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GCSE BiologyAQA, Edexcel and OCR

Biology required practicals: GCSE questions with answers

40 questions on the biology required practicals. Pick your answer, then tap to see if you were right and why. Every answer comes from the Show Working science bank, checked against the GCSE content every board in England teaches.

Quiz yourself: 40 questions 6 6-mark questions

01 / Quiz yourselfTry it, then tap to check

Biology required practicals questions to try

Choose your answer before you open it. Then read why, so it sticks.

01 / Biology required practicalsBoth tiers

Put these steps for focusing a light microscope in order.

  1. ARefocus using only the fine focus
  2. BUse the fine focus to make the image sharp
  3. CClip the slide on the stage, with the lowest-power objective in place
  4. DUse the coarse focus to get the cells roughly clear
  5. ESwap to a higher-power objective lens
Show the answer

The right order:

  1. Clip the slide on the stage, with the lowest-power objective in place
  2. Use the coarse focus to get the cells roughly clear
  3. Use the fine focus to make the image sharp
  4. Swap to a higher-power objective lens
  5. Refocus using only the fine focus

Start on the lowest power, where the field of view is widest, so the cells are easy to find. Coarse focus gets them roughly clear and fine focus sharpens them. Only then move up to a higher power.

Checked against OpenStax Biology 2e, 4.1 Studying Cells

02 / Biology required practicalsBoth tiers

Why must a piece of onion skin on a slide be only one cell thick?

  1. ASo the cells stay alive longer
  2. BSo it magnifies the cells more
  3. CSo the stain cannot reach it
  4. DSo light can pass through it
Show the answer

Answer: D So light can pass through it

A light microscope shines light up through the specimen, so it must be thin enough for light to pass through. Lowering the coverslip from one side pushes the air out, so no bubbles hide the cells.

Checked against OpenStax Biology 2e, 4.1 Studying Cells

03 / Biology required practicalsBoth tiers

Why is a stain such as iodine solution added to onion cells on a slide?

  1. ATo kill bacteria on the slide
  2. BTo make the cells swell up
  3. CTo make parts of the cells show up
  4. DTo stick the coverslip down
Show the answer

Answer: C To make parts of the cells show up

Most cell parts are almost colourless, so a stain is added to give contrast. Iodine solution is used on onion cells and methylene blue on cheek cells, which makes the nucleus stand out.

Checked against OpenStax Biology 2e, 4.1 Studying Cells

04 / Biology required practicalsBoth tiers

How is the total magnification of a light microscope found?

  1. AObjective − eyepiece magnification
  2. BObjective ÷ eyepiece magnification
  3. CEyepiece × objective magnification
  4. DEyepiece + objective magnification
Show the answer

Answer: C Eyepiece × objective magnification

The eyepiece and objective lenses each magnify, so their magnifications multiply: a ×10 eyepiece with a ×40 objective gives ×400. Magnification is also the size of the image divided by the real size of the object.

Checked against OpenStax Biology 2e, 4.1 Studying Cells

05 / Biology required practicalsBoth tiers

Which is a rule for a labelled biological drawing?

  1. ADraw it in several colours of pen
  2. BDraw each cell in 3D
  3. CUse clear, unbroken pencil lines
  4. DShade in the darker parts
Show the answer

Answer: C Use clear, unbroken pencil lines

Scientific drawings use sharp pencil, smooth unbroken outlines and no shading, with straight label lines drawn with a ruler that never cross. A magnification or scale bar lets anyone work out the real size.

Checked against AQA GCSE Biology (8461) specification, Required practical activity 1

06 / Biology required practicalsBoth tiers

Why can a school light microscope not show ribosomes?

  1. ARibosomes are see-through
  2. BIts resolution is too low
  3. CThe stain washes them away
  4. DIts eyepiece is too large
Show the answer

Answer: B Its resolution is too low

Resolution is how close two points can be and still be seen as separate. Ribosomes are far smaller than a light microscope can resolve, so they only show up with an electron microscope.

Checked against OpenStax Biology 2e, 4.1 Studying Cells

07 / Biology required practicalsBoth tiers

Why do you start with the lowest-power objective lens?

  1. AIt gives the biggest image
  2. BIt needs no light to work
  3. CIt is easier to find the cells
  4. DIt kills microbes on the slide
Show the answer

Answer: C It is easier to find the cells

The low-power lens shows a wide field of view, so the specimen is easy to find. The high-power lens sits very close to the slide, so big turns of the coarse focus could drive it into the slide.

