Decomposition (biology only) — AQA GCSE Biology

SPEC 4.7.2.3 • PAPER 2

BIOLOGY ONLY (8461)

REQUIRED PRACTICAL 10

Decomposition

The three factors that control the rate of decay, the compost and biogas applications, and the RP10 milk experiment — pH, tangents, and the rate calculation that examiners reward.

The Big Idea

In lesson 7.06 you saw that microorganisms recycle carbon and mineral ions. This lesson explains how fast they do it — and why decay can be sped up or slowed down by changing three specific conditions.

Definition

Rate of decay = how fast biological material is broken down by decomposer microorganisms (bacteria and fungi). Their enzymes digest large molecules; the microorganisms then respire.

What The Spec Wants

You must be able to:

Explain how temperature, water and availability of oxygen affect rate of decay.

Calculate rate changes; translate data between numerical and graphical form; plot graphs.

Apply this to compost (rapid decay for fertiliser) and biogas (anaerobic decay for methane fuel).

Carry out RP10 — effect of temperature on rate of decay of fresh milk by pH change.

Biology Only

Section 4.7.2.3 is biology only — appears on GCSE Biology paper 8461/2 but not on Combined Science. RP10 is also biology-only.

The Three Factors

The spec names exactly three factors affecting the rate of decay. Memorise all three — they appear in mark schemes as named MS points.

FactorEffect On Rate Of DecayWhy

TemperatureIncreases up to an optimum, then falls sharply.Microbe enzymes have an optimum temperature; too high → enzymes denature .

WaterMore water (within reason) → faster decay.Decomposers need water for enzyme action , to dissolve substances, and to live/reproduce.

Availability of oxygenMore oxygen → faster aerobic decay.Most decomposers respire aerobically — they need O₂ to release energy from food.

Spec Wording

"Students should be able to explain how temperature, water and availability of oxygen affect the rate of decay of biological material."

Factor 1: Temperature

Three Zones — Memorise This Shape

Low temperature (fridge ~4 °C): low kinetic energy → enzymes work very slowly → microbes barely active → slow decay.

Optimum (~30–40 °C): enzymes work fastest, microbes reproduce rapidly → fastest decay.

High temperature (>50–60 °C): enzymes denature (active site changes shape) → microbes die → decay slows or stops. This is why cooking preserves food.

Factors 2 & 3: Water and Oxygen

Water

Decomposers need water to:

Live and reproduce — cells are mostly water.

Dissolve substances so they can be absorbed.

Allow enzyme activity — enzymes need an aqueous environment.

More water → faster decay (up to a point).

Dry conditions → almost no decay (why dried food keeps).

Availability of Oxygen

Most decomposers respire aerobically:

O₂ + glucose → CO₂ + H₂O + (energy)

Aerobic respiration releases more energy than anaerobic.

More O₂ available → faster microbial growth → faster decay.

Low O₂ (e.g. waterlogged soil, sealed containers) → anaerobic decay → slower, produces methane.

Linking The Factors

Notice: too much water can reduce oxygen (waterlogged soil has little air). So the optimum for compost is damp but not wet. The three factors interact.

Application 1: Compost

Gardeners and farmers maximise rate of decay to produce compost — a natural fertiliser made from decomposed waste biological material.

Spec Statement

"Gardeners and farmers try to provide optimum conditions for rapid decay of waste biological material. The compost produced is used as a natural fertiliser for growing garden plants or crops."

Optimum Compost Conditions

FactorWhat gardeners doWhyWarmInsulated bin / large heap that retains heat from microbial respirationHigher T → enzymes work fasterDampWater added if dry, but not waterloggedWater needed for enzyme action and microbial lifeOxygen-richTurn the heap; mix in twiggy material for air spacesMaintains aerobic respiration → faster decay

Why Compost = Fertiliser

Decomposition releases mineral ions (e.g. nitrates) into the compost. When added to soil, plants absorb these ions through their roots — synoptic link to lesson 7.06 (the role of microorganisms).

