How are carbon and water recycled through living things and the environment?
The carbon cycle (photosynthesis, respiration, decomposition and combustion), the water cycle (evaporation, transpiration, condensation and precipitation), and how human activities such as burning fossil fuels affect the carbon cycle.
A focused answer to the OCR Gateway GCSE Biology A topic B4 on the carbon and water cycles, covering photosynthesis, respiration, decomposition and combustion in the carbon cycle, the stages of the water cycle, and the human impact of burning fossil fuels.
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What this dot point is asking
OCR wants you to describe the carbon cycle (the processes that remove and return carbon dioxide), describe the water cycle, and explain how human activities such as burning fossil fuels affect the carbon cycle.
The carbon cycle
Carbon is found in carbon dioxide in the air and in the carbon compounds that make up living things. The carbon cycle keeps recycling it between the air, living organisms and the ground.
Carbon dioxide is removed from the air by:
- Photosynthesis. Plants and algae take in carbon dioxide and use it to make glucose. The carbon becomes part of their bodies (as carbohydrates, proteins and fats) and passes along food chains when animals eat them.
Carbon dioxide is returned to the air by:
- Respiration. Plants, animals and microorganisms respire, releasing carbon dioxide.
- Decomposition. When organisms die, decomposers (bacteria and fungi) break down the dead material and respire, returning carbon dioxide to the air.
- Combustion. Burning wood and fossil fuels releases the carbon stored in them as carbon dioxide.
The water cycle
The water cycle recycles water between the sea, the air and the land, driven by the Sun's energy:
- Evaporation. The Sun's energy evaporates water from the sea, rivers and land, turning it into water vapour. Plants also release water vapour by transpiration.
- Condensation. The water vapour rises, cools and condenses into tiny droplets, forming clouds.
- Precipitation. The water falls as rain, snow or hail.
- The water flows over and through the land into rivers and back to the sea, and the cycle repeats.
The water cycle provides the fresh water that plants and animals need, and links to transport in plants (the transpiration stream) from topic B2.
Human impact on the carbon cycle
Human activities can upset the balance of the carbon cycle:
- Burning fossil fuels (coal, oil and gas) releases carbon dioxide that was locked away for millions of years, adding it to the air faster than photosynthesis can remove it.
- Deforestation removes trees that would have absorbed carbon dioxide by photosynthesis, and burning them releases more.
Together these raise the concentration of carbon dioxide in the atmosphere. Carbon dioxide is a greenhouse gas, so this contributes to global warming and climate change, which you study further in topic B6.
Exam-style practice questions
Practice questions written in the style of OCR exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
OCR 20196 marksDescribe how carbon is recycled through the carbon cycle, naming the processes that remove carbon dioxide from the air and the processes that return it.Show worked answer →
A 6-mark extended response. Mark it for a complete cycle naming the key processes.
Removing carbon dioxide from the air: photosynthesis removes carbon dioxide from the air. Plants (and algae) use it to make glucose, which becomes the carbon compounds in their bodies. This carbon passes along food chains when animals eat plants.
Returning carbon dioxide to the air: respiration by plants, animals and microorganisms releases carbon dioxide back into the air. When organisms die, decomposers (bacteria and fungi) break down the dead material and respire, returning carbon dioxide. Combustion (burning) of wood and fossil fuels also returns carbon dioxide to the air.
Markers reward photosynthesis as the removal process and at least two of respiration, decomposition and combustion as return processes, set out as a cycle.
OCR 20214 marksDescribe the main stages of the water cycle, and explain the part played by transpiration from plants.Show worked answer →
A 4-mark question on the water cycle.
Stages: the Sun's energy causes water to evaporate from the sea, rivers and land, forming water vapour. The vapour rises, cools and condenses into clouds (condensation). The water then falls as precipitation (rain, snow). It runs back into rivers and the sea, and the cycle repeats.
Transpiration: plants take up water through their roots and lose water vapour from their leaves by transpiration. This adds water vapour to the air, contributing to the evaporation stage of the cycle. Markers reward evaporation, condensation and precipitation in order, plus the point that transpiration releases water vapour from plants into the air.
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