Checked against OpenStax Biology 2e, 4.1 Studying Cells

08 / Biology required practicalsBoth tiers

What should you do with a cotton bud used to take cheek cells?

  1. APut it straight into disinfectant
  2. BLeave it on the bench
  3. CRinse it under the tap and use it again
  4. DPut it back in the box
Show the answer

Answer: A Put it straight into disinfectant

Cheek cells may carry microorganisms, so used swabs go straight into disinfectant. A mirror can reflect sunlight through the lenses and focus it into your eye, which can cause lasting damage.

Checked against AQA GCSE Biology (8461) specification, Required practical activity 1

09 / Biology required practicalsBoth tiers

Put these steps of the potato osmosis practical in order.

  1. ABlot each cylinder dry and weigh it again
  2. BBlot each cylinder dry and weigh it
  3. CCut potato cylinders of the same size
  4. DPut each cylinder in a different sugar solution
  5. ELeave them all for the same length of time
Show the answer

The right order:

  1. Cut potato cylinders of the same size
  2. Blot each cylinder dry and weigh it
  3. Put each cylinder in a different sugar solution
  4. Leave them all for the same length of time
  5. Blot each cylinder dry and weigh it again

Cylinders of the same size and type of potato are weighed, left in solutions of different concentrations for the same time, then dried and weighed again. The change in mass shows how much water moved in or out.

Checked against Pearson Edexcel GCSE Biology (1BI0) specification, core practical 1.16

10 / Biology required practicalsBoth tiers

In the potato osmosis practical, what is the independent variable?

  1. AThe temperature of the room
  2. BThe change in mass of each potato cylinder
  3. CThe volume of each solution
  4. DThe concentration of the solution
Show the answer

Answer: D The concentration of the solution

The concentration of the sugar or salt solution is what you change, and the change in mass is what you measure. The volume of solution, the time, the temperature and the size and type of potato are kept the same.

Checked against AQA GCSE Biology (8461) specification, Required practical activity 3

11 / Biology required practicalsBoth tiers

Why work out the percentage change in mass, not just the change?

  1. AThe cylinders started with different masses
  2. BIt means the cylinders do not need to be blotted dry
  3. CIt makes the balance more accurate
  4. DIt shows the sugar concentration
Show the answer

Answer: A The cylinders started with different masses

Cylinders are never exactly the same mass, so percentage change lets you compare them fairly. Solution clinging to the outside would add mass that did not come from osmosis, so each one is blotted gently in the same way.

Checked against Pearson Edexcel GCSE Biology (1BI0) specification, core practical 1.16

12 / Biology required practicalsBoth tiers

A potato cylinder in pure water gains mass. Why?

  1. ASugar moves in by active transport
  2. BSugar moves in by osmosis
  3. CWater moves in by osmosis
  4. DWater moves out by osmosis
Show the answer

Answer: C Water moves in by osmosis

Water moves by osmosis from a dilute solution to a more concentrated one, across the partially permeable cell membranes. Pure water is more dilute than the cell contents, so water moves in; a strong sugar solution draws water out.

Checked against OpenStax Biology 2e, 5.2 Passive Transport

13 / Biology required practicalsBoth tiers

On the graph of the osmosis results, what goes on the y-axis?

  1. AConcentration of the solution
  2. BStarting mass of potato
  3. CPercentage change in mass
  4. DTime in the solution
Show the answer

Answer: C Percentage change in mass

Concentration goes on the x-axis and percentage change in mass on the y-axis. Where the line crosses zero, water is not moving in or out overall, so that solution is about as concentrated as the inside of the potato cells.

Checked against Pearson Edexcel GCSE Biology (1BI0) specification, core practical 1.16

14 / Biology required practicalsBoth tiers

Why use a balance that reads to 0.01 g rather than to 1 g?

  1. AIt stops water moving
  2. BIt makes the potato heavier
  3. CSmall mass changes can be detected
  4. DIt means no repeats or mean are needed
Show the answer

Answer: C Small mass changes can be detected

The masses change by small amounts, so a balance with a finer resolution shows them. A result that does not fit the pattern is anomalous: leave it out of the mean and, if you can, repeat that test.

Checked against AQA GCSE Biology (8461) specification, Required practical activity 3

15 / Biology required practicalsBoth tiers

How should potato cylinders be cut safely?

