Pearson Edexcel International GCSE in Biology · 4BI1 · Free · no account needed
Biology required practicals
All 14 required practicals, in 16 procedures, for Pearson Edexcel International GCSE Biology (4BI1) on one page — the method step by step, the independent, dependent and control variables with the reason each one is held constant, and the sources of error classified as systematic, random or judgement. Each is taught inside its lesson, linked below.
The specification lists them as practical investigations; Pearson's UK GCSE calls the equivalent set core practicals, and students call them required practicals — this is that list for the International GCSE. They are examined in writing: "state a control variable and explain why it must be kept constant", "suggest an improvement", "explain why the result is anomalous". Every mark in those questions is a reason, not a step, which is why each practical here carries the reason beside each variable and the fix beside each error. Learn the method here; then open the lesson, understand the science the practical is testing, prove it in five marked questions, teach it back to an AI pupil and defend it in a short debate — the four things a method sheet cannot ask of you. Completely free.
The 14 practicals, 16 procedures
- food tests — Topic 2
- the effect of TEMPERATURE on enzyme activity — 2.12
- the effect of pH on enzyme activity (Paper 2 only) — 2.14
- the NON-LIVING system: diffusion into agar cubes — 2.17
- the LIVING system: osmosis in potato cylinders — 2.17
- measuring the rate of photosynthesis with pondweed — 2.23
- proving what photosynthesis requires, using the starch test — 2.23
- the energy content of a food sample — 2.33
- carbon dioxide and heat from respiring seeds — 2.39
- investigating breathing and the effect of exercise — 2.50
- measuring transpiration with a potometer — 2.58
- light and net gas exchange, using hydrogencarbonate indicator — 2.45
- the conditions needed for seed germination — 3.5
- estimating population size in two areas using quadrats — 4.2
- investigating distribution and biodiversity — 4.4
- investigating anaerobic respiration by yeast — 5.6
Topic 2 · Structures and functions in living organisms
1. Required practical: food tests
Specification statement 2.9 · Topic 2 · taught in Carbohydrates, Proteins and Lipids
Method
- Glucose (a reducing sugar) — add Benedict's solution and heat in a water bath. Blue → brick red / orange if present.
- Starch — add iodine solution. Orange-brown → blue-black if present.
- Protein — add Biuret solution (sodium hydroxide then copper sulfate). Blue → purple/lilac if present.
- Fat — the emulsion test: dissolve in ethanol, then add water. A cloudy white emulsion forms if present.
Variables
Independent (changed) — which food sample is being tested (and which reagent is used on it)
Dependent (measured) — the final colour — a qualitative result, present or absent, not a measurement
| Control variable | Why it must be held constant |
|---|---|
| Volume of food sample | more sample gives a stronger colour, so two foods could not be compared |
| Volume of reagent | too little reagent gives a weak or absent colour even when the food is present |
| Heating time and water-bath temperature for Benedict's | an unheated or briefly heated tube reads negative whatever is in it |
| Grinding and filtering the sample the same way | an unfiltered suspension is cloudy already, which hides the emulsion test result |
Sources of error
| Type | What goes wrong | What to do |
|---|---|---|
| Systematic | Benedict's solution not heated, or not heated long enough. The tube reads negative even when reducing sugar is present, and it does so every time. | Heat every tube in an 80 °C water bath for a fixed five minutes. |
| Judgement | Deciding whether a colour counts as 'brick red' or 'orange', or whether a lilac tinge is really purple. | Compare against a printed colour standard, or use a colorimeter. One person judges every tube in the same light. |
| Systematic | A cloudy or coloured food sample makes the emulsion test unreadable, since a positive result IS cloudiness. | Filter the sample first, and run an ethanol-and-water control with no food in it. |
| Random | Benedict's gives a graded colour — green, yellow, orange, brick red — with the amount of sugar, so a weak positive is easily called negative. | Note that the test is SEMI-quantitative and record the colour reached rather than only 'positive' or 'negative'. |
Understand the science behind it → · Prove it · Teach it · Defend it
2. Required practical 2.12 — the effect of TEMPERATURE on enzyme activity
Specification statement 2.12 · Topic 2 · taught in Enzymes: Catalysts With a Shape
Method
- Place starch solution and amylase in separate tubes in the water bath for five minutes before mixing, so both reach the set temperature.
