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In the water cycle, the sun heats water so it evaporates from oceans, rivers and lakes as water vapour. Free GCSE Biology notes and 13 FAQs.
The carbon and water cycles is part of Organisation of an ecosystem in GCSE Edexcel Higher Biology.
3 min
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41
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GCSE Edexcel Higher
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In the water cycle, the sun heats water so it evaporates from oceans, rivers and lakes as water vapour. Free GCSE Biology notes and 13 FAQs.
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Use this lesson to stay inside Organisation of an ecosystem while connecting the detail back to the wider Topic 9: Ecosystems and material cycles topic.
Read the core explanations from The carbon and water cycles before testing yourself from memory.
Picture Earth without these cycles. Plants would run out of carbon dioxide for photosynthesis. Animals would die of thirst as water stayed locked in the oceans. Nothing would decompose, so dead material would pile up everywhere.
The carbon and water cycles are Earth's recycling systems. They move essential materials between living things and the environment, keeping ecosystems running. Without them, life as we know it couldn't exist.
Carbon moves in a giant loop. Plants take carbon dioxide from the air during photosynthesis, turning it into glucose and other compounds. Animals eat plants, getting this carbon into their bodies.
When organisms respire, they release carbon dioxide back to the atmosphere. When they die and decompose, bacteria and fungi break them down, releasing more carbon dioxide. This completes the cycle.
Trap: Many students think carbon 'disappears' when animals eat plants. Actually, the carbon atoms just move from one organism to another - they're never destroyed, only recycled.
The sun drives the water cycle by providing energy for evaporation. Water evaporates from oceans, rivers and lakes, rising as invisible water vapour. As it rises higher, it cools and condenses into tiny droplets, forming clouds.
When droplets in clouds get too heavy, they fall as precipitation - rain, snow, sleet or hail. This water flows into rivers and streams, eventually returning to the sea. Some soaks into the ground, providing fresh water for plant roots.
Worked example: Tracing Water Through the Cycle
Follow a water molecule from the ocean to a plant's roots
Answer: Ocean → atmosphere → clouds → rain → soil → plant roots
This process is crucial because it desalinates water. Seawater is too salty for most plants and animals, but when it evaporates, the salt stays behind. Only pure water vapour rises, creating fresh water when it falls as rain.
These cycles don't work in isolation - they're interconnected. Plants need both water and carbon dioxide for photosynthesis. The water cycle provides the water, while the carbon cycle provides the carbon dioxide.
Photosynthesis equation: $6CO_2 + 6H_2O \rightarrow C_6H_{12}O_6 + 6O_2$
Notice how both cycles contribute: carbon dioxide from the carbon cycle and water from the water cycle combine to make glucose. When organisms respire, they use oxygen and glucose to release energy, producing carbon dioxide and water - feeding both cycles again.
Exam questions often ask you to 'describe' or 'explain' these cycles. Remember the difference: describe means state what happens, explain means give reasons why.
Exam depth extension: In the carbon and water cycles, connect why these cycles matter to the carbon cycle - nature's recycling system using clear cause-and-effect statements. When you explain biological processes, include what changes, why it changes, and the consequence for cells, organisms, or populations. If calculations appear, show each step, include units, and end with a sentence that interprets the result in biological context.
Common mistake: Students often forget that decomposition releases carbon dioxide. Remember: when organisms die, bacteria and fungi break them down, returning carbon to the atmosphere.
Master these cycles and you'll understand how Earth maintains the conditions needed for life. Every breath you take and every drop you drink has been recycled countless times through these amazing natural systems.
Published by Revise Right. Use the official specification reference to check the requirements for your course. Content updated: 2026-03-23.
Move to the related lessons or back to the topic page when you need the wider sequence before exam-style practice.
Precipitation is water falling from clouds as rain, snow, sleet or hail. It happens when droplets in clouds combine and get too heavy, so gravity pulls them down to Earth.
Condensation in the water cycle is when rising water vapour cools and turns into tiny droplets, forming clouds. Temperature drops with altitude, causing the water vapour to condense.
Infiltration is when rainwater soaks into the ground and reaches plant roots. The soil acts like a sponge, storing fresh water for plants.
In the water cycle, the sun heats water so it evaporates from oceans, rivers and lakes as water vapour. The vapour rises, cools and condenses into droplets forming clouds, which fall as precipitation. Water flows back to the sea or soaks into the ground, providing fresh water for plant roots.
The carbon cycle is essential because without it plants would run out of carbon dioxide for photosynthesis and dead material would pile up everywhere. It recycles carbon between living things and the environment, keeping ecosystems running.
Photosynthesis takes carbon out of the air because plants take in carbon dioxide and turn it into glucose and other compounds: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂. Animals then get this carbon by eating plants.
In the carbon cycle, plants take carbon dioxide from the air in photosynthesis, turning it into glucose. Animals eat plants, getting this carbon into their bodies. When organisms respire, they release carbon dioxide back to the atmosphere, and when organisms die, bacteria and fungi decompose them, releasing more carbon dioxide.
The sun drives the water cycle by providing energy for evaporation. Water evaporates from oceans, rivers and lakes as invisible water vapour, then cools, condenses into clouds and falls as precipitation.
The water cycle provides fresh water because evaporation desalinates seawater. When seawater evaporates, the salt stays behind and only pure water vapour rises, so it falls as fresh water in precipitation.
In decomposition, bacteria and fungi break down dead organisms, releasing carbon dioxide and nutrients back to the environment. This returns carbon to the atmosphere so plants can use it again in photosynthesis.
The carbon and water cycles are linked through photosynthesis. Plants combine carbon dioxide from the carbon cycle with water from the water cycle to make glucose, and when organisms respire they release carbon dioxide and water, feeding both cycles again.
Carbon is never destroyed in the carbon cycle; the carbon atoms just move from one organism to another. When animals eat plants, the carbon passes into their bodies and is later released when organisms respire or decompose.
When organisms respire, they release carbon dioxide back to the atmosphere. Organisms use oxygen and glucose to release energy, producing carbon dioxide and water, which feeds both the carbon cycle and the water cycle.