Launch practical
About 60 minutes
- Set up the bench with the 13 items of apparatus.
- Take the readings yourself, into the results table you draw first.
- Process your results.
- Answer exam questions on your own results, which you mark against the mark scheme.
Runs in your browser: a 3D bench with 2D close-ups of each instrument for exact readings. Nothing to install and no account.
What you measure, and how
One run takes 11 readings. You take every one yourself, the way you would in class.
- On the 3D bench (Fig. 1) you set out the apparatus and read each instrument where it stands.
- In the 2D close-up (Fig. 2) the same instrument is drawn large, from the same run, for an exact reading. It is also how the practical runs on a device that cannot show 3D.
- In your table you write what you read, to the instrument’s resolution, and the notebook checks it against the instrument.
How to read it. Cheek cells at ×400, stained with methylene blue.
What it shows
magnification = size of image ÷ size of real object
A light microscope magnifies twice: the objective (×4, ×10 or ×40) and the eyepiece (×10), so the total is ×40, ×100 or ×400. The eyepiece scale has no units of its own, so it is calibrated against a stage micrometer for each objective. Light passes up through the specimen, so it must be thin; a stain makes the colourless parts show.
Recall: learn it. Convert both sizes to the same unit first (1 mm = 1000 µm).
Apparatus
| Item | What it is for | Resolution |
|---|---|---|
| Light microscope (eyepiece ×10; objectives ×4, ×10 and ×40) with a lamp | Magnifies the cells ×40, ×100 or ×400. | — |
| Eyepiece graticule (a 100-division scale in the eyepiece) | A scale seen on top of the cells, to measure them. | 1 division |
| Stage micrometer slide (1 mm in 100 divisions) | A known length to calibrate the graticule. | 0.01 mm |
| Microscope slides and coverslips | Hold the specimen flat under the microscope. | — |
| Onion, white tile and scalpel | The plant tissue, and a safe surface to cut on. | — |
| Forceps and a mounted needle | Peel and flatten the tissue; lower the coverslip. | — |
| Dropping pipette and water | One drop of water to mount the tissue in. | — |
| Iodine solution in a dropper bottle | Stains the nucleus and cytoplasm yellow-brown. | — |
| Filter paper | Soaks up extra liquid from round the coverslip. | — |
| Sterile cotton buds, methylene blue and a beaker of disinfectant | Cheek cells: collect, stain, then dispose of the buds safely. | — |
| Prepared slide of cheek cells | Animal cells, if your school does not take cheek cell samples. | — |
| Safety goggles | Protect your eyes from iodine solution. | — |
| Sharp pencil, ruler and plain paper | For the labelled drawing. | — |
Method
In our own words, as the Lab runs it. Your school's method sheet may differ in detail.
Which version are you doing?
Boards set this practical slightly differently. Your course setting picks one; you can change it before you start.
Why: Both use a light microscope, a calibrated eyepiece scale and a labelled drawing; what you look at, and the exam questions, follow your board.
Put on safety gogglesSafety
Put your goggles on before you open the iodine solution.
Why: Iodine solution irritates the eyes, and a dropper can flick a drop up.
Put a drop of water on the slide
Use a dropping pipette to put one drop of water in the middle of a clean microscope slide.
Why: The tissue is mounted in water so it stays flat and does not dry out, and light passes through it evenly.
Take a thin layer of onion
Separate one of the thick layers of the onion. Peel the thin, see-through skin (the epidermis) off its inner surface with forceps and put it on the drop of water.
Do: Peel the thin epidermis with forceps.
Why: Light has to pass through the tissue. A layer one cell thick lets you see individual cells.
Make sure the layer is flat
Use the forceps and a mounted needle to unfold the layer so it lies flat in the water with no creases.
Why: A fold puts two layers on top of each other and stretches the cells along it.
Stain the onion
Put two drops of iodine solution onto the onion tissue.
Do: Two drops of iodine solution.
Why: Onion cells are almost colourless. Iodine stains the nucleus and cytoplasm yellow-brown so you can see them; the cell walls stand out against them.
Lower the coverslip
Rest one edge of the coverslip on the slide, hold the other edge up with a mounted needle, and lower it slowly.
Do: At an angle, lowered with a mounted needle.
Why: Lowering it at an angle pushes the air out ahead of the liquid, so no air bubbles are trapped.
Soak up the extra liquid
Touch a piece of filter paper to the edge of the coverslip to soak up any liquid that has spread out.
Why: Extra liquid lets the coverslip float and slide about, and iodine on top of the coverslip gets onto the stage and the objective lens.
Choose your animal cells
The activity asks for plant and animal cells. Use your own cheek cells, or a prepared slide of cheek cells if your school does not take cheek cell samples.