- Mix, start the stopwatch, and return the tube to the bath.
- Every 10 s, remove a drop and add it to iodine solution on a spotting tile.
- The end point is the first drop that stays orange-brown: all the starch has been digested.
- Repeat at each temperature across the range, and calculate rate = 1 ÷ time.
- Control: a tube of BOILED amylase should show no colour change at all, proving the starch is broken down by the enzyme and not by the heat or the water.
Variables
Independent (changed) — temperature of the water bath
Dependent (measured) — time for the starch to disappear, converted to rate = 1 ÷ time
| Control variable | Why it must be held constant |
|---|---|
| pH — use a buffer | pH independently changes the rate, so letting it vary would make the result invalid |
| Enzyme concentration and volume | more enzyme means more active sites, so a faster rate at every temperature |
| Starch concentration and volume | more starch takes longer to digest whatever the temperature |
| Sampling interval | a longer interval adds uncertainty to every reading equally, but changing it mid-run makes readings incomparable |
| Five minutes in the bath before mixing | if the solutions have not reached the set temperature, the recorded temperature is not the one the reaction ran at |
Sources of error
| Type | What goes wrong | What to do |
|---|---|---|
| Judgement | The end point — the first drop that stays orange-brown — is a matter of opinion. | One person judges every tube, against a white tile in the same light. Repeat and take a mean. |
| Systematic | Sampling only every 10 s means the true end point can be up to 10 s before the one recorded, and always in that direction. | Sample every 5 s, or use a colorimeter and data logger. |
| Random | Water baths drift by a degree or two during a run. | Monitor with a thermometer throughout and quote the mean temperature rather than the dial setting. |
| Systematic | Without the boiled-amylase control, starch breaking down for some other reason would be read as enzyme activity. | Run the boiled tube alongside. If it shows no colour change, the digestion in the others is the enzyme's. |
Understand the science behind it → · Prove it · Teach it · Defend it
3. Required practical 2.14 — the effect of pH on enzyme activity (Paper 2 only)
Specification statement 2.14 · Topic 2 · taught in Enzymes and pH
Method
- Use the starch–amylase method from the temperature practical, but hold the temperature constant in a water bath at 37 °C.
- Add a measured volume of buffer solution to set the pH of each tube: pH 3, 5, 7, 9 and 11.
- The buffer holds the pH steady even though the reaction itself would otherwise shift it slightly.
- Mix, start the stopwatch, and sample a drop onto iodine solution on a spotting tile every 10 s.
- The end point is the first drop that stays orange-brown — all the starch has been digested.
- Calculate rate = 1 ÷ time and plot rate against pH. The optimum is the pH directly beneath the peak.
Variables
Independent (changed) — pH, set by the buffer solution
Dependent (measured) — time for the starch to disappear, converted to rate = 1 ÷ time
| Control variable | Why it must be held constant |
|---|---|
| Temperature | temperature independently changes the rate, so letting it vary would make the result invalid |
| Volume of buffer added | the buffer adds volume, so different volumes dilute the reactants by different amounts |
| Enzyme concentration and volume | more enzyme means more active sites, so a faster rate at every pH |
| Starch concentration and volume | more starch takes longer to digest whatever the pH |
| Sampling interval | changing it mid-run makes the readings incomparable with one another |
Sources of error
| Type | What goes wrong | What to do |
|---|---|---|
| Systematic | Adding different volumes of buffer dilutes the reactants by different amounts, and the resulting change in rate is read as a pH effect. | Add the same total volume of buffer to every tube. |
| Systematic | Without a buffer the pH drifts during the run, so the pH recorded is not the pH the reaction actually happened at. | Always use a buffer, and check the pH of each tube at the end as well as the start. |
| Judgement | The end point is decided by eye against a spotting tile. | One person judges every tube in the same light. Repeat and take a mean. |
| Random | Five pH values give a coarse curve, so the peak may fall between two measured points and the optimum be misread. | Once the approximate peak is known, repeat with narrower intervals around it — pH 6, 6.5, 7, 7.5, 8. |
Understand the science behind it → · Prove it · Teach it · Defend it
4. Required practical 2.17 (a) — the NON-LIVING system: diffusion into agar cubes
Specification statement 2.17 · Topic 2 · taught in Diffusion, Osmosis and Active Transport
Method
- Cut cubes of agar containing dilute sodium hydroxide and universal indicator (purple) with sides of 1 cm, 2 cm and 3 cm.
