17.2 Materials Cycling

A free GCSE Biology lesson for AQA (8461) from OBA School: why materials must be recycled, the role of decomposers, and the water cycle. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

• Explain why materials such as carbon, water and mineral ions must be recycled
• Describe the role of microorganisms (decomposers) in cycling materials
• Describe the processes of the water cycle
• Explain why the water cycle matters for living things on land

Key words

• Decomposer: A microorganism, such as a bacterium or fungus, that breaks down dead material and waste.
• Decay: The breakdown of dead material by decomposers, using enzymes.
• Mineral ions: Simple inorganic substances, such as nitrate and phosphate ions, that plants absorb from the soil.
• Evaporation: Liquid water changing into water vapour, using energy from the Sun.
• Transpiration: Loss of water vapour from plant leaves, mainly through the stomata.
• Condensation: Water vapour cooling and turning back into liquid droplets, forming clouds.
• Precipitation: Water falling from clouds as rain, sleet, snow or hail.

Materials are recycled

Living things are made of materials taken from the environment: carbon, oxygen, hydrogen, nitrogen and many mineral ions. Earth has a fixed amount of these. They must be recycled so they can be used again by new organisms. Materials cycle between the living (biotic) and non-living (abiotic) parts of an ecosystem.

Key fact: Energy is not recycled. It enters as light and is eventually lost as heat. Materials are recycled.

The role of decomposers

When plants and animals die, or produce waste, the materials in their bodies are locked up. Decomposers – mainly bacteria and fungi – release them.

• Decomposers release enzymes onto dead material and absorb the small soluble molecules produced.
• They use these molecules in respiration, which releases carbon dioxide into the atmosphere.
• Decay releases mineral ions into the soil, which plants take up through their roots.
• Animals such as earthworms and woodlice (detritus feeders) break dead material into smaller pieces, which speeds up decay.

Worked example: how fast is litter recycled?

In a wood, 420 g of dead leaves fall onto each square metre of ground in a year. In the same year, 336 g of leaf litter on each square metre is decomposed.
Percentage decomposed = 336 ÷ 420 × 100 = 80%.

Answer: 80% of a year’s leaf fall is recycled within the year

The water cycle

• Evaporation; What happens The Sun heats water in seas, lakes and the ground, which turns into water vapour.
• Transpiration; What happens Plants lose water vapour from their leaves.
• Condensation; What happens Water vapour rises, cools and condenses into droplets, forming clouds.
• Precipitation; What happens Droplets join and fall as rain, snow, sleet or hail.
• Run-off and percolation; What happens Water flows over the land into rivers and seas, or soaks into soil and rock.

Precipitation provides fresh water for plants and animals on land, before it drains back into the seas. Animals also return water to the environment in sweat, urine, faeces and breath.

Exam tip: When water evaporates from the sea, the salt is left behind, so rain is fresh water. This is why the water cycle is vital for life on land.

Common mistakes

• Saying decomposers “eat” dead matter like animals do. They secrete enzymes and absorb the products.
• Forgetting that decomposers release carbon dioxide by respiration.
• Mixing up evaporation (from surfaces such as the sea) and transpiration (from plant leaves).
• Saying energy is recycled. Only materials are recycled.

Revision questions

Try these, then check your answers below each one.

Question 1. Which organisms are the main decomposers? (Choose from: Bacteria and fungi, Plants and algae, Viruses and bacteria, Earthworms and beetles.)

Answer: Bacteria and fungi

Working: Bacteria and fungi are decomposers. Earthworms are detritus feeders that help decay.

Question 2. True or false? Energy is recycled in ecosystems in the same way as carbon and water.

Answer: False

Working: False. Energy enters as light and is lost as heat; materials are recycled.

Question 3. How do decomposers return carbon to the atmosphere? (Choose from: By respiration, releasing carbon dioxide, By photosynthesis, releasing oxygen, By transpiration, releasing water vapour, By combustion, releasing heat.)

Answer: By respiration, releasing carbon dioxide

Working: Decomposers respire the products of digestion, releasing carbon dioxide.

Related lessons

• Previous lesson: 17.1 Feeding Relationships
• Next lesson: 17.3 The Carbon Cycle
• In this chapter: 17.4 Rates of Decomposition
• In this chapter: 17.5 Required Practicals: Organising an Ecosystem

Continue in the full course

This lesson is section 17.2 (AGCBH1702) of our free GCSE Biology course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Biology course.