Cycling of Materials

All the elements that make up living organisms are recycled through ecosystems. Materials pass between the living and non-living parts of the environment in continuous cycles. This lesson covers the carbon cycle, water cycle, decomposition and the nitrogen cycle.

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Why Do Materials Cycle?

The Earth is essentially a closed system — very little new matter enters or leaves the planet. This means that the elements needed for life (carbon, nitrogen, oxygen, hydrogen, etc.) are finite and must be recycled.

• Materials are taken from the environment by living organisms
• They are passed along food chains through feeding
• They are returned to the environment through respiration, excretion, death and decomposition
• These materials are then available for new organisms to use

Without recycling, elements would become locked up in dead organisms and waste, and life would eventually run out of raw materials.

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The Carbon Cycle

The carbon cycle describes how carbon moves between the atmosphere, living organisms, the oceans and the Earth's crust.

Carbon Enters the Atmosphere (as CO₂)

Process	How carbon is released
Respiration	All living organisms (plants, animals, fungi, bacteria) respire, releasing CO₂ — this is the main natural source
Combustion (burning)	Burning fossil fuels (coal, oil, natural gas) and wood releases CO₂ that was locked away
Decomposition	When decomposers (bacteria and fungi) break down dead organisms and waste, they respire and release CO₂
Volcanic activity	Releases CO₂ stored deep in the Earth

Carbon is Removed from the Atmosphere

Process	How carbon is removed
Photosynthesis	Green plants and algae absorb CO₂ from the atmosphere and convert it into glucose (organic molecules) — this is the main process that removes CO₂
Dissolving in oceans	CO₂ dissolves in seawater; some is used by marine organisms to make calcium carbonate shells

Carbon Passes Through Food Chains

1. Producers (plants) fix carbon from CO₂ into organic molecules (glucose, starch, cellulose, proteins, lipids) through photosynthesis
2. Primary consumers eat plants — carbon is transferred in the organic molecules they consume
3. Secondary and tertiary consumers eat other animals — carbon continues to be transferred
4. At each trophic level, some carbon is returned to the atmosphere as CO₂ through respiration

Carbon Returns to the Soil and Atmosphere

• When organisms die, their bodies contain carbon compounds
• Decomposers (bacteria and fungi) break down dead matter and waste
• During decomposition, decomposers respire, releasing CO₂ back into the atmosphere
• Some carbon is returned to the soil as humus (organic matter in soil)

Fossil Fuels and Carbon

Millions of years ago, some dead organisms did not fully decompose (e.g. in anaerobic conditions like swamps or ocean floors):

• Dead plant material formed coal (a fossil fuel)
• Dead marine organisms formed oil and natural gas (fossil fuels)
• Carbon was locked away (sequestered) in these fossil fuels for millions of years
• When humans burn fossil fuels, this stored carbon is released as CO₂
• This is adding extra CO₂ to the atmosphere faster than natural processes can remove it
• This is the main cause of the enhanced greenhouse effect and climate change

Summary Diagram of the Carbon Cycle

graph TD
    A["CO2 in Atmosphere"] -->|"Photosynthesis"| B["Plants and Algae"]
    B -->|"Feeding"| C["Animals"]
    B -->|"Respiration"| A
    C -->|"Respiration"| A
    B -->|"Death"| D["Dead Organisms and Waste"]
    C -->|"Death"| D
    D -->|"Decomposition - microorganisms respire"| A
    D -->|"Fossilisation over millions of years"| E["Fossil Fuels"]
    E -->|"Combustion"| A
    A -->|"Dissolving"| F["Oceans"]

    style A fill:#85C1E9,color:#333
    style B fill:#2ecc71,color:#fff
    style C fill:#e67e22,color:#fff
    style D fill:#8B4513,color:#fff
    style E fill:#2c3e50,color:#fff
    style F fill:#3498db,color:#fff

The carbon cycle can be summarised as a balance between processes that ADD CO₂ to the atmosphere and those that REMOVE it:

Adding CO₂ to atmosphere	Removing CO₂ from atmosphere
Respiration (all organisms)	Photosynthesis (plants and algae)
Combustion (burning fuels/wood)	Dissolving in oceans
Decomposition	Formation of fossil fuels (very slow)
Volcanic activity	Formation of carbonate rocks (very slow)

Exam tip: When drawing or describing the carbon cycle, make sure you include ALL the major processes: photosynthesis, respiration, combustion, decomposition, and feeding. A very common mistake is to forget that plants ALSO respire — they do not just photosynthesise. Plants both absorb CO₂ (photosynthesis) and release CO₂ (respiration).

