Decomposition (biology only) — AQA GCSE Biology
Test yourself on Decomposition (biology only) with AQA GCSE practice questions.
7 days Premium · Then free forever · No card, no charge
Decomposition (biology only) explained
Decay is the breakdown of biological material by microorganisms such as bacteria and fungi.
Read the full explanation
Their enzymes digest the material, and the rate depends on conditions. Warmth increases enzyme activity and microbial growth, so decay is faster up to an optimum; very high temperatures denature enzymes and slow or stop decay. Moisture is needed because microorganisms and their enzymes work in solution, so damp conditions speed up decay while dry conditions slow it. Oxygen is needed for aerobic respiration by decomposers, so plenty of oxygen increases the rate; waterlogged or sealed conditions limit oxygen and slow aerobic decay. These principles explain why food is refrigerated, dried, salted or vacuum-packed to preserve it.
Students should be able to: • calculate rate changes in the decay of biological material • translate information between numerical and graphical form • plot and draw appropriate graphs selecting appropriate scales for the axes.
Decay is the breakdown of biological material by microorganisms such as bacteria and fungi. The rate of decay changes with temperature, moisture, oxygen availability and the type of material. To calculate a rate change, measure how a quantity such as mass, temperature or gas volume alters over a timed interval, then divide the change by the time taken. For example, if a piece of fruit loses 12 g in 4 days, the average rate is 12 g ÷ 4 days = 3 g per day. You must also move between numbers and graphs: read values from a plotted curve, use a table to plot points, and choose axes with equal, sensible scales so the pattern is clear. A well-drawn graph lets you compare rates by comparing gradients.
Gardeners and farmers try to provide optimum conditions for rapid decay of waste biological material.
Decay is carried out by microorganisms such as bacteria and fungi. These microorganisms need warmth, moisture and oxygen to respire and break down waste biological material rapidly. Gardeners and farmers create compost heaps or use compost bins to provide these optimum conditions. A warm, moist, well-aerated heap allows microorganisms to decompose waste quickly, producing compost. If conditions are too cold, too dry or anaerobic, decay slows down. Turning a compost heap mixes in oxygen and distributes heat and moisture, while adding water prevents drying out. The aim is to maximise the rate of decay so that useful compost is produced in a shorter time.
The compost produced is used as a natural fertiliser for growing garden plants or crops.
When microorganisms have broken down waste biological material, the remaining dark, crumbly material is compost. Compost is rich in mineral ions and other nutrients that plants need for healthy growth. Gardeners dig compost into soil or spread it around plants, and farmers may spread it on fields. The mineral ions released, such as nitrate ions, are absorbed by plant roots and used to make proteins and other compounds. Compost therefore acts as a natural fertiliser, improving soil structure and supplying nutrients without relying only on manufactured fertilisers. Using compost also recycles waste biological material, reducing the amount sent to landfill.
Anaerobic decay produces methane gas. Biogas generators can be used to produce methane gas as a fuel.
When microorganisms break down biological material without oxygen, the process is called anaerobic decay. Anaerobic decay produces methane gas, which is a useful fuel. Biogas generators are designed to carry out anaerobic decay on a large scale. Waste biological material, such as animal manure or crop waste, is placed in an airtight digester so that oxygen is excluded. Microorganisms decompose the waste anaerobically and release methane, which is collected and burned as a fuel for heating, cooking or generating electricity. The remaining material can be used as fertiliser. The rate of methane production depends on temperature and the type of waste.
Required practical activity 10: investigate the effect of temperature on the rate of decay of fresh milk by measuring pH change.
Decay is the breakdown of biological material by microorganisms such as bacteria and fungi. These microorganisms respire and release enzymes that digest the milk's lactose into acids, so the pH of fresh milk falls over time. In this practical you set up several samples of fresh milk, each kept at a different controlled temperature, and measure pH at regular intervals. A faster fall in pH indicates a faster rate of decay. You should use a pH meter or universal indicator solution and record readings at fixed times, for example every 24 hours. Plot pH against time for each temperature and compare the gradients. The temperature that produces the steepest fall in pH shows the fastest decay, usually around the optimum for the microorganisms involved.
AT skills covered by this practical activity: AT 1, 3, 4 and 5.
This practical develops four Apparatus and Techniques (AT) skills. AT 1 involves using appropriate apparatus to make and record measurements accurately, such as using a water bath for temperature and a pH meter or indicator. AT 3 covers using apparatus and techniques for the observation and measurement of biological material, like observing the decay of milk. AT 4 requires the safe and ethical use of living organisms to measure physiological functions, which here involves the bacteria naturally present in milk causing decay. AT 5 involves measuring rates of reaction by a variety of methods, specifically using a colour change of an indicator (like Cresol red or phenolphthalein) to measure the rate of decay as pH drops.
