International A Level Biology / Environment and climate change

The carbon cycle revision notes

Edexcel International A Level Biology (YBI11), chapter 16: Environment and climate change, specification point 5.18. Free lesson.

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The carbon cycle shows where carbon is stored and how it moves between stores. Understanding its sinks and fluxes explains changes in atmospheric carbon dioxide and how those changes can be reduced.

How sinks and fluxes differ

A carbon sink is a store that takes in and holds carbon. A carbon flux is a movement of carbon between stores. Sinks are measured as a mass of carbon; fluxes are measured as a rate, the mass moved per year. A sink lowers atmospheric carbon dioxide only while it takes in more carbon than it releases. Sinks include oceans, forests, soils, peat and fossil fuels. Fluxes include photosynthesis, respiration and combustion.

Where carbon is stored and how it moves

The atmosphere stores carbon as carbon dioxide gas. Forests hold it in wood, leaves and roots. Soil holds it in dead organic matter; peat is soil where very little has decayed. Oceans hold dissolved carbon dioxide, while fossil fuels are coal, oil and natural gas formed from buried remains. Photosynthesis takes carbon dioxide out of the air. Respiration and decomposition return carbon dioxide to the air. Decomposition is the breakdown of dead material by decomposers. Dead remains can be buried, moving carbon into long-term stores. Combustion, or burning, releases carbon from…

How carbon moves through living things

Plants, algae and phytoplankton are producers. They use photosynthesis to fix carbon dioxide into sugars and other organic molecules. Feeding moves carbon into consumers, such as animals, because eaten material becomes part of their bodies. Dead bodies, fallen leaves and faeces still contain carbon. Decomposers, mostly bacteria and fungi, feed on dead material and release carbon dioxide through respiration. Every organism respires, including producers, consumers and microorganisms. More organisms can therefore mean more carbon dioxide returned to the atmosphere.

How oceans and fossil fuels store carbon

Carbon dioxide dissolves at the ocean surface. Some stays dissolved; some becomes part of marine organisms’ calcium carbonate shells, which can sink and form sediment. In places such as swamps or the sea floor, dead organisms may be left without oxygen and not decompose. Buried remains can slowly form fossil fuels. Carbon can stay in deep ocean water for centuries, and much longer once sediment becomes rock. Fossil fuels store carbon for millions of years. Carbon dioxide can leave the ocean surface again. Burning fossil fuels releases quickly carbon that took millions of years to store.…

Where plants get their carbon

Plants do not take carbon from soil for photosynthesis. Carbon dioxide from the air diffuses into leaves through stomata, which are small openings. Roots take up water and mineral ions. Soil carbon is stored carbon, not a supply for photosynthesis; decomposers return some of it to the air as carbon dioxide when they respire.

How the cycle can lower atmospheric CO₂

Atmospheric carbon dioxide rises when more carbon flows into the air than out. Burning fossil fuels and clearing forests have tipped this balance. To lower the level, cut fluxes into the air, boost fluxes out, and keep the removed carbon stored. Burning less fossil fuel cuts a flux into the atmosphere; more photosynthesis boosts a flux out. If new biomass is burnt or dies and decomposes, its carbon returns to the air.