Do: Your own cheek cells, stained with methylene blue.
Why: Comparing the two shows which structures plant cells have that animal cells do not.
Take the readings
Calibrate the graticule at ×10, then measure five onion cells along their length in eyepiece divisions and convert each to µm.
Repeats: AQA’s method measures a cell’s length once. You measure five different cells and take a mean, because cells vary in size (biological variation): a bigger sample gives a more typical length. Measuring the same cell again would only reduce your reading error.
Make safe and clear upSafety
Turn back to the ×4 objective and lower the stage. Switch off the lamp, unplug it and put the cover on the microscope. Put used slides and coverslips in the glass waste, and cotton buds in the disinfectant. Wipe up any iodine, wash the tile and scalpel, then wash your hands.
While you take readings
Focus at high power
At ×400 the objective nearly touches the coverslip. Focus with the fine focus knob only.
Do: Fine focus only.
Why: The coarse knob moves the stage a long way for each turn. At high power it drives the slide into the objective.
Calibrate the eyepiece graticule
The scale in the eyepiece has no units. Put a stage micrometer (1 mm in 100 divisions) on the stage, line its zero up with the scale’s zero, and see how many eyepiece divisions match a known length. Do it with the objective you will measure with.
Do: Calibrate at ×10, the objective you measure with.
Why: What one eyepiece division is worth depends on the objective: about 10 µm at ×10, but only about 2.5 µm at ×40.
Measure along the cell
Turn the eyepiece so the scale lies along the length of the cell, then move the slide so one end wall sits on a division.
Do: Turn the scale along each cell.
Why: The length of a cell is its longest dimension, wall to wall.
Hazards and control measures
| Hazard | What could happen | Control measure |
|---|---|---|
| Scalpel (or knife) used to cut the onion | Cuts to the fingers or hand | Cut on a white tile, away from your body, with your fingers behind the blade. |
| Iodine solution | Irritates the eyes, stains the skin and is harmful if swallowed | Wear goggles, add it from a dropper bottle and wash any off your skin straight away. |
| Glass slides and coverslips | Cuts from broken glass | Hold slides by their edges, lower the coverslip with a mounted needle and use only the fine focus at ×400. |
| Cheek cell sample on a cotton bud | Infection from someone else’s saliva or cells | Use only your own cells and a new sterile cotton bud, and put used buds and slides straight into disinfectant. |
| Microscope lamp on a mains lead, near water and stain | Electric shock | Keep liquids away from the lamp and its lead, dry any spill at once and switch off at the socket before moving it. |
Variables
- Independent: you change
- The cell measured (five different cells)
- Dependent: you measure
- Length of each cell in µm
- Control: you keep the same
- Objective. Calibrate and measure with the same objective (×10).
- Direction measured. Measure every cell along its length, from end wall to end wall.
- The slide. Measure cells from the same layer of onion epidermis.
Results from a specimen run
| Looking at | What you saw |
|---|---|
| Onion slide, ×40 | A single layer of brick-shaped cells, their nuclei and cytoplasm stained yellow-brown by the iodine |
| Air bubbles, ×100 | One or two air bubbles: round, with thick black edges |
| Onion cells, ×400 | Cell walls, a thin layer of cytoplasm and a nucleus in each cell |
| Cheek cells, ×400 | Flat, irregular cells, each with a blue nucleus near the middle and no cell wall |
| Calibration | Value |
|---|---|
| Eyepiece divisions equal to 0.50 mm (×10) | 49 |
| Length of one eyepiece division in µm | 10.20 |
| Cell number | Length of cell in eyepiece divisions | Length of cell in µm |
|---|---|---|
| 1 | 24 | 245 |
| 2 | 28 | 286 |
| 3 | 24 | 245 |
| 4 | 214 | |
| 5 | 19 | 194 |
| Mean | 237 |
| Field of view | Value |
|---|---|
| Diameter of the field of view in µm | 1800 |
| Cells end to end across it | 7 |
| Estimated length of one cell in µm | 257 |
Anomaly spotted: Length of cell 4, retaken.
Processing the data
- One eyepiece division
- 10.2 µm
- Lengths in µm
- Cell 1 245 · Cell 2 286 · Cell 3 245 · Cell 4 214 · Cell 5 194
- Mean length
- 237 µm
- Is it sensible?
- Yes: it is in the usual range for onion epidermis cells
- Estimate from the field of view
- 257 µm
- Draw and label
- Labelled drawing; cell 1 drawn 82 mm long
- Magnification of your drawing
- 335
- Plant and animal cells
- A cell wall and a large vacuole
| Quantity | Specimen run | Accepted |
|---|---|---|
| Mean length of your onion cells | 237 µm | 235 µm |
| One eyepiece division at ×10 | 10.2 µm | 10.2 µm |
Across 120 simulated careful groups, mean length came out between 204 and 305 µm (median 247). That is the simulation's model, not a promise of what a real class gets.