- Drop all three into dilute hydrochloric acid at the same moment, each fully covered.
- Acid diffuses in and neutralises the alkali, turning the agar yellow from the outside inwards.
- Time how long each cube takes to turn yellow all the way to the centre.
- The smallest cube changes completely first.
Variables
Independent (changed) — cube size — and so the surface area to volume ratio
Dependent (measured) — time to turn yellow all the way to the centre
| Control variable | Why it must be held constant |
|---|---|
| Acid concentration | a stronger acid diffuses in faster |
| Volume of acid | too little is used up and the gradient falls |
| Temperature | warmer particles diffuse faster |
| Shape — cubes only | a slab of equal volume has a different ratio |
Sources of error
| Type | What goes wrong | What to do |
|---|---|---|
| Judgement | The moment the last purple disappears is a matter of opinion. | One person judges every cube; repeat and take a mean. |
| Random | Cubes cut by hand are not exactly the size intended. | Measure each cube and reject any that is out. |
| Systematic | Cubes added one at a time are in the acid for different lengths of time. | Add all three at the same moment and start one clock. |
Understand the science behind it → · Prove it · Teach it · Defend it
5. Required practical 2.17 (b) — the LIVING system: osmosis in potato cylinders
Specification statement 2.17 · Topic 2 · taught in Diffusion, Osmosis and Active Transport
Method
- Cut cylinders with the same cork borer, trim to equal length, and remove the waterproof skin.
- Blot each cylinder the same number of times and record its initial mass to 0.01 g.
- Place one cylinder in each sucrose concentration, all for the same length of time.
- Remove, blot identically, and reweigh.
- Percentage change in mass = (final mass − initial mass) ÷ initial mass × 100.
- Plot percentage change against sucrose concentration and draw a line of best fit.
Variables
Independent (changed) — sucrose concentration
Dependent (measured) — percentage change in mass
| Control variable | Why it must be held constant |
|---|---|
| Time in the solution | longer means more water moves |
| Temperature | warmer speeds up osmosis |
| Volume of solution | too little is diluted by water leaving the potato, so the gradient falls mid-run |
| Cylinder dimensions | surface area changes how fast water crosses |
| Same potato tuber | different potatoes have different cell sap concentrations |
Sources of error
| Type | What goes wrong | What to do |
|---|---|---|
| Systematic | Inconsistent blotting leaves surface water on, adding mass every time in the same direction. | Blot every cylinder identically. Repeating cannot remove this one. |
| Random | Balance readings and small differences between cylinders scatter the points. | Three repeats and a mean at each concentration. |
| Systematic | Cylinders are not identical to start with, so grams gained is not comparable between them. | Plot percentage change, not change in grams. |
| Judgement | One point sits well off the line of best fit. | Name it as anomalous, leave it out of the line, and suggest a reason. |
Understand the science behind it → · Prove it · Teach it · Defend it
6. Required practical 2.23 (a) — measuring the rate of photosynthesis with pondweed
Specification statement 2.23 · Topic 2 · taught in What Limits Photosynthesis
Method
- Place a piece of pondweed, cut end upwards, in a boiling tube of sodium hydrogencarbonate solution, which supplies carbon dioxide.
- Stand the tube in a large beaker of water. This water bath absorbs the heat from the lamp, so light intensity can be changed without changing the temperature.
- Set the lamp at a measured distance and leave the plant for five minutes to settle to a steady rate before counting anything.