Your focus
- Explain how temperature affects the rate of decay and the role of enzyme denaturation.
- Describe how water and oxygen availability affect the rate of decomposition.
- Apply these factors to explain methods of food preservation.
Show all 21 objectives
- Calculate the rate of decay from measurements of change over time.
- Convert numerical data into a correctly plotted graph and read values from a graph.
- Choose suitable scales and axes when plotting biological data.
- State the conditions microorganisms need for rapid decay of waste biological material.
- Explain how gardeners and farmers provide optimum conditions in a compost heap.
- Relate changes in temperature, moisture or oxygen to changes in the rate of decay.
- Describe compost as the product of microbial decay of waste biological material.
- Explain how compost acts as a natural fertiliser by supplying mineral ions to plants.
- Give examples of how gardeners and farmers use compost for growing plants or crops.
- State that anaerobic decay of biological material produces methane gas.
- Explain how a biogas generator produces methane as a fuel.
- Describe uses of methane from biogas generators and the fate of the remaining material.
- Set up milk samples at different temperatures while controlling other variables.
- Measure and record pH at regular time intervals.
- Compare rates of pH change to identify the effect of temperature on decay.
- Select and use appropriate apparatus to accurately measure temperature, volume and pH (AT 1).
- Safely handle biological material and living organisms to observe physiological processes (AT 3, AT 4).
- Measure the rate of a biological reaction using the colour change of an indicator (AT 5).
Decomposition (biology only) exam tips
Marking Points
- Warm temperatures increase enzyme activity and microbial growth, so decay is faster up to an optimum temperature.
- Very high temperatures denature the enzymes of decomposers, so the rate of decay falls.
- Moist conditions allow microorganisms and their enzymes to work, so decay is faster in damp material than in dry material.
- Availability of oxygen supports aerobic respiration in decomposers, so more oxygen increases the rate of decay.
- Limited oxygen, for example in waterlogged or sealed conditions, slows aerobic decay and can favour anaerobic decomposers.
- Rate of decay is calculated by dividing the change in a measured quantity by the time taken, for example mass lost ÷ time or gas produced ÷ time.
- A steeper gradient on a mass-loss or gas-volume graph indicates a faster rate of decay, while a flatter gradient indicates a slower rate.
- Numerical data from a table can be translated into a graph by plotting time on the x-axis and the measured quantity on the y-axis.
- When plotting graphs, choose a scale that uses most of the available axis and gives evenly spaced major intervals, then label each axis with quantity and unit.
- Changes in temperature, moisture and oxygen availability alter the rate of decay because they affect the activity of decomposers.
- Identify warmth, moisture and oxygen as conditions needed by microorganisms for rapid decay.
- Explain that microorganisms respire and break down waste biological material.
- Describe how gardeners and farmers provide optimum conditions, for example by using a compost heap, adding water and turning the heap.
- Explain why decay is slower in cold, dry or anaerobic conditions.
- Link rapid decay to the faster production of compost for use on gardens or crops.
- State that compost is the product of the decay of waste biological material by microorganisms.
- Identify compost as a natural fertiliser that supplies mineral ions to plants.
- Explain that plants absorb mineral ions from the soil through their roots for healthy growth.
- Describe a use of compost, such as digging it into garden soil or spreading it on crop fields.
- Recognise that using compost recycles nutrients and reduces waste.
- State that anaerobic decay occurs without oxygen and produces methane gas.
- Describe biogas generators as airtight vessels in which waste biological material undergoes anaerobic decay.
- Explain that methane produced by biogas generators is collected and burned as a fuel.
- Give uses of methane as a fuel, such as heating, cooking or generating electricity.
- Recognise that the material remaining after anaerobic decay can be used as fertiliser.
- Compare anaerobic decay with aerobic decay, noting that aerobic decay does not produce methane as a fuel.
- State that microorganisms in the milk break down lactose and produce acid, causing pH to fall.
- Describe setting up several milk samples at different temperatures while keeping other variables such as volume of milk and starting pH the same.
- Measure pH at regular, stated time intervals using a pH meter or universal indicator and record results in a suitable table.
- Calculate or compare the rate of decay from the change in pH divided by the time taken, or from the gradient of a pH-time graph.
- Identify the temperature giving the fastest pH fall as the temperature with the fastest rate of decay.
- Explain that very high temperatures may kill or denature the microorganisms' enzymes, so decay can slow again.
- AT 1: Use appropriate apparatus to make and record a range of measurements accurately, including temperature using a water bath and thermometer, and volume using syringes.
- AT 3: Use appropriate apparatus and techniques for the observation and measurement of biological material, such as monitoring the decay process in milk.
- AT 4: Demonstrate safe and ethical use of living organisms to measure physiological functions, specifically the bacteria in milk that respire and produce lactic acid.