Application 2: Anaerobic Decay → Biogas

Without oxygen, decay still happens — but anaerobically. It's slower and produces a different product: methane gas (CH₄).

Spec Statement

"Anaerobic decay produces methane gas. Biogas generators can be used to produce methane gas as a fuel."

Don't Confuse

Compost = aerobic decay (with oxygen), produces fertiliser. Biogas = anaerobic decay (no oxygen), produces methane fuel. Different conditions → different products.

RP10 Part 1: The Method

The question: How does temperature affect the rate of decay of fresh milk, measured by pH change?

Why pH Falls

Bacteria in the milk break down lactose (the milk sugar) into lactic acid. Lactic acid lowers the pH. So faster decay = faster pH drop.

Milk also contains lipids (fats). Microbial enzymes break these down into fatty acids, which also contribute to lowering the pH.

RP10 Part 2: Variables & Reliability

Independent Variable (IV)

Temperature — set using water baths (e.g. 5, 20, 35, 50 °C).

Dependent Variable (DV)

pH — measured with a pH probe / pH meter (or universal indicator solution), at fixed intervals over several days.

Control Variables (Mark-Scheme Verified)

Volume of milk in each tube.

Type of milk (same source, same processing — e.g. all whole milk from one batch).

Freshness / age of the milk at the start (same batch, opened the same day).

Exposure to air / oxygen — e.g. same bungs on each tube, or all sealed.

Same time of day for each pH measurement.

Starting pH of the milk.

Mark Scheme Trap

"Same pH probe / meter" is NOT credited as a control variable — mark schemes explicitly ignore it. Likewise, "pH paper" unqualified is not accepted as a measurement method — say "pH probe", "pH meter", or "universal indicator".

Reliability & AT Skills

At least 3 repeats at each temperature → calculate the mean.

AT skills tested: AT 1 (apparatus), AT 3 (heating safely), AT 4 (accurate measurement), AT 5 (measuring pH).

Calculating Rate From A pH Graph

Examiners love testing the tangent method for finding rate at a specific time point.

The Method (4 marks typical)

Draw a tangent to the curve at the time point asked (a straight line just touching the curve at one point).

Read off two pH values from the tangent at clearly-marked time values.

Calculate ΔpH ÷ Δtime using those values.

State the answer with units (pH units per day).

Worked Example (from the diagram)

Tangent gives ΔpH = 1.1 over Δt = 1.3 days. Rate ≈ 1.1 ÷ 1.3 = 0.85 pH units per day. Always show the working — mark schemes typically award marks for the tangent, the two values, and the division.

Common Errors

Wrong

"Cold kills microorganisms in the fridge."

Right

"Cold slows microorganisms — enzymes work slower; microbes reproduce slower. Most are not killed."

Wrong

"Enzymes melt at high temperature."

Right

"Enzymes denature — the active site changes shape so the substrate no longer fits."

Wrong

"Decay needs sunlight."

Right

"Decay needs temperature, water, and oxygen. Sunlight is not a named factor."

Wrong

"Anaerobic decay produces gas."

Right

"Anaerobic decay produces methane (CH₄) gas — use the specific name."

Wrong

"pH falls because milk goes off."

Right

"Bacteria break down lactose into lactic acid, which lowers the pH."

Wrong

"Viruses cause the milk / food to decay."

Right

Bacteria and fungi (microorganisms) cause decay. Viruses are NOT decomposers — mark schemes explicitly reject them.

Examiner Alerts

Alert 1 — Name All Three Factors

When asked "what affects rate of decay?", name temperature, water and availability of oxygen. Each factor is its own MS point. Don't merge them or substitute generic terms like "weather".

Alert 2 — "Denature" Not "Kill" Or "Melt"

At high temperatures, enzymes denature — active site shape changes. Don't say enzymes "die" (they're not alive) or "melt". Mark schemes are strict on this term.

Alert 3 — Show The Tangent

For rate-from-graph questions, draw the tangent on the graph and label the values you read from it. Just writing "rate ≈ 0.5" without showing the tangent loses method marks even if the final number is right.