- Count the bubbles released in one minute. Repeat three times at that distance and take a mean.
- Repeat across a range of lamp distances, then plot mean bubbles per minute against distance — or against 1 ÷ distance², which is proportional to light intensity.
Variables
Independent (changed) — light intensity, changed by moving the lamp to measured distances
Dependent (measured) — number of oxygen bubbles released per minute
| Control variable | Why it must be held constant |
|---|---|
| Temperature | photosynthesis is enzyme-controlled, so warming changes the rate |
| Carbon dioxide concentration | otherwise it becomes the limiting factor instead of light |
| The same piece of pondweed | different pieces have different numbers of chloroplasts |
| Counting time | a longer count gives more bubbles whatever the rate |
Sources of error
| Type | What goes wrong | What to do |
|---|---|---|
| Judgement | Bubbles of different sizes are counted as though equal. | Collect the gas in a capillary tube and measure its volume instead. |
| Systematic | Room lighting reaches the plant as well as the lamp. | Work in a darkened room so the lamp is the only source. |
| Random | Bubbles are released unevenly, so a single count is unrepresentative. | Take three counts at each distance and use the mean. |
Understand the science behind it → · Prove it · Teach it · Defend it
7. Required practical 2.23 (b) — proving what photosynthesis requires, using the starch test
Specification statement 2.23 · Topic 2 · taught in What Limits Photosynthesis
Method
- Destarch the plant first by leaving it in darkness for 48 hours, so that any starch found afterwards must have been made during the experiment.
- Set up the one condition being tested: foil over part of a leaf removes LIGHT; a variegated leaf tests for CHLOROPHYLL; sealing a leaf in a flask with soda lime removes CARBON DIOXIDE.
- Leave the plant in bright light for several hours.
- Test the leaf for starch: boil in water to kill it, then boil in ethanol in a water bath with the flame off, as ethanol is highly flammable, to remove the chlorophyll.
- Rinse in warm water to soften the brittle leaf, spread it out and add iodine solution.
- Blue-black means starch is present and photosynthesis has happened; orange-brown means it has not.
Variables
Independent (changed) — the single condition removed — light, chlorophyll or carbon dioxide
Dependent (measured) — whether the leaf turns blue-black with iodine solution
| Control variable | Why it must be held constant |
|---|---|
| Destarching time | starch left over from before would give a false positive |
| Time in the light | too short and even a working leaf makes no detectable starch |
| The same plant | different plants photosynthesise at different rates |
| Temperature | it changes the rate at which starch is made |
Sources of error
| Type | What goes wrong | What to do |
|---|---|---|
| Systematic | The plant was not fully destarched, so every leaf tests positive. | Test one leaf before starting, to confirm destarching worked. |
| Judgement | Faint blue-black and dark orange-brown are hard to tell apart. | Rinse thoroughly and compare against a known positive leaf. |
| Random | The foil slips and lets light onto the covered area. | Fix the foil on both surfaces and check it before testing. |
Understand the science behind it → · Prove it · Teach it · Defend it
8. Required practical 2.33 — the energy content of a food sample
Specification statement 2.33 · Topic 2 · taught in Absorption and the Energy in Food
Method
- Measure a fixed volume of water into a boiling tube — 25 cm³ has a mass of 25 g — and clamp the tube.
- Record the mass of the food sample and the starting temperature of the water.
- Set light to the food and immediately hold it under the boiling tube, relighting it if it goes out before it is fully burnt.
- Stir the water and record the highest temperature reached.
- Energy transferred = mass of water × 4.2 × temperature rise. Divide by the mass of food to get the energy per gram.
