AQA GCSE Combined Science Foundation (8464), Foundation tier · Biology › Ecology › Organisation of an ecosystem
Practise How materials are cycled. 7 exam-style questions plus unlimited generated ones on this subtopic, at up to four difficulty levels, with full mark schemes and a progress tracker. Free, no account needed.
Materials such as carbon and water are constantly recycled between living things and the non-living environment. Expect to name or explain the processes on carbon cycle and water cycle diagrams, and to explain how microorganisms return carbon dioxide to the air and mineral ions to the soil.
Key facts
Photosynthesis removes CO2 from the air
Respiration (by plants, animals and microorganisms) releases CO2
Combustion of fossil fuels and wood releases CO2
Feeding passes carbon compounds along food chains
Decomposers respire → CO2 to the air; mineral ions → the soil
Water cycle: evaporation and transpiration → condensation → precipitation → drainage to the sea
The nitrogen cycle is not required
Notes
Why materials are cycled
The materials that make up living things are used again and again: they are recycled to build new organisms.
Many different materials, including carbon, water and mineral ions, cycle between the biotic (living) and abiotic (non-living) parts of an ecosystem.
You do not need to learn the nitrogen cycle.
The carbon cycle
diagram
Photosynthesis: plants and algae take in carbon dioxide and use it to make glucose and other carbon compounds.
Feeding: carbon compounds pass along food chains when animals eat plants and other animals.
Respiration: plants, animals and microorganisms all respire, releasing carbon dioxide into the air.
Decay: when organisms die or produce waste, microorganisms (decomposers) break the material down. Their respiration releases carbon dioxide.
Combustion: burning fossil fuels and wood releases carbon dioxide.
Some dead material does not decay and, over millions of years, forms fossil fuels, which store carbon until they are burned.
So the carbon cycle returns carbon from organisms to the atmosphere as carbon dioxide, which plants use again in photosynthesis.
The carbon cycle. Only photosynthesis (green) takes carbon dioxide out of the air.
The water cycle
diagram
Energy from the Sun makes water evaporate from seas, lakes and land, and plants lose water vapour from their leaves by transpiration.
Water vapour cools and condenses to form clouds.
Water falls as precipitation (rain, snow or hail), providing fresh water for plants and animals on land.
Water drains through the soil and flows in rivers back to the sea, and the cycle continues.
The water cycle is driven by energy from the Sun.
The role of microorganisms
Decomposers return carbon to the atmosphere as carbon dioxide and return mineral ions to the soil, where plant roots can absorb them.
Without decomposers, dead material would build up and the carbon and mineral ions in it would stay locked away.
How to answer each type of question
Name a process on a carbon cycle diagram
1 mark eachGrade 4
Arrow from the air into plants: photosynthesis.
Arrow from living organisms or decomposers into the air: respiration.
Arrow from fossil fuels or wood into the air: combustion.
Arrow from plants to animals: feeding (eating).
Example. A carbon cycle diagram has three arrows labelled P, Q and R.
P goes from carbon dioxide in the air to green plants. Q goes from animals to carbon dioxide in the air. R goes from coal to carbon dioxide in the air. Name processes P, Q and R.
Start with evaporation from the sea (and transpiration from plants).
Water vapour condenses to form clouds.
Precipitation brings fresh water to the land.
Water drains through the soil and flows back to the sea.
Example. Describe how water from the sea can reach the roots of a plant growing on land. (4 marks)
Show the model answerHide the model answer
Water evaporates from the sea, heated by the Sun (1). The water vapour cools and condenses to form clouds (1). The water falls on the land as rain / precipitation (1). The rainwater soaks into the soil, where the roots can absorb it (1).
Don’t lose marks
Saying decomposers 'release carbon dioxide' without saying how. They release it by respiration.
Thinking plants only photosynthesise. Plants respire all the time too, releasing carbon dioxide.
Mixing up evaporation (liquid to vapour) and condensation (vapour to liquid).
Writing that mineral ions go into the air. Decomposers return mineral ions to the soil and carbon dioxide to the air.
More tips
Memory tricks
Only one process takes CO2 out of the air: photosynthesis. Respiration (including by decomposers) and combustion put it back.
Carbon moves by five processes: photosynthesis, feeding, respiration, decay and combustion.
Water cycle order: Evaporate, Condense, Precipitate, Drain.
Exam technique
On cycle diagrams, check the direction of each arrow before naming the process.
Use precise names: photosynthesis, respiration, combustion, evaporation, transpiration, condensation, precipitation.
In any answer about decomposers, include the word respiration.
What each grade needs
What you need to be able to do, from the first marks up to the top grade.
Grade 3
State that materials are recycled in ecosystemsMaterials cycle through the living and non-living parts of an ecosystem, providing the building blocks for new organisms.
Grade 4
Name the processes in the water cycleEvaporation, transpiration, condensation, precipitation, and water draining back into rivers and seas.
Grade 5
Name processes that add or remove carbon dioxidePhotosynthesis removes carbon dioxide from the air; respiration and combustion release it.
Quick recall
Cover the answers and test yourself. The app has these as flashcards that come back just before you'd forget them.
Name the type of organism that returns carbon from dead plants and animals to the atmosphere.
Microorganisms (decomposers)
Name the process by which plants lose water vapour from their leaves.
Transpiration
Microorganisms also return substances to the soil that plants need for healthy growth. Name this type of substance.
Mineral ions
Sample questions
Written for this site in the style of AQA exam questions. They are not taken from real past papers.
Question 1Easy5 marks
Carbon is cycled between living organisms and the atmosphere.
(a) Which process removes carbon dioxide from the atmosphere? Tick (✓) one box.[1]
Combustion
Decay
Photosynthesis
Respiration
(b) Which two processes return carbon dioxide to the atmosphere? Tick (✓) two boxes.[2]
Combustion of fossil fuels
Feeding
Photosynthesis
Respiration
Transpiration
(c) Name the type of organism that returns carbon from dead plants and animals to the atmosphere.[1]
(d) Why is it important that materials such as carbon are recycled in the living world?[1]
Show the answer and mark scheme
(a)Answer: Photosynthesis
(b)Answer: Combustion of fossil fuels; Respiration
(c)Answer: Microorganisms (decomposers)
microorganisms / decomposers (bacteria or fungi)
(d)
to provide the building blocks for future organisms / so that materials are available for new organisms
Question 2Medium8 marks
The water cycle provides fresh water for plants and animals on land.
(a) Name the process by which plants lose water vapour from their leaves.[1]
(b) Describe how water from the sea can become rain that falls on land.[3]
(c) Explain why the water cycle is important to living organisms on land.[2]
(d) Give two ways in which animals return water to the environment.[2]
Show the answer and mark scheme
(a)Answer: Transpiration
transpiration
(b)
water evaporates from the sea (heated by the Sun)
the water vapour rises, cools and condenses to form clouds
the clouds are blown over the land by the wind
the water falls as precipitation / rain
(c)
it provides fresh water on land
water is needed by plants for photosynthesis / to keep cells turgid / for transport
animals need water for drinking / for chemical reactions in cells
water drains back to the seas and is continuously evaporated and precipitated, so the supply is renewed