Decomposition

Decomposition is the process by which dead organisms and waste products are broken down by decomposers — mainly bacteria and fungi. This process is vital for recycling nutrients in ecosystems and is an important topic in AQA GCSE Biology. This lesson also covers the Required Practical on the effect of temperature on the rate of decomposition.

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What Is Decomposition?

Decomposition is the breakdown of dead plant and animal material (and waste such as faeces and urine) by microorganisms, mainly bacteria and fungi. These organisms secrete enzymes onto the dead material, which digest it externally, and then absorb the soluble products.

Decomposition returns nutrients and mineral ions to the soil, where they can be taken up by plant roots and used for growth. Without decomposition, dead material would accumulate indefinitely and the supply of minerals in the soil would eventually run out.

Key Term	Definition
Decomposition	The breakdown of dead organic material by microorganisms
Decomposer	An organism that breaks down dead material (bacteria, fungi)
Detritivore	An organism that feeds on dead material (detritus), speeding up decomposition
Detritus	Dead organic material (dead leaves, animal remains, faeces)

Decomposers vs Detritivores

Feature	Decomposers	Detritivores
Examples	Bacteria, fungi	Earthworms, woodlice, maggots, millipedes
How they feed	Secrete enzymes externally, absorb soluble products	Eat and physically break down dead material
Role	Chemically break down organic matter	Break material into smaller pieces, increasing surface area for decomposers

Exam Tip: Do not confuse decomposers with detritivores. Decomposers (bacteria and fungi) chemically break down dead material using enzymes. Detritivores (earthworms, woodlice) physically eat and break up the material. Detritivores speed up decomposition by increasing the surface area available for decomposers to work on.

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Conditions Affecting the Rate of Decomposition

The rate of decomposition is affected by several environmental conditions. All of these relate to the conditions needed by the decomposing microorganisms (bacteria and fungi) to function effectively.

Factor	Effect on Decomposition Rate	Explanation
Temperature	Increases rate up to an optimum (around 37-40 degrees C for bacteria), then decreases rapidly	Enzymes in decomposers work faster at higher temperatures; above the optimum, enzymes denature
Moisture	Higher moisture increases the rate of decomposition	Decomposers need water for metabolic reactions and to dissolve nutrients
Oxygen (aerobic conditions)	Decomposition is faster in aerobic conditions	Aerobic respiration by decomposers releases more energy than anaerobic respiration
pH	Most decomposers work best at neutral pH (around 7)	Extreme pH values denature enzymes
Surface area	Smaller pieces decompose faster	More surface area is available for enzymes to act on

graph LR
    A["Rate of<br/>Decomposition"] --> B["Temperature<br/>(up to optimum)"]
    A --> C["Moisture<br/>(higher = faster)"]
    A --> D["Oxygen<br/>(aerobic = faster)"]
    A --> E["pH<br/>(neutral = optimal)"]
    A --> F["Surface Area<br/>(greater = faster)"]
    style A fill:#fff9c4,stroke:#f9a825
    style B fill:#ffccbc,stroke:#d84315
    style C fill:#bbdefb,stroke:#1565c0
    style D fill:#c8e6c9,stroke:#2e7d32
    style E fill:#e1bee7,stroke:#6a1b9a
    style F fill:#d7ccc8,stroke:#5d4037

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Required Practical: Effect of Temperature on the Rate of Decomposition

This practical investigates how temperature affects the rate of decomposition using milk as the organic material and lipase (an enzyme) as a model for decomposer enzymes. Alternatively, some schools use fresh milk with a pH indicator (such as cresol red or phenolphthalein) to detect the acid produced when milk is decomposed by bacteria.

Method Using Lipase and Milk

1. Set up water baths at different temperatures (e.g. 10, 20, 30, 40, 50, 60 degrees C)
2. Add 5 cm3 of full-fat milk and 5 cm3 of sodium carbonate solution (to make it alkaline) to a test tube, along with a few drops of phenolphthalein indicator (turns pink in alkaline conditions)
3. Add 1 cm3 of lipase solution to each test tube
4. Place each test tube in the appropriate water bath
5. Start a timer and wait for the colour to change from pink to white/colourless
6. Record the time taken for the colour change at each temperature
7. The colour change indicates that the lipase has broken down the fat in the milk, producing fatty acids that neutralise the sodium carbonate, turning the indicator colourless

Expected Results

Temperature (degrees C)	Time for Colour Change (s)	Rate of Reaction (1/time)
10	Very long (e.g. 600+)	Very slow
20	Long (e.g. 300)	Slow
30	Moderate (e.g. 120)	Moderate
40	Short (e.g. 60)	Fast (near optimum)
50	Moderate (e.g. 150)	Declining
60	Very long or no change	Very slow or zero

Variables

Variable Type	Variable
Independent	Temperature of the water bath
Dependent	Time taken for the colour change (indicator turning colourless)
Control variables	Volume of milk, volume of lipase, volume of sodium carbonate, concentration of each solution, type of indicator

Interpreting the Results

• The rate of decomposition increases as temperature rises towards the optimum (around 37-40 degrees C)
• Above the optimum, the rate decreases rapidly because the enzymes in the decomposers become denatured — their active site changes shape and can no longer bind to the substrate
• At very low temperatures, enzymes have very little kinetic energy, so the rate of reaction is very slow (but enzymes are not denatured — they are just inactive)

Exam Tip: Remember to distinguish between enzymes being inactive at low temperatures (they still have the correct shape but lack kinetic energy) and enzymes being denatured at high temperatures (their shape has permanently changed). This distinction is commonly tested.

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Decomposition in Gardening and Agriculture

Humans exploit decomposition in several practical applications:

Composting

Composting is the process of collecting plant waste (grass clippings, vegetable peelings, leaves) in a heap or bin and allowing decomposers to break it down into a nutrient-rich material called compost or humus.

Condition	Why It Is Important for Composting
Warmth	Decomposers work faster at higher temperatures (up to their optimum)
Moisture	Decomposers need water for metabolic reactions
Oxygen (air)	Aerobic decomposition is faster than anaerobic decomposition
Turning the compost	Mixes in air (oxygen) and distributes decomposers throughout the heap
Shredding material	Increases surface area, speeding up decomposition

The compost produced is rich in mineral ions and can be added to soil to improve fertility and structure.

Biogas Production (Anaerobic Decomposition)

When decomposition occurs without oxygen (anaerobically), it produces biogas — a mixture of gases, mainly methane (CH4) and carbon dioxide (CO2).

Biogas can be produced in a sealed container called a biogas generator or anaerobic digester:

graph TD
    A["Organic waste<br/>(sewage, manure,<br/>crop waste)"] --> B["Anaerobic Digester<br/>(sealed container,<br/>no oxygen)"]
    B --> C["Biogas<br/>(methane + CO2)"]
    B --> D["Digestate<br/>(nutrient-rich fertiliser)"]
    C --> E["Burned as fuel<br/>(heating, electricity)"]
    style A fill:#d7ccc8,stroke:#5d4037
    style B fill:#546e7a,stroke:#37474f,color:#fff
    style C fill:#fff9c4,stroke:#f9a825
    style D fill:#c8e6c9,stroke:#2e7d32
    style E fill:#ffccbc,stroke:#d84315