Variables
Independent (changed) — the type of food burnt
Dependent (measured) — temperature rise of the water, used to calculate energy per gram
| Control variable | Why it must be held constant |
|---|---|
| Volume of water | more water rises less for the same energy |
| Distance from flame to tube | further away means more heat lost to the air |
| Starting temperature | a hotter start loses heat faster to the room |
| Same thermometer | two thermometers may read slightly differently |
Sources of error
| Type | What goes wrong | What to do |
|---|---|---|
| Systematic | Much of the heat goes into the air, the glass and the clamp. | Shield the apparatus with a draught screen and insulate the tube. |
| Systematic | The food does not burn completely, so not all its energy is released. | Relight until the sample will no longer catch at all. |
| Judgement | Deciding the highest temperature as the reading drifts. | Stir continuously and record the maximum steady value. |
| Random | Food samples differ from one piece to the next. | Repeat with several samples of each food and take a mean. |
Understand the science behind it → · Prove it · Teach it · Defend it
9. Required practical 2.39 — carbon dioxide and heat from respiring seeds
Specification statement 2.39 · Topic 2 · taught in Anaerobic Respiration
Method
- Rinse two batches of seeds in disinfectant, so microbes on their surfaces cannot respire and affect the results.
- Boil one batch and allow it to cool. These dead seeds are the control.
- Place each batch in its own vacuum flask with a thermometer, and plug the neck with cotton wool so air can enter but heat is not easily lost.
- Record the temperature in each flask at the same time each day for several days.
- For carbon dioxide, draw air slowly through each flask and bubble it through limewater; limewater turning milky shows carbon dioxide is being released.
Variables
Independent (changed) — whether the seeds are alive or have been boiled
Dependent (measured) — temperature rise, and whether the limewater turns milky
| Control variable | Why it must be held constant |
|---|---|
| Mass of seeds | more seeds respire more and release more heat |
| Type of flask | different insulation loses heat at different rates |
| Starting temperature | a warmer flask loses heat to the room faster |
| Disinfectant treatment | surface microbes would respire and add heat themselves |
Sources of error
| Type | What goes wrong | What to do |
|---|---|---|
| Systematic | Heat escapes through the neck of the flask. | Plug the neck with cotton wool and insulate the flask. |
| Systematic | Microbes on the seed surfaces respire and release heat too. | Rinse every batch in disinfectant before setting up. |
| Judgement | Deciding when limewater has turned milky enough. | Compare against an untouched tube of limewater. |
| Random | Thermometer readings vary slightly from day to day. | Read at eye level at the same time and repeat. |
Understand the science behind it → · Prove it · Teach it · Defend it
10. Required practical 2.50 — investigating breathing and the effect of exercise
Specification statement 2.50 · Topic 2 · taught in Alveoli, Exercise and Smoking
Method
- Measure resting breathing rate by counting breaths for one minute while sitting still, and repeat three times to obtain a mean.
- Exercise for a fixed time — for example three minutes of step-ups at a set rate — then immediately count breaths for one minute again.
- Continue counting each minute afterwards until the rate returns to the resting value, and record the recovery time.
- To show carbon dioxide is released, breathe out through a delivery tube into limewater; it turns milky, while limewater that room air is drawn through stays clear.
- Plot breathing rate against time, marking the exercise period, and compare rest, exercise and recovery.
Variables
Independent (changed) — whether the person is at rest or has exercised, and the time since exercise
Dependent (measured) — breathing rate in breaths per minute
| Control variable | Why it must be held constant |
|---|---|
| Type of exercise | different activities demand different amounts of energy |
| Duration of exercise | a longer effort produces a larger oxygen debt |
| The same person | fitness and lung capacity differ between individuals |
| Counting period | a longer count records more breaths regardless of rate |
Sources of error
| Type | What goes wrong | What to do |
|---|---|---|
| Judgement | Deciding where one breath ends and the next begins. | Count only the rises of the chest, and use one counter throughout. |
| Systematic | Being aware of the count makes a person breathe differently. | Have someone else count, without telling the subject when. |
| Random | Breathing rate varies naturally from minute to minute. | Take three counts at rest and use the mean. |
Understand the science behind it → · Prove it · Teach it · Defend it
11. Required practical 2.58 — measuring transpiration with a potometer
Specification statement 2.58 · Topic 2 · taught in Water Uptake and Transpiration
Method
- Cut the leafy shoot under water and assemble the potometer under water too, so no air bubble enters the xylem and breaks the column.
- Dry the leaves, check the apparatus is airtight, and introduce a single air bubble into the capillary tube.