  1. ACutting towards the hand that is holding the potato
  2. BOn a tile, cutting away from your fingers
  3. CWith a blunt knife so it slips
  4. DHolding the potato up in the air
Show the answer

Answer: B On a tile, cutting away from your fingers

Cork borers and scalpels are sharp. Press down onto a white tile and cut away from your body and fingers, so a slip does not cut you.

Checked against Health and Safety Executive, Managing risks and risk assessment

16 / Biology required practicalsBoth tiers

Put these steps for making a food sample to test in order.

  1. AAdd a little distilled water and stir
  2. BGrind the food with a pestle and mortar
  3. CFilter the mixture
  4. DAdd the test reagent to the liquid
Show the answer

The right order:

  1. Grind the food with a pestle and mortar
  2. Add a little distilled water and stir
  3. Filter the mixture
  4. Add the test reagent to the liquid

Grinding breaks the food up so its molecules can dissolve or spread out in the water. Filtering removes lumps, leaving a clear liquid in which colour changes are easy to see.

Checked against AQA GCSE Biology (8461) specification, Required practical activity 4

17 / Biology required practicalsBoth tiers

Iodine solution turns blue-black with a food. What does the food contain?

  1. AStarch
  2. BLipid
  3. CProtein
  4. DGlucose
Show the answer

Answer: A Starch

Iodine solution is orange-brown. With starch it turns blue-black, so this is the test for starch. It does not show sugars, proteins or fats.

Checked against OpenStax Biology 2e, 3.2 Carbohydrates

18 / Biology required practicalsBoth tiers

Which reagent is used to test for sugars such as glucose?

  1. ALimewater
  2. BBiuret reagent
  3. CIodine solution
  4. DBenedict’s solution
Show the answer

Answer: D Benedict’s solution

Benedict’s solution is blue. Heated with sugars such as glucose it turns green, yellow, orange or brick red, depending on how much sugar there is. Brick red means a lot.

Checked against AQA GCSE Biology (8461) specification, Required practical activity 4

19 / Biology required practicalsBoth tiers

How is the Benedict’s test heated safely?

  1. AIn a sealed bottle
  2. BStraight over a Bunsen flame
  3. CIn a hot water bath
  4. DOn a hot plate with no water
Show the answer

Answer: C In a hot water bath

The Benedict’s test only works when heated, but a test tube held in a flame can boil suddenly and spit hot liquid. Standing it in a beaker of hot water heats it evenly and safely.

Checked against Pearson Edexcel GCSE Biology (1BI0) specification, core practical 1.13B

20 / Biology required practicalsBoth tiers

Which test shows that a food contains protein?

  1. AIodine test
  2. BBenedict’s test
  3. CLimewater test
  4. DBiuret test
Show the answer

Answer: D Biuret test

Biuret reagent is a blue mixture of an alkali and copper sulfate. It turns purple when protein is present. The alkali can harm eyes, so wear eye protection.

Checked against Pearson Edexcel GCSE Biology (1BI0) specification, core practical 1.13B

21 / Biology required practicalsBoth tiers

Food tests use qualitative reagents. What does qualitative mean here?

  1. AThey show the energy in the food
  2. BThey give the exact mass of each substance
  3. CThey measure a rate of reaction
  4. DThey show whether a substance is there
Show the answer

Answer: D They show whether a substance is there

A qualitative test tells you whether a substance is present from a colour change. It does not give an exact amount, though the Benedict’s colour gives a rough idea of how much sugar there is.

Checked against AQA GCSE Biology (8461) specification, Required practical activity 4

22 / Biology required practicalsBoth tiers

Put these steps of the amylase and pH practical in order.

  1. AAdd a drop of the mixture to iodine at regular intervals
  2. BNote the time when the iodine stays orange-brown
  3. CWait for them to reach the bath temperature
  4. DPut the starch, amylase and buffer in a water bath
  5. EMix them and start the stopwatch
Show the answer

The right order:

  1. Put the starch, amylase and buffer in a water bath
  2. Wait for them to reach the bath temperature
  3. Mix them and start the stopwatch
  4. Add a drop of the mixture to iodine at regular intervals
  5. Note the time when the iodine stays orange-brown

The solutions are warmed to the same temperature before mixing. Drops of the mixture are tested with iodine at set intervals, which is continuous sampling. When the iodine no longer turns blue-black, the starch has all been digested.

