How Are Carbon and Water Cycled in Nature?
Learn how carbon and water cycle between living organisms and the environment for AQA GCSE Biology, including photosynthesis, respiration and rainfall.
Carbon and water are cycled in nature: they move between living organisms and the non-living environment, then become available to living organisms again. In AQA GCSE Biology, the key idea is that materials are recycled, even though energy passes through ecosystems. This explanation assumes the separate GCSE Biology course and applies to both Foundation and Higher tier.
The carbon cycle
Plants and algae take in carbon dioxide from the air during photosynthesis. They use its carbon to make glucose and other molecules. When an animal eats a plant, some of that carbon passes into the animal. Carbon can then move along a food chain as animals eat other animals.
Plants, animals and microorganisms release carbon dioxide back into the air through respiration. When organisms die or produce waste, decomposers such as bacteria and fungi break down the material. As decomposers respire, they also return carbon dioxide to the atmosphere. Plants can take in that carbon dioxide again.
The water cycle
Water evaporates from seas, lakes and other surfaces into the air as water vapour. Plants also release water vapour from their leaves through transpiration. As water vapour cools, it condenses into tiny droplets that form clouds. It returns to Earth as precipitation, such as rain. Water can then flow into rivers and seas or be taken up by plants. Animals obtain water by drinking and eating.
Exam mistake to avoid: do not say that plants get their carbon from the soil. In the carbon cycle, they take in carbon dioxide from the air during photosynthesis. Also, do not confuse the two cycles: carbon dioxide is returned by respiration, whereas evaporation and precipitation move water.