- Allow the shoot to equilibrate for several minutes, then record the distance the bubble moves in a fixed time.
- Reset the bubble using the reservoir tap and repeat three times, taking a mean.
- Change one condition — a fan for wind, a lamp for light, a plastic bag for humidity — and repeat.
- Volume of water taken up = πr² × distance moved, where r is the radius of the capillary tube.
Variables
Independent (changed) — the environmental factor being changed — wind speed, light intensity, humidity or temperature
Dependent (measured) — distance the bubble moves in a fixed time, converted to a volume
| Control variable | Why it must be held constant |
|---|---|
| The same leafy shoot | different shoots have different numbers of stomata |
| Leaf surface area | more leaf area transpires more water |
| Time measured | a longer run moves the bubble further whatever the rate |
| The other three factors | each of them changes the rate on its own |
Sources of error
| Type | What goes wrong | What to do |
|---|---|---|
| Systematic | A potometer measures water UPTAKE, not water lost. | State the assumption that uptake is approximately equal to transpiration. |
| Systematic | An air leak at a joint lets the bubble move without transpiration. | Seal every joint and assemble the apparatus under water. |
| Random | The bubble moves unevenly between repeats. | Take three readings at each condition and use the mean. |
| Judgement | Deciding exactly where the bubble's edge sits on the scale. | Read the same edge each time, at eye level. |
Understand the science behind it → · Prove it · Teach it · Defend it
12. Required practical 2.45 — light and net gas exchange, using hydrogencarbonate indicator
Specification statement 2.45 · Topic 2 · taught in Gas Exchange in Plants, Day and Night
Method
- Hydrogencarbonate indicator is RED when in equilibrium with ordinary air. More carbon dioxide turns it YELLOW; less turns it PURPLE.
- Put an equal volume of indicator into four boiling tubes and equilibrate each with the air by shaking.
- Place a leaf of the same species and similar size in three of the tubes, supported clear of the liquid, and seal all four with bungs.
- Tube A: in bright light. Tube B: wrapped in aluminium foil, in darkness. Tube C: wrapped in one layer of gauze, in dim light. Tube D: no leaf — the control.
- Leave all four for the same length of time, typically one to two hours, at the same temperature.
- Record the final colour of each: A turns purple, B turns yellow, C stays red, and D stays red.
Variables
Independent (changed) — light intensity reaching the leaf
Dependent (measured) — colour of the hydrogencarbonate indicator — that is, the change in carbon dioxide concentration
| Control variable | Why it must be held constant |
|---|---|
| Species, age and size of leaf | a bigger or younger leaf has a different rate per tube, so the colours would not be comparable |
| Temperature | both respiration and photosynthesis are enzyme-controlled, so warming a tube changes both rates |
| Volume of indicator | the same amount of carbon dioxide changes a small volume more than a large one |
| Time left before reading | the colour goes on changing, so a longer run gives a further shift |
Sources of error
| Type | What goes wrong | What to do |
|---|---|---|
| Systematic | The lamp warms the bright-light tube, raising both rates in that tube only. | Put a glass tank of water between lamp and tube as a heat filter, or use an LED lamp. |
| Judgement | Deciding where red ends and purple begins is a matter of opinion. | Compare against a printed colour standard, or measure with a colorimeter. |
| Random | Leaves differ in area and age, so the rate per tube varies. | Use leaves of one species and similar size, and repeat with several sets. |
| Systematic | Breathing near an open tube adds carbon dioxide and shifts it towards yellow. | Seal the tubes promptly and handle them by the rim, away from the face. |
Understand the science behind it → · Prove it · Teach it · Defend it
Topic 3 · Reproduction and inheritance
13. Required practical 3.5 — the conditions needed for seed germination
Specification statement 3.5 · Topic 3 · taught in Seeds and Germination
Method
- Set up four boiling tubes or dishes, each containing 10 cress or mung bean seeds on cotton wool.
- Tube A — the full set of conditions: damp cotton wool, air, at room temperature (about 20 °C).
- Tube B — no water: dry cotton wool, air, room temperature.