Checked against AQA GCSE Biology (8461) specification, Required practical activity 5

23 / Biology required practicalsBoth tiers

In the amylase practical, how do you know all the starch has been digested?

  1. AThe mixture starts to fizz
  2. BThe iodine turns blue-black
  3. CThe mixture turns purple
  4. DThe iodine stays orange-brown
Show the answer

Answer: D The iodine stays orange-brown

Amylase breaks starch down into sugar. While starch is left, a drop turns iodine blue-black; once it has gone, the iodine stays orange-brown. Each buffer holds the mixture at one pH.

Checked against Pearson Edexcel GCSE Biology (1BI0) specification, core practical 1.10

24 / Biology required practicalsBoth tiers

In the amylase and pH practical, which is the dependent variable?

  1. ATime for the starch to be digested
  2. BThe volume of amylase
  3. CThe pH of the buffer
  4. DThe temperature of the water bath used
Show the answer

Answer: A Time for the starch to be digested

pH is the independent variable and the time for the starch to disappear is the dependent variable. Temperature, and the volumes and concentrations of amylase and starch, are kept the same, because each of them also changes the rate.

Checked against AQA GCSE Biology (8461) specification, Required practical activity 5

25 / Biology required practicalsBoth tiers

At which pH does the starch disappear in the shortest time?

  1. AAt the lowest pH tested
  2. BAt the highest pH tested
  3. CIt is the same at every pH
  4. DAt the optimum pH
Show the answer

Answer: D At the optimum pH

Amylase works fastest at its optimum pH, so the starch goes in the shortest time there. The rate is found from 1 ÷ time, so a shorter time means a faster rate.

Checked against OpenStax Biology 2e, 6.5 Enzymes

26 / Biology required practicalsBoth tiers

Far from its optimum pH, amylase digests starch slowly or not at all. Why?

  1. AThe shape of its active site changes
  2. BIt turns into a different enzyme that digests fat
  3. CIts particles move more slowly
  4. DIt runs out of energy
Show the answer

Answer: A The shape of its active site changes

A pH far from the optimum changes the shape of the active site, so starch no longer fits it. The enzyme is denatured. This is not about kinetic energy, which is the temperature effect.

Checked against OpenStax Biology 2e, 6.5 Enzymes

27 / Biology required practicalsBoth tiers

Drops are tested every 30 seconds. How could the time be found more precisely?

  1. AUse a bigger spotting tile
  2. BTest drops more often
  3. CPut more iodine in each well
  4. DTest drops less often
Show the answer

Answer: B Test drops more often

With 30-second samples, the starch may have gone at any time in the last 30 seconds, so sampling more often narrows the time. Iodine solution and enzymes can irritate the eyes, so eye protection is worn.

Checked against AQA GCSE Biology (8461) specification, Required practical activity 5

28 / Biology required practicalsBoth tiers

In the light intensity practical, how is the light intensity changed?

  1. AMove the lamp to different distances
  2. BChange the volume of water in the beaker
  3. CChange the water temperature
  4. DUse a different plant each time
Show the answer

Answer: A Move the lamp to different distances

Moving the lamp further away lowers the light intensity reaching the plant. Light intensity is what you change; the rate of photosynthesis is what you measure.

Checked against Pearson Edexcel GCSE Biology (1BI0) specification, core practical 6.5

29 / Biology required practicalsBoth tiers

What is used to measure the distance from the lamp to the plant?

  1. AA measuring cylinder
  2. BA top-pan balance
  3. CA metre ruler
  4. DA thermometer
Show the answer

Answer: C A metre ruler

The distance between the lamp and the plant is measured with a metre ruler. Other light, from windows or room lights, would change the light intensity, so the room is darkened.

Checked against AQA GCSE Biology (8461) specification, Required practical activity 6

30 / Biology required practicalsBoth tiers

Why might a filament lamp close to the plant spoil the results?

  1. AIt changes the pH of the water
  2. BIt uses up the oxygen
  3. CIt adds carbon dioxide
  4. DIt warms the water as well
Show the answer

Answer: D It warms the water as well

Temperature also changes the rate of photosynthesis, so it must stay the same. A filament lamp gives off a lot of heat; an LED gives off very little, or a beaker of water can be placed between lamp and plant as a heat shield.