Common mistakes and the fix
Mistakes to check in this practical, and what to do instead. In a write-up or an exam answer, the mark goes to naming the step and saying what it does to the readings.
Method: Skip them
No effect on your slide, but your eyes are not protected from a splash of iodine: flagged in the safety review.
The fix: Put on safety goggles. Iodine solution irritates the eyes, and a dropper can flick a drop up.
Method: Cut a thin slice with the scalpel
Even a thin slice is several cells thick. The layers overlap, so the view is dark, no nucleus shows at ×400, and walls from the layer underneath cut across each cell: your lengths come out about a quarter too short (a systematic error).
The fix: Take a thin layer of onion. Light has to pass through the tissue. A layer one cell thick lets you see individual cells.
Method: Leave it as it landed
The layer folds over on itself. Along the crease cells overlap, and cells next to it tear and stretch, so one of the cells you measure is not one whole cell: an anomaly to spot.
The fix: Make sure the layer is flat. A fold puts two layers on top of each other and stretches the cells along it.
Your slide at ×40: Recorded “A single layer of brick-shaped cells, their nuclei and cytoplasm stained yellow-brown by the iodine”
The fix: Look again: is it really one clean layer?
Your slide at ×40: Recorded “A single layer of cells, with a dark crease where the tissue folded over”
The fix: Look again down the eyepiece: that is not what the view shows.
Exam answer: Leaving out: Thin layer: so light can pass through / so individual cells can be seen (allow one cell thick)
The fix: A thin layer lets light pass through, so you can see individual cells.
Exam answer: Leaving out: Iodine: to stain the nucleus / cell wall / parts of the cell so they can be seen (ignore "to stain the cell")
The fix: Say what the iodine does: it stains the nucleus and cytoplasm so you can see them. "To stain the cell" is not enough.
Worked exam-style questions
Written for Study Hack in the style of exam questions, on the specimen run above. They are not taken from any exam board's papers. Try each one before you open the worked answer.
- Question 1Preparing the slide · Applying knowledge[3 marks]
A student made a slide of onion cells. Three of the steps were:
1. Peel a thin layer of epidermis from the onion.
2. Add two drops of iodine solution.
3. Lower the coverslip at an angle.
Give one reason for each step.
Worked answer and how the marks are given
A full-mark answer
Thin layer: so light can pass through and individual cells can be seen. Iodine: stains the nucleus and cytoplasm so they can be seen. Coverslip at an angle: so no air bubbles are trapped.
Mark scheme
- Thin layer: so light can pass through / so individual cells can be seen (allow one cell thick)
- Iodine: to stain the nucleus / cell wall / parts of the cell so they can be seen (ignore "to stain the cell") Do not accept “chloroplast…”.
- Coverslip at an angle: to prevent (or reduce) air bubbles
Where marks are usually lost
- Missing: Thin layer: so light can pass through / so individual cells can be seen (allow one cell thick). A thin layer lets light pass through, so you can see individual cells.
- Missing: Iodine: to stain the nucleus / cell wall / parts of the cell so they can be seen (ignore "to stain the cell"). Say what the iodine does: it stains the nucleus and cytoplasm so you can see them. "To stain the cell" is not enough.
- Missing: Coverslip at an angle: to prevent (or reduce) air bubbles. Lowering the coverslip at an angle stops air bubbles being trapped.
- Question 2Calibrating the graticule · Applying knowledge[2 marks]
When you calibrated the eyepiece graticule with the ×10 objective, 0.50 mm on the stage micrometer lined up with 49 eyepiece divisions.
Calculate the length of one eyepiece division in micrometres (µm).
One eyepiece division = µm
Worked answer and how the marks are given
A full-mark answer
0.50 mm = 500 µm
500 ÷ 49 = 10.2 µmMark scheme
- 0.50 mm ÷ 49 (or 500 µm ÷ 49): 1 mm = 1000 µm (method)
- = 10.2 µm
- Allow answers from 10.10 to 10.31 µm (the marker's tolerance; correct rounding of the student's own working is also accepted).
- Allow the unit written as um, μm, micrometres, micrometers, micrometre, micrometer and 2 other spellings.
- A correct answer with no working scores full marks.
Where marks are usually lost
- Answer 0.0102 µm (gains 1 mark). That is in millimetres. 1 mm = 1000 µm, so 0.50 mm = 500 µm and one division = 500 ÷ 49 = 10.2 µm.