- AT 5: Measure rates of reaction by a variety of methods, such as timing the colour change of an indicator to calculate the rate of decay.
Examiner Tips
- 💡Use the word 'optimum' when describing temperature and explain what happens above it.
- 💡Link each factor to the microorganisms: enzymes, respiration or growth, rather than treating decay as a simple chemical reaction.
- 💡When explaining food preservation, name the factor that is controlled and state how it slows decay.
- 💡Always include units in rate calculations, such as g per day or cm³ per minute, and show the division clearly.
- 💡When reading a graph, use a ruler to line up the value on the axis with the curve, then read across to the other axis.
- 💡Check that the scale on each axis increases by equal steps and that the axes are labelled with both the quantity and its unit.
- 💡Name the microorganisms responsible for decay, such as bacteria and fungi, rather than referring only to germs.
- 💡Use the phrase optimum conditions and state each condition with its effect on the rate of decay.
- 💡Link each condition to a practical action, for example turning a compost heap to increase oxygen supply.
- 💡Use the term mineral ions rather than simply food when describing what plants take from compost.
- 💡Link compost to plant growth by naming a mineral ion, such as nitrate, and its use in the plant.
- 💡Mention recycling of waste biological material as an advantage of composting.
- 💡Use the term anaerobic precisely and state that oxygen is absent.
- 💡Name methane as the fuel gas and give at least one practical use, such as heating or electricity generation.
- 💡Link the design of the biogas generator, especially its airtight nature, to the need for anaerobic conditions.
- 💡Name the independent variable as temperature and the dependent variable as pH, and state at least two control variables.
- 💡When describing rate, use the change in pH divided by the time taken, and include the units.
- 💡Link your conclusion back to enzyme activity and microorganism respiration rather than simply saying 'it decayed faster'.
- 💡When asked about the method, explicitly mention the apparatus used (AT 1), such as using a thermostatically controlled water bath to maintain a constant temperature.
- 💡Be prepared to explain how the colour change of the indicator (AT 5) relates to the biological process of bacteria producing acidic products during decay.
Common Mistakes
- Saying higher temperature always increases decay; the correction is that decay increases up to an optimum, then enzymes denature and the rate falls.
- Thinking water is a reactant that is used up; the correction is that water provides the moist conditions needed for microbial enzymes to work.
- Ignoring oxygen and assuming decay is the same in all conditions; the correction is that oxygen availability strongly affects the rate of aerobic decomposition.
- Dividing time by the change in mass instead of dividing the change in mass by time; correct this by keeping the rate as change in quantity per unit time.
- Plotting points without checking the scale, so the line is cramped or runs off the grid; correct this by choosing a scale that fits the largest value and uses most of the axis.
- Joining points with straight lines when the pattern is a smooth curve, or drawing a line of best fit through scattered points without judging the trend; correct this by using a smooth curve or a straight line of best fit as appropriate.
- Thinking that decay is a purely chemical process with no living organisms involved; correct this by stating that microorganisms carry out the decay.
- Believing that sealing waste from air speeds up decay; correct this by explaining that oxygen is needed for rapid aerobic decay.
- Confusing optimum conditions for decay with conditions that prevent decay, such as freezing or drying; correct this by listing warmth, moisture and oxygen as the requirements.
- Stating that compost is a manufactured fertiliser; correct this by describing it as a natural fertiliser produced by decay.
- Thinking that plants absorb compost directly as food; correct this by explaining that microorganisms release mineral ions which plant roots absorb.
- Assuming compost only improves soil texture and supplies no nutrients; correct this by stating that it also provides mineral ions such as nitrates.
- Stating that methane is produced by aerobic decay; correct this by linking methane production specifically to anaerobic decay.
- Describing biogas generators as open containers; correct this by explaining that they must be airtight to exclude oxygen.
- Confusing methane with carbon dioxide as the fuel gas; correct this by naming methane as the combustible gas produced.
- Recording only a single pH reading at the end instead of taking repeated readings over time; correct this by measuring pH at regular intervals so a rate can be calculated.
- Changing several variables at once, such as using different milk volumes at each temperature; correct this by keeping all variables except temperature constant.
- Assuming the highest temperature always gives the fastest decay; correct this by explaining that enzymes and microorganisms have an optimum temperature and may be destroyed above it.
- Confusing AT skills with Working Scientifically (WS) skills; correct this by remembering AT skills refer to specific practical apparatus and techniques, not general data analysis.
- Forgetting that bacteria are living organisms in this practical (AT 4); correct this by recognising that the decay is a biological process carried out by microorganisms.
- Misidentifying the method of measuring the rate (AT 5); correct this by stating that the rate is measured by timing how long it takes for the indicator to change colour due to pH change.