- Tube C — no oxygen: damp cotton wool covered with a layer of boiled, cooled water and a film of oil to seal out air, room temperature.
- Tube D — low temperature: damp cotton wool, air, in a refrigerator at about 4 °C.
- Leave for one week and count how many seeds in each tube have germinated.
- Result: only tube A shows substantial germination. Each of B, C and D lacks one condition and shows little or none.
Variables
Independent (changed) — which single condition is removed — water, oxygen, or a suitable temperature
Dependent (measured) — number of seeds that germinate (or the percentage germination)
| Control variable | Why it must be held constant |
|---|---|
| Number and species of seeds | different species and different numbers give different germination rates, so the counts would not be comparable |
| Age and source of the seeds | older seeds germinate less reliably, which would look like an effect of the condition being tested |
| Time left before counting | germination continues, so a tube left longer shows a higher count regardless of conditions |
| Everything except the one condition removed | if two conditions differ between tubes, no result can be attributed to either |
Sources of error
| Type | What goes wrong | What to do |
|---|---|---|
| Systematic | The refrigerated tube is both colder and darker than the others. | Put the room-temperature control in the dark too, so temperature is the only difference. |
| Judgement | Deciding whether a seed has 'germinated' is a matter of opinion. | Set one criterion in advance — a visible root over 2 mm — and apply it everywhere. |
| Random | Some seeds are dead or infertile before the experiment begins. | Use at least 10 seeds per tube, repeat the whole experiment, and work in percentages. |
| Systematic | Ordinary water contains dissolved oxygen, so the 'no oxygen' tube is not truly anaerobic. | Boil the water first to drive out dissolved air, cool it, and seal the surface with a layer of oil. |
Understand the science behind it → · Prove it · Teach it · Defend it
Topic 4 · Ecology and the environment
14. Required practical 4.2 — estimating population size in two areas using quadrats
Specification statement 4.2 · Topic 4 · taught in Populations, Ecosystems and Sampling with Quadrats
Method
- Lay two tape measures at right angles along adjacent edges of the first area, to act as the axes of a grid.
- Generate two random numbers and use them as coordinates to place a quadrat.
- Count the number of individuals of the chosen species inside the quadrat, using a fixed rule for plants lying on a boundary — for example, count those overlapping the top and left edges only.
- Repeat with new random coordinates for at least ten quadrats.
- Calculate the mean number per quadrat, then the mean per square metre by dividing by the quadrat's area.
- Multiply by the total area of the habitat to estimate the population size.
- Repeat the whole procedure in the second area, and compare.
Variables
Independent (changed) — the area being sampled — for example mown lawn against unmown meadow
Dependent (measured) — number of individuals of the chosen species per quadrat
| Control variable | Why it must be held constant |
|---|---|
| Size of quadrat | a larger quadrat catches more individuals, so counts from different sizes cannot be compared |
| Number of quadrats | a mean from more samples is more reliable, so unequal numbers make the two areas unequally reliable |
| Time and day of sampling | growth, weather and animal activity change with time, so sampling on different days introduces a second variable |
| The rule for boundary individuals | counting plants on the edge in one area and not the other shifts every count in one direction |
Sources of error
| Type | What goes wrong | What to do |
|---|---|---|
| Systematic | Quadrats placed where the experimenter thinks the area looks typical. | Place them using random coordinates from a random number generator. |
| Random | The species is unevenly distributed, so counts vary widely between quadrats. | Take many more quadrats and use the mean. |
| Judgement | Deciding whether a plant on the boundary is inside the quadrat. | Fix one rule in advance and apply it to every quadrat. |
| Judgement | Deciding where one clumped plant ends and the next begins. | Count percentage cover instead of individuals for such species. |
Understand the science behind it → · Prove it · Teach it · Defend it
15. Required practical 4.4 — investigating distribution and biodiversity
Specification statement 4.4 · Topic 4 · taught in Biodiversity, Distribution and Limiting Factors
Method
- FOR BIODIVERSITY: place at least ten quadrats at random, using coordinates from random numbers, in each area to be compared.