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The Water Cycle

The water cycle describes the continuous movement of water between the atmosphere, land and oceans.

Stages of the Water Cycle

Stage	Description
Evaporation	Water is heated by the Sun and changes from a liquid to a gas (water vapour); mainly from oceans, lakes and rivers
Transpiration	Water evaporates from the leaves of plants through stomata — this is a significant contributor to atmospheric water vapour
Condensation	Water vapour rises, cools and condenses into tiny water droplets, forming clouds
Precipitation	Water droplets in clouds combine and fall as rain, snow, sleet or hail
Collection/runoff	Water collects in rivers, lakes and oceans; some seeps into the ground (percolation) and becomes groundwater
The cycle repeats	Water is continuously cycled — no new water is created

Key Points

• The Sun provides the energy that drives the water cycle (evaporation)
• Transpiration from plants is a major source of water vapour — deforestation reduces transpiration, which can affect local rainfall patterns
• Water is a universal solvent — it carries dissolved minerals through the soil to plant roots and carries nutrients through organisms

Exam tip: Remember that transpiration is part of the water cycle. Plants take up water from the soil through their roots and lose it from their leaves as water vapour. This is essentially evaporation from a biological source.

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Decomposition

Decomposition is the breakdown of dead organisms and waste materials by decomposers — mainly bacteria and fungi.

How Decomposition Works

1. Decomposers secrete enzymes onto dead organic matter
2. The enzymes break down complex organic molecules (proteins, carbohydrates, lipids) into simpler, soluble substances
3. The decomposers absorb these soluble products for their own nutrition
4. This process is called saprotrophic nutrition (or saprophytic nutrition)
5. During decomposition, decomposers respire, releasing CO₂ and returning minerals to the soil

Why Decomposition is Important

• Recycles nutrients — returns carbon, nitrogen, phosphorus and other elements to the soil and atmosphere
• Makes minerals available for plants to absorb through their roots
• Without decomposition, dead matter would accumulate and nutrients would be locked up permanently

Factors Affecting the Rate of Decomposition

Factor	Effect	Explanation
Temperature	Warm temperatures increase rate (up to ~45°C); very high temperatures decrease rate	Enzymes work faster at higher temperatures; above optimum, enzymes denature
Moisture	Moist conditions increase rate; dry conditions decrease rate	Decomposers need water for metabolic reactions and enzyme function; water helps transport dissolved substances
Oxygen availability	Aerobic conditions increase rate	Aerobic respiration by decomposers releases more energy, allowing them to be more active; in anaerobic conditions, decomposition is much slower
pH	Optimal pH varies, but extreme pH reduces rate	Enzymes have an optimal pH; extreme acidity or alkalinity denatures them
Surface area	Smaller pieces decompose faster	More surface area exposed to decomposer enzymes

Exam tip: When explaining how a factor affects decomposition rate, always link it to ENZYME ACTIVITY. For example: "Increasing temperature increases the kinetic energy of enzyme and substrate molecules, so they collide more frequently, increasing the rate of decomposition."

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Core Practical: Investigating the Effect of Temperature on Decomposition

Using Milk and Lipase (or Similar)

Aim: To investigate how temperature affects the rate of decomposition.

Method (using lipase enzyme and milk):

1. Add a set volume of full-fat milk to several test tubes
2. Add a few drops of phenolphthalein indicator (pink in alkaline conditions) and a small amount of sodium carbonate solution (to make the milk alkaline and turn the indicator pink)
3. Add a set volume of lipase enzyme to each tube
4. Place each tube in a water bath at a different temperature (e.g. 10°C, 20°C, 30°C, 40°C, 50°C, 60°C)
5. Time how long it takes for the solution to turn from pink to white/colourless
6. The colour change indicates that lipase has broken down the fat in the milk, producing fatty acids that lower the pH and cause the indicator to become colourless

Variables:

Variable	Details
Independent	Temperature of the water bath
Dependent	Time taken for the colour change (pink to colourless)
Control variables	Volume of milk, volume of lipase, concentration of lipase, volume of indicator, volume of sodium carbonate

Expected results:

• As temperature increases from 10°C to about 40°C, the time decreases (decomposition gets faster)
• At very high temperatures (above ~45°C), the time increases dramatically or no change occurs (enzymes are denatured)
• The fastest decomposition occurs around 35–40°C (the enzyme's optimum temperature)

Alternative method: Investigate the rate of decomposition of compost at different temperatures by measuring the volume of CO₂ produced.

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The Nitrogen Cycle (Higher Tier)