Decay, the carbon cycle and the water cycle – GCSE Combined Science Biology

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Nature recycles. The carbon in your body was once in the air, and the water you drink has fallen as rain before. This lesson follows three recycling stories: decay, the water cycle and the carbon cycle.

It matters because without recycling, the nutrients plants need would be locked inside dead things. It also helps you to understand compost and why carbon dioxide is in the news.

By the end you will be able to explain what decomposers do and what speeds up decay. You will describe the water cycle and the carbon cycle. You will also explain how compost uses decay.

Decay and decomposers

When a plant or animal dies, it does not stay as it is. Decomposers break it down. Decomposers are living things that feed on dead material. The main ones are bacteria and fungi.

They release enzymes onto the dead material. Enzymes are chemicals that break big molecules into small ones. The decomposers then absorb the nutrients. They also break down waste such as faeces.

The mineral ions return to the soil, where plants take them up again. Think of decomposers as the recycling team of nature.

What speeds up decay

Decay is faster in warm conditions, because enzymes work faster when warm. It is faster when there is enough water, because decomposers need moisture. It is faster with plenty of oxygen, because aerobic respiration releases more energy.

Very hot conditions slow decay, because the enzymes are damaged. Very dry conditions slow it too. Cold slows it down, which is why food keeps in a fridge.

The best conditions are warm, damp and with plenty of air.

Compost

Compost is decayed plant material that is used as a natural fertiliser. A compost heap needs air, warmth and moisture, the same conditions that speed up decay. Gardeners and farmers use it to return nutrients to the soil.

The water cycle

The water cycle gives plants and animals fresh water. Energy from the Sun evaporates water from the sea, rivers and soil. Evaporation means a liquid turning into a gas. Plants also lose water vapour from their leaves. This is called transpiration.

The water vapour rises and cools. Condensation turns it back into liquid droplets, which form clouds. Water then falls as precipitation, which means rain, snow or hail.

Some water runs off the land into rivers and the sea. Some soaks into the ground. The cycle then starts again.

The carbon cycle: taking carbon in

Carbon is recycled between living things and the air. The air holds carbon dioxide, and this is the main store of carbon for living things. Think of it as a shared bank account that everyone pays into and takes from.

Plants take carbon dioxide from the air for photosynthesis. The carbon becomes glucose and then other compounds in the plant. Animals get their carbon by eating plants, or by eating other animals.

Photosynthesis is the way carbon dioxide leaves the air.

The carbon cycle: giving carbon back

Carbon dioxide returns to the air in three main ways. Plants, animals and decomposers all release it by respiration. Burning wood and fossil fuels releases it by combustion.

When living things die, decomposers break them down. They respire too, so the carbon goes back into the air. Over millions of years some remains become fossil fuels, such as coal and oil.

The same carbon is used again and again. It is not used up.

Recap

Decomposers, mainly bacteria and fungi, break down dead material. Decay is fastest when warm, moist and with oxygen.

Water cycle: evaporation, transpiration, condensation, precipitation.

Carbon cycle: photosynthesis takes carbon dioxide out of the air. Respiration, combustion and decay put it back.

Compost uses decay with air.