- Record every different species present in each quadrat, and the abundance of each as a count or as percentage cover.
- The total number of different species recorded across all the quadrats is the measure of biodiversity for that area. Repeat in the second area and compare.
- FOR DISTRIBUTION: lay a tape measure as a transect along the gradient being investigated — for example from open ground into the shade of a hedge.
- Place a quadrat at regular intervals along the line, such as every two metres.
- At each position record the abundance of the chosen species AND the abiotic factor, using a light meter, thermometer or pH probe.
- Plot abundance against distance along the transect, with the abiotic measurement on the same axis, and describe how they change together.
Variables
Independent (changed) — distance along the transect — or, for biodiversity, the area being sampled
Dependent (measured) — abundance of the species, or the number of different species present
| Control variable | Why it must be held constant |
|---|---|
| Quadrat size | a larger quadrat catches more species, so counts from different sizes cannot be compared |
| Number of quadrats | more quadrats find more species, so unequal numbers make one area appear more diverse than it is |
| Time of day and date | light and temperature change through the day, so readings taken at different times are not comparable |
| The person identifying the species | different people split or lump species differently, which changes the count without anything in the field changing |
Sources of error
| Type | What goes wrong | What to do |
|---|---|---|
| Judgement | Species that are hard to tell apart are recorded as one, or as two. | Use a printed identification key, and have one person identify throughout. |
| Systematic | A transect laid along the gradient rather than across it. | Lay the line at right angles to the boundary being investigated. |
| Random | Species are patchy, so a single transect may miss them entirely. | Run several parallel transects and take a mean at each distance. |
| Systematic | Light readings taken as clouds pass, so the light meter drifts. | Take all readings within a short period, and repeat each three times. |
Understand the science behind it → · Prove it · Teach it · Defend it
Topic 5 · Use of biological resources
16. Required practical 5.6 — investigating anaerobic respiration by yeast
Specification statement 5.6 · Topic 5 · taught in Yeast, Bread and Fermentation
Method
- Mix dried yeast with glucose solution in a boiling tube, and add a thin layer of oil on the surface to keep oxygen out so that respiration is anaerobic.
- Seal the tube with a bung and delivery tube leading into limewater, or into an inverted measuring cylinder of water to collect the gas.
- Place the tube in a water bath at the first temperature and allow five minutes for the contents to reach it before starting to record.
- Count the bubbles produced per minute, or measure the volume of gas collected in a fixed time.
- Repeat at a range of temperatures — for example 20, 30, 40, 50 and 60 °C — keeping everything else the same.
- Plot rate of gas production against temperature. The rate rises to an optimum around 40 °C and then falls sharply as the enzymes are denatured.
Variables
Independent (changed) — temperature of the water bath
Dependent (measured) — rate of carbon dioxide production — bubbles per minute, or volume of gas per minute
| Control variable | Why it must be held constant |
|---|---|
| Volume and concentration of glucose solution | more substrate gives a faster rate, which would be mistaken for an effect of temperature |
| Mass of yeast used | more yeast cells means more respiration, so the counts would not be comparable |
| pH of the mixture | the enzymes of respiration have an optimum pH, so a change in pH changes the rate independently |
| Time allowed to reach the set temperature | a tube not yet at the water bath's temperature is being measured at the wrong one |
Sources of error
| Type | What goes wrong | What to do |
|---|---|---|
| Judgement | Bubbles vary in size, so counting them is not a consistent measure of volume. | Collect the gas in a measuring cylinder and record its volume instead. |
| Systematic | Gas escapes through a loose bung, so every reading is too low. | Check the seal, and test the apparatus for leaks before starting. |
| Systematic | Readings taken before the tube has reached the water bath's temperature. | Leave each tube five minutes to equilibrate before recording. |
| Random | The rate varies between repeats with the same yeast suspension. | Take three readings at each temperature and calculate a mean. |
Understand the science behind it → · Prove it · Teach it · Defend it
From the method to the mark
A method is a list until you can say why each line is there. Each lesson linked above explains the science first, then asks you to prove it, teach it and defend it.
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