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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. Unlike energy, which flows through an ecosystem once and is lost as heat, materials such as carbon are used over and over again — and the carbon cycle describes how. This lesson, part of Topic B4 of OCR Gateway Science A, traces carbon as it is removed from the air by photosynthesis, passed along food chains, and returned to the air by respiration, decomposition and combustion. Understanding the cycle ties together photosynthesis, respiration and decomposition, and explains why these processes keep ecosystems going.
By the end of this lesson you should be able to 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 where carbon is stored in fossil fuels.
Carbon is the element on which all life is built. The large molecules that make up living things — carbohydrates, proteins, lipids and DNA — are all based on chains and rings of carbon atoms. To build their bodies, organisms must obtain carbon; and when they die or respire, that carbon must be returned to the environment so it can be used again. There is only a fixed amount of carbon on Earth, so it has to be recycled continually between the living and non-living worlds. The store of carbon that most of the cycle revolves around is the carbon dioxide (CO₂) in the atmosphere (and dissolved in the oceans).
Carbon is found in several stores around the cycle, and it helps to keep these in mind as you follow it. There is carbon as CO₂ gas in the air and dissolved in the oceans; carbon in living organisms, locked into their carbohydrates, proteins and fats; carbon in dead organisms and waste, before decomposers break it down; and carbon in fossil fuels (coal, oil and gas), which is the most long-term store of all. The carbon cycle is simply the set of processes that move carbon between these stores — photosynthesis, feeding, respiration, decomposition and combustion. Each store can hold carbon for very different lengths of time: an atom may stay in a fast-growing plant for only a season, in a tree for decades, or in a fossil fuel for hundreds of millions of years.
It is easiest to follow the cycle as a sequence 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.
Four 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 mistake 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 of the carbon cycle. When a plant or animal dies, it still contains all the carbon (and minerals) locked up in 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 that the living community depends on would dwindle. Decomposers therefore keep the whole cycle turning — a theme you will study in detail in the next lesson on the rate of decay.
It is worth noting that decomposers work fastest in warm, moist conditions with plenty of oxygen, because they are living microorganisms whose enzymes need these conditions. This means the carbon and minerals in dead material are recycled quickly in a warm, damp habitat such as a tropical forest, but very slowly in cold or waterlogged places — which is exactly why, in waterlogged bogs, dead plant material does not fully decompose and builds up as peat instead of returning its carbon to the air. The speed of decomposition therefore controls how fast carbon is returned to the cycle, linking this lesson directly to the rate of decay you study next.
Some carbon is removed from the active cycle for very long periods. Hundreds of millions of years ago, the remains of plants and microscopic sea organisms were buried before they could fully decompose. Over enormous spans of time, under heat and pressure, this carbon-rich material became the fossil fuels — coal (from ancient plants) and oil and natural gas (from ancient marine organisms). The carbon in these organisms was therefore locked away underground rather than being returned to the air.
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