How Materials are Cycled

The carbon cycle and the water cycle — what AQA wants you to know, and the role of microorganisms in keeping the building blocks of life moving.

SPEC 4.7.2.2 • PAPER 2 • BOTH TIERS

The Big Idea

Every atom of carbon in your body, every molecule of water in a leaf — has been here for billions of years. Nothing new is made. Materials are recycled between living things and their environment.

Spec Statement

"All materials in the living world are recycled to provide the building blocks for future organisms."

Two Cycles On Your Spec

The carbon cycle — returns carbon to the atmosphere as CO₂ for photosynthesis.

The water cycle — provides fresh water for plants and animals on land.

Key Term

Decomposer — a microorganism (bacterium or fungus) that breaks down dead organic matter, returning carbon to the atmosphere as CO₂ and mineral ions to the soil.

Not On Your Spec

The nitrogen cycle — AQA spec explicitly states students are not expected to study it. Don't waste revision time on it.

Abiotic ↔ Biotic

Materials cycle between the non-living (abiotic) and living (biotic) parts of an ecosystem.

Abiotic Stores

Atmosphere (CO₂, water vapour)

Oceans, lakes, rivers

Soil (mineral ions)

Rocks & fossil fuels

Biotic Stores

Plants (glucose, cellulose, starch)

Animals (proteins, fats)

Microorganisms (in soil, water)

Dead organic matter

Exam Cue

If a question mentions an arrow on a cycle diagram going into a living thing → think photosynthesis or feeding. Arrow going out → think respiration, decay, or death.

The Water Cycle

Driven by energy from the Sun. Water is continuously evaporated and precipitated.

Water Cycle: The Named Processes

ProcessWhat HappensDriven By

EvaporationLiquid water → water vapour. Mainly from oceans, lakes, rivers.Sun's energy

TranspirationWater vapour released from plant leaves through stomata.Sun's energy

CondensationWater vapour cools → liquid droplets → forms clouds.Cooling at altitude

PrecipitationRain, snow, sleet, hail falls back to surface.Gravity

Why It Matters (Spec)

The water cycle provides fresh water for plants and animals on land before draining into the seas. Water is continuously evaporated and precipitated.

The Carbon Cycle

Returns carbon from organisms to the atmosphere as CO₂ to be used by plants in photosynthesis.

Carbon: The Four Named Processes

ProcessCO₂ Added to AtmosphereCO₂ Removed from Atmosphere

Photosynthesis (plants, algae)✓

Respiration (all organisms)✓

Decomposition (microorganisms respire)✓

Combustion (burning fuels)✓

The Spec Statement

"The carbon cycle returns carbon from organisms to the atmosphere as carbon dioxide to be used by plants in photosynthesis." — learn this sentence.

Synoptic Link

Photosynthesis uses CO₂ to make glucose. Glucose is then used to build cellulose (cell walls), starch (storage) and amino acids → proteins (Module 4 link).

The Role of Microorganisms

Without decomposer microorganisms, dead matter would pile up and nutrients would stay locked away. They break down dead plants AND dead animals — anything organic. The spec asks you to explain their two specific roles:

Role 1 — Returning Carbon

Microorganisms (bacteria and fungi) break down dead plants and animals.

They respire, releasing CO₂ back to the atmosphere.

Role 2 — Returning Mineral Ions

Decomposition releases mineral ions (e.g. nitrates, magnesium, potassium) back into the soil.

Plants then absorb these through their roots.

Verbatim Spec Wording

"Students should be able to explain the role of microorganisms in cycling materials through an ecosystem by returning carbon to the atmosphere as carbon dioxide and mineral ions to the soil."

Mark Scheme Cue

If asked "describe how microorganisms cycle carbon" — you must mention both: (1) breaking down dead matter, and (2) respiring to release CO₂. "Decomposing" alone is not enough — examiners want the respiration step named.

Reading Cycle Diagrams (WS 1.2)

AQA expects you to interpret and explain processes shown in diagrams of the carbon and water cycle. They give you a diagram with letters or arrows — you name the process.

Diagram Identification Checklist

Arrow from sea/lake going up → evaporation

Arrow from plant leaves going up → transpiration

Arrow from cloud going down → precipitation

Arrow from atmosphere into plant → photosynthesis

Arrow from organism into atmosphere → respiration

Arrow from fossil fuels into atmosphere → combustion

Arrow from dead matter into atmosphere → decomposition (microorganisms respiring)

Pro Tip

If the arrow points into the atmosphere from a living thing, it's almost always respiration. If it points out of the atmosphere into a plant, it's photosynthesis. Memorise these two and you'll save marks on every cycle diagram.

Common Errors

Wrong

"Plants release water through pores."

Right

"Plants release water vapour by transpiration through stomata."

Wrong

"Decomposers break down dead things and that releases CO₂."

Right

"Microorganisms respire, releasing CO₂ as a product of respiration."