Checked against AQA GCSE Biology (8461) specification, Required practical activity 6

31 / Biology required practicalsBoth tiers

As the lamp is moved further from the plant, the rate of photosynthesis:

  1. AStays the same
  2. BGoes up, then down
  3. CGoes up
  4. DGoes down
Show the answer

Answer: D Goes down

Further from the lamp, less light reaches the plant, so less energy is available for photosynthesis and the rate falls. The rate only stops rising with more light when another factor, such as carbon dioxide or temperature, holds it back.

Checked against OpenStax Biology 2e, 8.2 The Light-Dependent Reactions of Photosynthesis

32 / Biology required practicalsBoth tiers

Why should water be kept away from the lamp and its plug?

  1. AThe lamp would add carbon dioxide
  2. BWater and mains electricity can give a shock
  3. CWater makes the bulb brighter
  4. DWater would cool the plant
Show the answer

Answer: B Water and mains electricity can give a shock

Spilt water near a mains lamp or socket can give an electric shock. Keep the lamp and plug dry, wipe up spills at once, and do not touch a hot bulb.

Checked against Health and Safety Executive, Managing risks and risk assessment

33 / Biology required practicalsBoth tiers

Put these steps for estimating a plant population in a field in order.

  1. ALay two tape measures at right angles to make a grid
  2. BPlace the quadrat at random coordinates on the grid
  3. CCount the plants inside the quadrat
  4. DFind the mean per quadrat and scale up to the field
  5. ERepeat for many quadrats
Show the answer

The right order:

  1. Lay two tape measures at right angles to make a grid
  2. Place the quadrat at random coordinates on the grid
  3. Count the plants inside the quadrat
  4. Repeat for many quadrats
  5. Find the mean per quadrat and scale up to the field

Random coordinates on a grid decide where each quadrat goes. Counting in many quadrats gives a mean per quadrat, which is scaled up to the area of the whole field to estimate the population.

Checked against AQA GCSE Biology (8461) specification, Required practical activity 9

34 / Biology required practicalsBoth tiers

Why are quadrats placed at random coordinates?

  1. ASo every quadrat has plants in
  2. BSo the sample is not biased
  3. CSo the quadrats do not overlap
  4. DSo fewer quadrats are needed
Show the answer

Answer: B So the sample is not biased

Choosing spots by eye tends to pick the most interesting patches, which biases the sample. Random coordinates give every part of the area an equal chance. A thrown quadrat still depends on where you stand and how you throw.

Checked against AQA GCSE Biology (8461) specification, Required practical activity 9

35 / Biology required practicalsBoth tiers

Why use many quadrats rather than one or two?

  1. AIt removes the need for a mean
  2. BIt makes the field smaller
  3. CThe sample better represents the area
  4. DEach quadrat then has fewer plants in it to count
Show the answer

Answer: C The sample better represents the area

One or two quadrats may land on unusual patches; many quadrats give a mean closer to the true value. Where single plants cannot be told apart, the percentage of the quadrat they cover is estimated instead.

Checked against OpenStax Biology 2e, 45.1 Population Demography

36 / Biology required practicalsBoth tiers

How is a transect used in fieldwork?

  1. AQuadrats are placed at intervals along a line
  2. BPlants are dug up and weighed
  3. COne quadrat is placed in the middle
  4. DQuadrats are placed only at random coordinates
Show the answer

Answer: A Quadrats are placed at intervals along a line

A transect runs a line across an area where conditions change, such as from shade into sunlight. Sampling at intervals along it shows how the distribution of a species changes with that factor.

Checked against Pearson Edexcel GCSE Biology (1BI0) specification, core practical 9.5

37 / Biology required practicalsBoth tiers

Which instrument measures light intensity at each quadrat?

  1. AA measuring cylinder
  2. BA light meter
  3. CA pH probe
  4. DA thermometer
Show the answer

Answer: B A light meter

Abiotic factors are measured with instruments, such as a light meter for light intensity. To compare two areas fairly, use the same size of quadrat and the same number of quadrats in each.

Checked against AQA GCSE Biology (8461) specification, Required practical activity 9

38 / Biology required practicalsBoth tiers

Which is a sensible precaution in fieldwork?

  1. AWork alone near deep water to keep it quiet
  2. BPick every plant in the quadrat
  3. CTaste plants to identify them
  4. DWash hands after touching soil and plants
Show the answer

Answer: D Wash hands after touching soil and plants

Soil and plants can carry microorganisms, and some plants sting or are poisonous. Wash your hands afterwards, work in groups, take care near water, and leave the habitat as you found it.