Alert 4 — Name The Right Instrument

For "how could pH be measured?" — accepted answers: pH probe, pH meter, or universal indicator (paper or solution). "pH paper" unqualified is NOT credited, and neither are litmus, phenolphthalein, or "data logger" alone.

Bonus: The Linked-Factor Trap

"Why does food keep longer in a sealed plastic bag?" — students often say "no oxygen". Better: low oxygen reduces aerobic decay; anaerobic decay still occurs but is slower. Specificity earns the mark.

Practice — Variables

A student investigates how the amount of water affects the rate of decay of grass clippings. They add different volumes of water to identical jars of grass and measure mass loss after 5 days.

13.1 Give three control variables the student must keep the same. 3 marks

13.2 Explain why decay is faster when more water is added (up to a point). 2 marks

13.1

Any three of: same mass of grass clippings; same type of grass (or same source); same temperature; same oxygen availability/jar lid condition; same size/particle size of clippings; same time/duration; same starting amount of microorganisms. (1 mark each)

13.2

Water is needed for enzyme activity / for decomposers to live and reproduce (1). Without enough water, the microorganisms cannot break down the grass (1). Optional cap: too much water reduces oxygen, slowing aerobic decay.

Practice — Calculate Rate

The graph below shows pH against time for milk decaying at 25 °C. Use a tangent drawn at day 3 to estimate the rate of pH change in pH units per day.

4 marks

1

Draw a tangent just touching the curve at day 3. (Mark 1)

2

Read two pH values from the tangent — e.g. at day 2 read pH ≈ 5.8; at day 4 read pH ≈ 4.2. (Mark 2)

3

Calculate ΔpH = 5.8 − 4.2 = 1.6; Δt = 4 − 2 = 2 days. (Mark 3)

4

Rate = ΔpH ÷ Δt = 1.6 ÷ 2 = 0.8 pH units per day (mark schemes typically accept a range — e.g. 0.6 to 1.0 — provided the tangent and working are shown). (Mark 4)

Tip: examiners accept values within a tolerance, provided the tangent is drawn and the values used are clear. Marks come from method, not just the final number.

Practice — 6-Marker

A farmer notices that their compost heap decomposes much faster in summer than in winter. The heap is well-aerated and the farmer adds water if it gets dry.

Explain why decomposition is faster in summer, referring to all three factors from the spec and the role of decomposer microorganisms.

6 marks (Level marked)

Level 3 (5–6): three factors linked to microorganism activity, with mechanism explained.

A

Decomposition is carried out by microorganisms (bacteria and fungi). They release enzymes to break down the waste, and they respire.

B

Temperature — in summer the heap is warmer. Higher temperature → more kinetic energy → enzymes work faster → microbes reproduce faster → faster decay.

C

Water — the farmer keeps the heap damp. Water is needed for enzyme activity and for microorganisms to live and reproduce.

D

Oxygen — the heap is well-aerated, so the decomposers can respire aerobically, which releases more energy and supports faster growth → faster decay.

E

Together, all three factors are near their optimum in summer, so decomposers are at their most active and decay is fastest.

Tip: a Level 3 answer needs all three factors LINKED to enzyme activity / microbial respiration. Listing the factors without mechanism caps at Level 2.

Recap & Next Up

3

Factors affecting decay

RP10

Milk + pH method

2

Applications: compost & biogas

Three Things To Take Away

1

The three factors — temperature, water, oxygen — all affect microorganism activity. Optimum conditions → fastest decay.

2

Applications — compost (aerobic, fast decay, natural fertiliser) vs biogas (anaerobic, methane fuel). Different conditions, different products.

3

RP10 — pH falls because bacteria turn lactose into lactic acid. Use the tangent method to calculate rate from a pH-time graph.

Next: Lesson 7.08

Impact of environmental change (biology only, HT only) — how changes in temperature, water and gas composition affect the distribution of species.