- Answer 1.02 µm (no marks). 0.50 mm is 500 µm, not 50 µm (1 mm = 1000 µm). One division = 500 ÷ 49 = 10.2 µm.
- Answer 0.098 µm (no marks). Upside down: divide the length by the number of divisions. 500 ÷ 49 = 10.2 µm.
- Question 3Describing the method · Knowledge[6 marks]
A student wants to find the length of an onion epidermis cell using a light microscope.
Describe how the student should prepare a slide of onion cells and use the light microscope to find the length of one cell.
Worked answer and how the marks are given
A full-mark answer
1. Peel a thin layer of epidermis from the inside of an onion layer with forceps.
2. Put it flat on a drop of water on a microscope slide.
3. Add two drops of iodine solution to stain it.
4. Lower a coverslip at an angle with a mounted needle so no air bubbles are trapped.
5. Put the slide on the stage with the lowest power objective. Watching from the side, use the coarse focus to bring the objective close to the slide, then look through the eyepiece and use the coarse focus to move them apart until the cells are in focus.
6. Change to a higher power objective and focus with the fine focus.
7. Calibrate the eyepiece graticule with a stage micrometer at that objective.
8. Count the eyepiece divisions along the length of a cell and multiply by the length of one division.
9. Measure several cells and calculate a mean.Mark scheme
Indicative content:
- Peel a thin layer of onion epidermis (with forceps)
- Place it (flat) on a drop of water on a slide
- Add iodine solution (a stain)
- Lower a coverslip (at an angle, with a mounted needle, to avoid air bubbles)
- Start with the lowest power objective and focus with the coarse focus
- Change to a higher power objective and focus with the fine focus
- Calibrate the eyepiece graticule with a stage micrometer
- Count the eyepiece divisions across the length of the cell and convert to µm (or use the field of view)
- Measure several cells and calculate a mean
Levels of response Level Marks What the answer does 3 5–6 A coherent method that would give valid results: all the important steps are there, in a sensible order. 2 3–4 A method that might not give valid results: most of the important steps are there, but some are missing or out of order. 1 1–2 A method that would not give valid results: some relevant steps are given, but they are not linked into a workable method. In Study Hack a six-mark answer is not given an automatic level: you tick the points your answer makes, then choose the level that fits it.
- No relevant content: 0 marks.
Evaluate
| Source of error | Type | Effect | Improvement |
|---|---|---|---|
| Graticule not calibrated, or calibrated with a different objective | Systematic | Every length is wrong by the same factor: about 10 times too small with no calibration, about 4 times too small calibrated at ×40 and used at ×10. | Calibrate the eyepiece graticule against a stage micrometer with the same objective you measure with. |
| Measuring across the cell at an angle instead of along its length | Systematic | The scale runs out through a side wall, so lengths come out too short. | Turn the eyepiece so the scale lies along each cell, from one end wall to the other. |
| Thick or overlapping tissue | Systematic | Walls of the layer underneath cut across each cell, so lengths are too short and the view is too dark to see the nucleus. | Peel a thin layer, one cell thick, and lay it flat. |
| Lining up the cell walls with the scale (resolution: one division, about 10 µm at ×10) | Random | Each length can be up to about half a division out either way, about 2–5 % of a cell’s length. | Measure several cells end to end in a row and divide by the number of cells, which shrinks the percentage uncertainty. |
| Cells differ in size (biological variation) | Random | One cell may not be typical, so a single length can be well above or below the mean. | Measure more cells, from different parts of the slide, and calculate a mean. |
| A torn cell or an air bubble measured as if it were one cell | Random | A length that does not fit the others (about twice as long, or round and the wrong size). | Lower the coverslip at an angle and lay the tissue flat; check each cell has a wall at both ends, and measure again any that do not fit. |
Repeatable. You measure the same five cells again with the same microscope and get almost the same lengths.
Reproducible. Another student, with a different microscope calibrated for its own objective, gets a similar mean length for cells from the same onion.
Other exam boards
How this practical lines up with other GCSE boards' required or core practicals. Codes only; check your own specification for the exact activity.
| Board | Practical | Match | What is different |
|---|---|---|---|
| AQA | Required practical 1 | This page | — |
| Edexcel | 1.6 | Same activity | — |
Print it
- Blank write-up: the nine-step lab book with the correct table headings, ready to print for a real lesson.
- Specimen lab book: this page's run written up in full, with its exam answers ticked against the mark scheme.
- Printable method card: the practical on one index card. Practise with the cards in Study Hack.
- Practical workbook: a full simulated data set to process on paper, with exam questions and answers.
- Teacher and technician pack: apparatus lists, technician notes and marking points.