Checked against Health and Safety Executive, Managing risks and risk assessment

39 / Biology required practicalsBoth tiers

What is an anomalous result?

  1. AThe mean of the results
  2. BOne that does not fit the pattern
  3. CThe largest result in the table
  4. DThe first result taken
Show the answer

Answer: B One that does not fit the pattern

An anomalous result lies well away from the pattern of the others. It is left out of the mean and the test repeated if possible. Repeating and taking a mean evens out random errors, but cannot remove a systematic error.

Checked against AQA GCSE Biology (8461) specification, section 2 Working scientifically

40 / Biology required practicalsBoth tiers

Why should a sample be representative?

  1. ASo every result is the same
  2. BSo conclusions apply to the whole population
  3. CSo it takes less time
  4. DSo it is as small and quick to collect as possible
Show the answer

Answer: B So conclusions apply to the whole population

You cannot usually measure every organism, so you sample. A large, randomly chosen sample is more likely to represent the whole population, so conclusions drawn from it are more valid.

Checked against AQA GCSE Biology (8461) specification, section 2 Working scientifically

02 / Longer answersMarked by levels

6-mark questions on this topic

Write your answer first, on paper. Then open it and tick off the points you made. How 6-mark questions are marked.

6 marks / Looking at onion cellsBoth tiers

Describe how to prepare a slide of onion cells and use a light microscope to make a labelled drawing of them.

What a full answer includes
  • Peel a thin layer of epidermis from the inside of an onion piece with forceps, so that light can pass through it.
  • Lay it flat in a drop of water on a clean microscope slide.
  • Add a drop of iodine solution to stain the cells, so the nucleus and cell wall show up clearly.
  • Lower a cover slip onto it at an angle, using a mounted needle, so no air bubbles are trapped.
  • Clip the slide on the stage and focus with the lowest-power objective lens first, using the coarse focusing knob, then the fine one.
  • Change to a higher-power objective lens and refocus using only the fine focusing knob.
  • Draw the cells with a sharp pencil in clear, unbroken lines with no shading, and label the cell wall, cytoplasm and nucleus with straight ruled lines.
  • Write the magnification on the drawing (eyepiece lens magnification times objective lens magnification) or add a scale bar.

Model answer

First I would use forceps to peel a thin layer of epidermis from the inside of a piece of onion. It has to be thin so that light can pass through it. I would lay it flat in a drop of water on a clean slide and add a drop of iodine solution, which stains the cells so the nucleus and cell wall can be seen clearly. Then I would lower a cover slip at an angle with a mounted needle so that no air bubbles are trapped. I would clip the slide onto the stage, select the lowest-power objective lens and focus using the coarse focusing knob, then the fine focusing knob. Next I would switch to a higher-power objective lens and refocus with the fine focusing knob only. I would draw a few cells with a sharp pencil, using clear continuous lines and no shading, and label the cell wall, cytoplasm and nucleus with ruled lines. Finally I would write the magnification, which is the eyepiece magnification multiplied by the objective magnification, or draw a scale bar.

Where marks slip

Many answers leave out the stain or just say it helps you “see the cells”. Say what it shows (the nucleus and cell wall), and remember total magnification is eyepiece times objective, not the two added together.

6 marks / Osmosis in potato piecesBoth tiers

Plan an investigation to find out how the concentration of sugar solution affects the mass of potato tissue.

What a full answer includes
  • Cut cylinders of potato with a cork borer and trim them to the same length with a scalpel, all from the same potato and with no skin.
  • Gently blot each cylinder dry and find its starting mass on a balance.
  • Put each cylinder in a boiling tube with the same volume of sugar solution, using a range of concentrations such as 0.0, 0.2, 0.4, 0.6, 0.8 and 1.0 mol/dm³, where 0.0 is distilled water.
  • Leave all the tubes for the same length of time at the same temperature.
  • Remove each cylinder, blot it dry in the same way and find its final mass.
  • Work out the percentage change in mass for each cylinder, because the starting masses are not exactly the same.
  • Repeat with at least three cylinders at each concentration and calculate a mean.
  • Plot percentage change in mass against concentration; where the line crosses zero, the solution has the same concentration as the potato cells.

