The Water and Carbon Cycles

The atoms in your body are not new. The carbon in the glucose you respired this morning may once have been in a dinosaur, a tree, or the shell of a sea creature. There is a fixed amount of each element on Earth, so living things cannot keep taking fresh supplies — the same atoms have to be used again and again.

This is the central idea of this topic:

Materials are recycled. Many different materials cycle through the abiotic (non-living) and biotic (living) parts of an ecosystem, providing the building blocks for future organisms.

Two terms are worth fixing now, because the whole topic rests on them:

• Abiotic components — the non-living parts of an ecosystem: the air, water, soil and minerals.
• Biotic components — the living parts: plants, animals, fungi and microorganisms.

A material such as carbon moves out of the abiotic world (the air), into the biotic world (a plant, then an animal), and eventually back out again. The two cycles AQA asks about — the carbon cycle and the water cycle — are just detailed maps of these movements. You are not required to study the nitrogen cycle for this specification.

Carbon is stored in the air as the gas carbon dioxide (CO₂). The carbon cycle describes how that carbon is taken out of the air, passed through living things, and returned to the air. There are really only two directions to learn:

• One process removes CO₂ from the air: photosynthesis.
• Three processes return CO₂ to the air: respiration, combustion, and decomposition.

Process	Carried out by	Effect on atmospheric carbon
Photosynthesis	Plants and algae	Removes CO₂ — carbon is locked into glucose
Respiration	Plants, animals and microorganisms	Returns CO₂
Combustion (burning)	Burning of wood and fossil fuels	Returns CO₂
Decomposition	Decomposers (microorganisms)	Returns CO₂

The single fact examiners most want to see stated clearly:

Photosynthesis is the only process that takes carbon dioxide out of the atmosphere. Respiration, combustion and decomposition all put it back.

Once carbon is inside a plant as glucose, it can be built into other molecules (starch, cellulose, fats, proteins) and passed on. The next slide follows it through the living world.

The clearest way to learn the carbon cycle is to trace a single carbon atom and name the process at every step. Start with one atom of carbon in a CO₂ molecule in the air.

1. Into a plant — photosynthesis. A green plant absorbs the CO₂ and uses light energy to combine it with water, making glucose. The carbon atom is now part of a sugar molecule inside the plant. CO₂ has been removed from the air.
2. Along a food chain — feeding. A rabbit eats the plant. The carbon atom, now in plant material, becomes part of the rabbit's body. Carbon in food passes along food chains from producers to consumers.
3. Back to the air — respiration. The rabbit respires the glucose to release energy. Carbon dioxide is produced as a waste product and breathed out. The carbon atom is back in the atmosphere.

That is one complete loop. But the carbon could have taken other return routes instead:

• Decomposition. If the rabbit died and was not eaten, decomposers (microorganisms) would feed on its remains and respire, releasing the carbon as CO₂.
• Combustion. If the original plant had been a tree that was cut down and burned — or buried over millions of years and turned into a fossil fuel — burning it would release the carbon as CO₂.

Notice that respiration, decomposition and combustion all reach the same destination: carbon dioxide back in the air, ready for a plant to photosynthesise again.

A clear way to picture the whole cycle is to put the atmosphere at the top, plants and animals in the middle, and the return arrows pointing back up to the air.

Reading the diagram: one arrow points down out of the atmosphere (photosynthesis), and every other arrow points up into it (respiration, combustion, decomposition). If you can label the four arrows correctly, you can answer almost any carbon-cycle question.

Decomposers are the reason dead leaves, dead animals and waste do not pile up forever. They are microorganisms — chiefly bacteria and fungi — that feed on dead organisms and waste material, breaking them down.

Decomposers do two jobs that the specification asks you to know:

• They return carbon to the atmosphere as carbon dioxide. Decomposers respire as they feed, so the carbon locked in dead bodies and waste is released back into the air as CO₂. Decomposition is therefore one of the three return routes in the carbon cycle.
• They return mineral ions to the soil. Breaking down dead material releases minerals — such as the ions plants need for healthy growth — back into the soil, where plant roots can absorb them again.

The role of decomposers (microorganisms): to recycle materials — returning carbon to the air as CO₂ and mineral ions to the soil — so the building blocks can be reused by the next generation of organisms.

Without decomposers, carbon and minerals would stay trapped in dead bodies, the soil would lose its fertility, and the cycles would grind to a halt. Their warmth-loving, moisture-loving activity is exactly why this topic links to composting and the breakdown of waste.

(Extra context — not required by AQA 8461.) Decomposition is fastest in warm, moist, oxygen-rich conditions because the microorganisms can respire aerobically and reproduce quickly; this is why gardeners turn and dampen compost heaps.

The water cycle supplies fresh water to plants and animals living on land, before that water drains back into the seas. Like carbon, water is not used up — the same water molecules move round and round between the sea, the air, the land and living things.

There are three key processes to name:

• Evaporation. Heat energy from the Sun turns liquid water — mainly from the surface of the seas — into water vapour, a gas. Plants also add water vapour to the air through transpiration (evaporation of water from leaves).
• Condensation. As the water vapour rises and cools, it condenses into tiny droplets, forming clouds.
• Precipitation. The droplets join, grow heavy, and fall as rain, snow or sleet — this is the fresh water that plants and animals on land depend on.

How the cycle provides fresh water — a walk-through. Sea water is salty, so it cannot be drunk or used by most land plants. When the Sun evaporates sea water, only the water turns to vapour — the salt is left behind in the sea. That vapour condenses into clouds and falls as precipitation over the land. Because the salt was left behind, the rain that falls is fresh (salt-free) water. Plants absorb it through their roots and animals drink it; the rest flows through rivers and drains back into the seas, where the whole cycle begins again.

1. Saying respiration removes carbon dioxide

It is the wrong way round. Respiration releases CO₂; only photosynthesis removes it from the air. Many marks are lost by swapping these two over.

2. Forgetting that decomposers respire

Decomposition returns carbon to the air because decomposers respire as they feed. If you only say "they break things down" you miss why CO₂ is released. State that decomposers respire and release CO₂.

3. Listing only one return route for carbon

Carbon returns to the air by three routes — respiration, combustion and decomposition. A question worth several marks usually expects more than one. Learn all three.

4. Confusing evaporation, condensation and precipitation

A common slip is to use the wrong word for the wrong step. Keep them in order: evaporation/transpiration (liquid → vapour) → condensation (vapour → droplets/clouds) → precipitation (droplets fall as rain or snow).

5. Saying the sea provides fresh water directly

The sea is salt water. It only becomes fresh water once it has evaporated (leaving the salt behind) and fallen again as precipitation. Always link "fresh water" to evaporation followed by precipitation.

6. Mentioning the nitrogen cycle

The nitrogen cycle is not on the AQA 8461 specification for this topic. Do not include it in your answer — stick to the carbon cycle and the water cycle.