume root nodules) convert nitrogen gas into nitrogen compounds.
• Decomposers break down dead organisms and waste, releasing ammonia.
• Nitrifying bacteria convert ammonia into nitrates, which plants absorb through their roots.
• Denitrifying bacteria convert nitrates back into nitrogen gas, returning it to the air.

Farmers increase available nitrogen by adding fertilisers, using crop rotation (growing legumes to fix nitrogen naturally), so crops grow better.

Decomposition (Biology only)

The rate at which decomposers break down dead material depends on three factors:

• Temperature: warmth speeds up the decomposers' enzyme-controlled reactions (until it is hot enough to denature them); cold slows decay.
• Water content: decomposers need water, so moist conditions decay faster than dry ones.
• Oxygen availability: decomposers need oxygen for aerobic respiration, so good oxygen supply speeds up decay.

This explains both food preservation (keeping food cold, dry, or sealed from air slows decomposers) and composting (gardeners keep a compost heap warm, moist and aerated to speed up decay).

Try this

Q1. Name the two processes that return carbon dioxide to the atmosphere in the carbon cycle. [2 marks]

• Cue. Respiration (including by decomposers) and combustion (burning).

Q2. State the type of bacteria that convert nitrogen gas into nitrogen compounds. [1 mark]

• Cue. Nitrogen-fixing bacteria.