Model answer

I would use a cork borer to cut cylinders from the same potato, remove any skin and trim them to the same length with a scalpel. I would blot each one dry and measure its mass on a balance. Then I would put each cylinder into a labelled boiling tube containing the same volume, say 10 cm³, of sugar solution. I would use a range of concentrations, for example 0.0, 0.2, 0.4, 0.6, 0.8 and 1.0 mol/dm³, with distilled water as the 0.0 tube. The tubes would all be left for the same time, such as 30 minutes, at room temperature. Afterwards I would take each cylinder out, blot it gently in the same way to remove surface liquid, and find its new mass. Because the cylinders do not all start with exactly the same mass, I would calculate the percentage change in mass. I would do three cylinders for each concentration and work out a mean. Finally I would plot a graph of percentage change in mass against concentration. Cylinders in dilute solutions gain mass as water moves in by osmosis, and those in concentrated solutions lose mass.

Where marks slip

Comparing the raw masses instead of the percentage change, when the cylinders start at different masses. Also, it is water that moves by osmosis, not the sugar or the solution.

6 marks / How pH affects amylaseBoth tiers

Describe a method to investigate how pH affects the rate at which amylase breaks down starch.

What a full answer includes
  • Put one drop of iodine solution into each well of a spotting tile.
  • Warm the starch solution, the amylase solution and a pH buffer in a water bath at the same temperature, such as 35 °C, for a few minutes.
  • Add the buffer to the starch, then add the amylase, mix in a test tube in the water bath and start a stop clock straight away.
  • Every 30 seconds, use a pipette to put a drop of the mixture into a fresh well of iodine.
  • Record the time when the iodine stays orange-brown instead of turning blue-black, which shows all the starch has been broken down.
  • Repeat with buffers of different pH, such as pH 4, 5, 6, 7 and 8, keeping the volumes, concentrations and temperature the same.
  • Repeat each pH three times and calculate a mean time.
  • The shortest time shows the fastest rate, which is at the optimum pH; the rate can be found as 1 divided by the time.

Model answer

First I would put a drop of iodine solution in each well of a spotting tile. I would place tubes of starch solution, amylase solution and a pH 5 buffer in a water bath at 35 °C for a few minutes so they all reach the same temperature. Then I would add 1 cm³ of the buffer to 2 cm³ of starch, add 2 cm³ of amylase, mix them and start a stop clock straight away. Every 30 seconds I would use a pipette to take a drop of the mixture and add it to a new well of iodine. While starch is still there the iodine turns blue-black. I would record the time when the iodine stays orange-brown, because then all the starch has been broken down. I would repeat this with buffers of pH 6, 7 and 8, keeping the temperature, the volumes and the concentrations of starch and amylase the same each time. I would do each pH three times and work out a mean time. The pH with the shortest time is where amylase works fastest, its optimum, and the rate can be worked out as 1 divided by the time.

Where marks slip

Saying the iodine goes “clear” when the starch has gone: it stays orange-brown. Also add the buffer before the starch and amylase meet, because the reaction starts as soon as they are mixed.

6 marks / Light intensity and photosynthesisBoth tiers

Plan an investigation into how light intensity affects the rate of photosynthesis in pondweed.

What a full answer includes
  • Put a piece of pondweed in a boiling tube of water with some sodium hydrogencarbonate added, so the pondweed has plenty of carbon dioxide.
  • Place a lamp at a measured distance from the tube, such as 10 cm, measured with a metre ruler.
  • Leave the pondweed for a few minutes to adjust to the light.
  • Count the bubbles of oxygen given off in one minute, or better, collect the gas in an upturned measuring cylinder or gas syringe over a set time and measure its volume.
  • Move the lamp to different distances, such as 20, 30, 40 and 50 cm, to change the light intensity.
  • Keep the temperature the same, for example by using an LED bulb or standing the tube in a beaker of water, and use the same pondweed and the same concentration of hydrogencarbonate.
  • Repeat at each distance and calculate a mean rate.
  • Plot the rate against distance; the closer the lamp, the higher the light intensity and the faster the rate, until another factor limits it.

Model answer

I would put a piece of pondweed into a boiling tube of water with a little sodium hydrogencarbonate dissolved in it, so carbon dioxide is not in short supply. I would place a lamp 10 cm from the tube, measuring the distance with a metre ruler, and leave the pondweed for five minutes to adjust. Then I would measure the rate of photosynthesis by collecting the oxygen given off in an upturned measuring cylinder for five minutes and reading the volume of gas. Counting bubbles is quicker but less accurate, because bubbles vary in size. I would repeat this with the lamp at 20, 30, 40 and 50 cm, so the light intensity is lower each time. The lamp can warm the water, so I would use an LED bulb and stand the tube in a beaker of water to keep the temperature the same. I would also use the same piece of pondweed throughout. I would repeat each distance three times and find the mean. A graph of rate against distance should show that the rate is fastest when the lamp is closest.

