The carbon cycle and the nitrogen cycle, the roles of photosynthesis, respiration, combustion and decomposition, the types of bacteria in the nitrogen cycle, and the conditions affecting decay.

What this dot point is asking

WJEC wants you to describe the carbon cycle and the nitrogen cycle, name the processes and bacteria involved, explain the role of decomposers, and state the conditions that affect the rate of decay.

The carbon cycle

Carbon is constantly recycled between the air, the soil and living things. The key processes are:

• Photosynthesis: plants remove carbon dioxide from the air and use the carbon to make glucose and other compounds.
• Feeding: carbon passes along food chains when animals eat plants (and other animals).
• Respiration: plants, animals and decomposers respire, releasing carbon dioxide back into the air.
• Decomposition: when organisms die, decomposers break them down and release carbon dioxide as they respire.
• Combustion: burning wood and fossil fuels releases carbon dioxide that was locked in them.

The nitrogen cycle

Plants and animals need nitrogen to make proteins and DNA, but they cannot use nitrogen gas from the air directly. The nitrogen cycle converts nitrogen into forms they can use.

• Nitrogen fixation: nitrogen-fixing bacteria (in the soil and in the root nodules of some plants, such as peas and beans) convert nitrogen gas from the air into nitrogen compounds (nitrates). Lightning can also fix some nitrogen.
• Assimilation: plants absorb nitrates from the soil and use them to make proteins. Animals get their nitrogen by eating plants.
• Decomposition: when plants and animals die, or produce waste, decomposers break down the proteins and release ammonia.
• Nitrification: nitrifying bacteria convert ammonia into nitrates, which plants can use again.
• Denitrification: denitrifying bacteria convert nitrates back into nitrogen gas, returning it to the air. This happens most in waterlogged soils with little oxygen.

The role of decomposers

Decomposers are bacteria and fungi that break down dead organisms and waste. They are essential to both cycles because they release the carbon (as carbon dioxide) and the nitrogen (as ammonia) locked in dead matter, returning these nutrients to the environment so they can be reused.

Conditions affecting the rate of decay

The rate at which decomposers break down dead material depends on the conditions.

• Temperature: warmth speeds up the decomposers' enzyme reactions, so decay is faster up to an optimum; too hot and the enzymes denature.
• Moisture (water): decomposers need water to live and for reactions; decay is slow in dry conditions.
• Oxygen: most decomposers respire aerobically, so good aeration (oxygen) speeds up decay.

This is why a compost heap is kept warm, moist and turned to add air, and why food is preserved by being dried, frozen or sealed (removing the conditions decomposers need).