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The carbon atoms in a leaf today may once have been part of a dinosaur, a lump of coal, or the air a Roman breathed. That is because carbon is constantly recycled between living things, the soil, the oceans and the atmosphere. Water behaves the same way: the rain that waters a field may have evaporated from an ocean on the other side of the world. Unlike energy — which flows through an ecosystem once and is lost as heat — materials such as carbon and water are used over and over again, and the carbon cycle and water cycle describe how. In this lesson of Topic B4 you will trace both cycles, see how decomposers drive the recycling of materials, and learn what controls the rate of decay. Understanding these cycles ties together photosynthesis, respiration and decomposition, and explains why they keep ecosystems going.
By the end of this lesson you should be able to describe the stages of the water cycle, describe how carbon is removed from and returned to the atmosphere, explain the roles of photosynthesis, respiration, decomposition and combustion, describe the part played by decomposers, and explain what affects the rate of decay.
This lesson mainly develops AO1 (recalling the stages of the carbon and water cycles), with AO2 when you apply the roles of the processes to explain how carbon moves, and AO3 when you interpret how factors such as temperature affect the rate of decay.
Living things are built from a small set of elements — above all carbon — and they depend on a constant supply of water. There is only a fixed amount of each on Earth, so both must be recycled continually between the living and non-living worlds. As organisms grow they take up carbon and water; as they respire, die and decay, these materials are returned to the environment to be used again. If they were not recycled, the raw materials of life would soon run out. The two cycles in this lesson are the routes by which that reuse happens.
The water cycle describes how water is continually recycled between the oceans, the air, the land and living things, providing the fresh water that plants and animals need. It is powered by energy from the Sun.
The stages are:
This cycle provides the fresh water (not salty) that land organisms depend on: when sea water evaporates, the salt is left behind, so the rain that falls is fresh.
Exam Tip: Use the four key terms in order — evaporation → condensation → precipitation → (return/runoff) — and remember that transpiration from plants also releases water vapour. A common mark is for noting that the water cycle provides fresh water because the salt is left behind when sea water evaporates.
Carbon is the element on which all life is built: carbohydrates, proteins, lipids and DNA are all based on carbon atoms. The store of carbon that most of the cycle turns around is the carbon dioxide (CO₂) in the atmosphere (and dissolved in the oceans). It is easiest to follow the cycle as a set of processes that remove CO₂ from the air and processes that return it.
The only process that removes carbon dioxide from the atmosphere in this cycle is photosynthesis. Green plants and algae take in CO₂ and, using light energy, convert it into glucose and other carbon compounds that make up their tissues. In this way carbon from the air becomes part of the producers' biomass. Carbon then passes to animals when they eat plants (or eat other animals), so it moves along the food chain locked up in carbohydrates, proteins and fats.
Three processes return carbon dioxide to the atmosphere:
Exam Tip: Remember that photosynthesis is the only process that removes CO₂ from the air, while respiration, decomposition (decay) and combustion all return it. A common misconception is to think plants only remove CO₂ — in fact plants also respire, returning some CO₂, so they do both.
Decomposers — mainly bacteria and fungi — are the recyclers behind both cycles. When a plant or animal dies, it still contains all the carbon (and minerals) locked into its body. Decomposers feed on this dead material and break it down. As they respire, the carbon is released back into the air as CO₂; and the mineral ions (such as nitrate) are returned to the soil, where plants can take them up again. Without decomposers, dead bodies and waste would pile up, the carbon in them would stay locked away, and the supply of CO₂ and minerals the living community depends on would dwindle. Decomposers therefore keep the whole cycle turning.
How quickly decomposers break down dead material — the rate of decay — is not fixed. Because decomposers are living microorganisms whose activity depends on enzymes, decay happens fastest in the conditions that suit them best. Three factors matter most:
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