Where marks slip

Forgetting that the lamp heats the water as well as lighting it. Say how you keep the temperature the same, or a change in rate could be caused by temperature, not light.

6 marks / Counting dandelions with quadratsBoth tiers

A class wants to estimate how many dandelions grow on their school field. They have 1 m² quadrats, two long tape measures and a random number generator.

Describe how they should estimate the number of dandelions on the whole field.

What a full answer includes
  • Lay the two tape measures at right angles along two edges of the field to make a grid.
  • Use the random number generator to pick coordinates for each quadrat, so the samples are random and not chosen.
  • Place the 1 m² quadrat at each pair of coordinates.
  • Count the dandelions inside the quadrat, with a fixed rule for plants on the edge, such as counting them only if more than half is inside.
  • Sample a large number of quadrats, such as 20, so the sample is representative of the whole field.
  • Calculate the mean number of dandelions per quadrat, which is the mean per m².
  • Measure the area of the field in m² and multiply it by the mean number per m² to estimate the total.

Model answer

They should lay the two tape measures at right angles along two edges of the field so they form the axes of a grid. Then they should use the random number generator to choose a pair of coordinates and place the quadrat there. Using random coordinates is better than throwing the quadrat, because people tend to aim, and the sample might not be fair. They count all the dandelions inside the quadrat and decide a rule for plants on the edge, for example counting a plant only if more than half of it is inside. They should repeat this for at least 20 random positions, because a larger sample is more likely to be representative of the whole field. Next they add up all the counts and divide by the number of quadrats to find the mean number of dandelions in 1 m². Finally they measure the length and width of the field to find its area in m², and multiply the area by the mean number per m². This gives an estimate of the total number of dandelions on the field.

Where marks slip

Multiplying the total count from all the quadrats by the area of the field. You must use the mean number per m², and the quadrats must be placed using random numbers, not thrown.

6 marks / Testing antibiotics on bacteriaBoth tiers · separate only

Describe how to investigate which of three antibiotics is best at stopping the growth of a bacterium, using aseptic technique.

What a full answer includes
  • Use a Petri dish and agar that have been sterilised, for example in an autoclave, so no other microorganisms are present.
  • Pass the inoculating loop through a Bunsen flame until it glows red, then let it cool, to kill any microorganisms on it.
  • Spread the bacteria evenly over the agar, lifting the lid only slightly and working near a Bunsen flame so microorganisms from the air do not get in.
  • Use sterile forceps to place paper discs soaked in each antibiotic, all the same size and concentration, on the agar, plus a control disc soaked in sterile water.
  • Secure the lid with a few pieces of tape and store the dish upside down, so condensation does not drip onto the agar.
  • Incubate at 25 °C for about two days, so bacteria that cause disease in humans, which grow best at body temperature, are less likely to grow.
  • Measure the diameter of the clear zone of inhibition around each disc; the biggest clear zone shows the most effective antibiotic.
  • Repeat with more plates and compare mean clear zones, which can be turned into areas using πr².

Model answer

I would use a sterile agar plate, because the Petri dish and agar have been heated in an autoclave to kill any microorganisms. I would sterilise an inoculating loop by passing it through a Bunsen flame until it glows red, then let it cool so it does not kill the bacteria I want. Working near the Bunsen flame, which carries microorganisms from the air upwards, I would lift the lid slightly and spread the bacteria evenly over the agar. Then, using sterile forceps, I would place three paper discs, each soaked in a different antibiotic of the same concentration, onto the agar, plus a control disc soaked in sterile water. I would tape the lid down with a few strips, but not seal it all round, and store the dish upside down so condensation does not fall onto the agar. I would incubate it at 25 °C for two days, because at this temperature pathogens that grow at body temperature are less likely to grow. Finally I would measure the diameter of the clear zone around each disc. The antibiotic with the largest clear zone is the most effective.

Where marks slip

Saying the plates should be incubated at 37 °C. In school they are kept at 25 °C so that bacteria that could cause disease in humans are less likely to grow. Also say why each aseptic step is done, not just what it is.

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