Wrong

"Carbon goes back into the atmosphere when things die."

Right

"After death, microorganisms decompose the matter and respire, returning carbon to the atmosphere as CO₂."

Wrong

"Water rises into the sky."

Right

"Water evaporates (liquid → vapour) using energy from the Sun."

Wrong

"Plants only respire at night / plants don't respire in the day."

Right

"Plants respire continuously (day and night). During the day, the rate of photosynthesis just exceeds the rate of respiration → net uptake of CO₂."

Examiner Alerts

Alert 1 — Name The Process

Mark schemes reward the specific process word: evaporation, transpiration, condensation, precipitation, photosynthesis, respiration, decomposition, combustion. Generic phrases like "water moves" or "things rot" score zero.

Alert 2 — Respiration Is The Verb For Decay

When microorganisms decompose dead matter, the CO₂ is released because they respire. State the respiration step explicitly — "they cause decay" without naming respiration loses the mark.

Alert 3 — Don't Confuse The Two Cycles

Water cycle = evaporation/transpiration/condensation/precipitation. Carbon cycle = photosynthesis/respiration/decomposition/combustion. Mixing terms (e.g. "carbon evaporates") is a guaranteed zero.

Alert 4 — "Net Uptake" Language

For carbon-balance questions (e.g. CO₂ levels day vs night), mark schemes credit phrases like "net uptake of CO₂" or "more photosynthesis than respiration". They explicitly reject "no respiration" — plants respire continuously, even in light.

Bonus: Synoptic Trap

If a 6-mark question links "dead leaves" to "new plant growth", you must chain: decay by microorganisms → enzymes break down large molecules → respiration → CO₂ to atmosphere → photosynthesis by plants → glucose → cellulose / amino acids / proteins → growth.

Practice — Name The Processes

The diagram below shows part of an ecosystem. Four processes are labelled A, B, C and D.

12.1 Name process A. 1 mark

12.2 Name process B. 1 mark

12.3 Name process C. 1 mark

12.4 Name process D, and identify the type of organism that carries it out. 2 marks

12.1

Photosynthesis. (1 mark) — CO₂ from atmosphere taken in by the grass.

12.2

Respiration. (1 mark) — CO₂ released by the cow.

12.3

Transpiration. (1 mark) — water vapour released from the leaves (note: not evaporation, because it's from a plant).

12.4

Decomposition / decay (1 mark). Carried out by microorganisms — bacteria and fungi (1 mark). The CO₂ is released because the microorganisms respire.

Practice — Explain (4 marks)

A farmer ploughs dead crop material back into the soil at the end of the harvest.

Explain how microorganisms in the soil return useful substances to the environment.

4 marks

1

Microorganisms (bacteria and fungi) decompose / break down the dead crop material. (1)

2

The microorganisms respire, releasing carbon dioxide into the atmosphere. (1)

3

Decomposition also releases mineral ions back into the soil. (1)

4

The mineral ions / CO₂ are then taken up by plants (ions by roots; CO₂ for photosynthesis) for growth. (1)

Practice — 6-Marker (Level)

A tree drops its leaves in autumn. Months later, a new sapling growing nearby is producing fresh leaves.

Explain how carbon from the dead leaves can end up in the new leaves of the sapling.

6 marks (Level marked)

Level 3 (5–6): a coherent, fully linked explanation covering decay → CO₂ release → photosynthesis → biological molecules → new growth.

A

Dead leaves contain carbon compounds (in cellulose, glucose, proteins).

B

Microorganisms (bacteria and fungi) decompose the dead leaves.

C

They release enzymes that digest large molecules (proteins, carbohydrates) into smaller, soluble ones.

D

The microorganisms absorb these small molecules and respire them.

E

Respiration releases the carbon as carbon dioxide into the atmosphere.

F

The sapling absorbs the CO₂ through stomata and uses it in photosynthesis.

G

Photosynthesis combines CO₂ and water to make glucose.

H

Glucose is converted into cellulose (cell walls) and amino acids → proteins, which are required for the growth of new leaves.

Tip: a Level 3 answer links these steps. Level 2 just lists them. Level 1 only names a couple. Mark schemes specifically credit the enzyme/digestion step — don't skip it.

Recap & Next Up

2

Cycles on the spec

4+4

Named processes total

2

Roles of microorganisms

Three Things To Take Away

1

Carbon cycle — photosynthesis takes CO₂ out of the atmosphere; respiration, decomposition and combustion put it back.

2

Water cycle — evaporation, transpiration, condensation, precipitation. Driven by the Sun. Provides fresh water on land.

3

Microorganisms — return carbon to atmosphere (as CO₂ via respiration) and mineral ions to the soil.

Next: Lesson 7.07

Decomposition — how temperature, water and oxygen affect the rate of decay, plus Required Practical 10 (effect of temperature on decay of milk).