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Better Equipped Practical Teaching Guides
A complete classroom guide for measuring biological cells using a light microscope, eyepiece graticule and stage micrometer, with apparatus, calibration method, slide preparation, calculations, risk assessment, troubleshooting, technician notes and exam support.
Teacher note: This resource supports practical teaching and equipment planning. Teachers should adapt the procedure and risk assessment to match their exam board specification, local laboratory rules and departmental policies.
This A Level Biology practical develops core microscopy skills by asking students to prepare a biological specimen, calibrate an eyepiece graticule using a stage micrometer, measure cell dimensions and process the data accurately. It links directly to cell ultrastructure, mathematical skills in biology, practical endorsement evidence and exam-style evaluation.
A light microscope magnifies specimens using objective and eyepiece lenses. Magnification increases image size, but resolution determines how much detail can be distinguished. To measure a cell accurately, students use an eyepiece graticule. The graticule scale has no fixed unit value until it is calibrated against a stage micrometer at the magnification being used.
The value of one eyepiece graticule division changes when magnification changes. Students must recalibrate for each objective lens before using the graticule to measure cells.
Ensure microscopes are clean, lenses focus smoothly, light sources work, graticules are visible and stage micrometers are handled carefully to avoid scratching.
The stage micrometer has a precisely known scale. A typical stage micrometer has 1 mm divided into 100 divisions, so one stage division equals 0.01 mm, or 10 micrometres. The eyepiece graticule must be aligned with this known scale so the value of one eyepiece division can be calculated.
If 40 eyepiece divisions align with 10 stage divisions:
10 stage divisions = 100 micrometres
1 eyepiece division = 100 ÷ 40 = 2.5 micrometres
Examination tip: Always show all working in calibration calculations. Students often lose marks for missing unit conversions or failing to state units.
Students should record the number of eyepiece divisions occupied by each cell, multiply by the calibration factor and calculate a mean cell size. Typical onion epidermal cells are approximately 50-80 micrometres wide and 70-120 micrometres long. Values far outside this range should prompt students to re-check calibration, unit conversions and whether the cells are folded or distorted.
| Cell number | Eyepiece units | Calibration factor (micrometres per division) | Cell size / micrometres |
|---|---|---|---|
| 1 | 32 | 2.5 | 80 |
| 2 | 31 | 2.5 | 77.5 |
| 3 | 33 | 2.5 | 82.5 |
| 4 | 30 | 2.5 | 75 |
| 5 | 34 | 2.5 | 85 |
Mean = total cell sizes ÷ number of measurements. Example: 80 + 77.5 + 82.5 + 75 + 85 = 400; mean = 400 ÷ 5 = 80 micrometres.
Magnification = image size ÷ actual size. Convert units first. For example, 40 mm = 40,000 micrometres; 40,000 ÷ 80 = x500.
| Structure | Approximate size | Scale |
|---|---|---|
| Ribosome | 20-30 nm | Nanometre scale |
| Virus | About 100 nm | Nanometre scale |
| Bacterium | 1-5 micrometres | Micrometre scale |
| Mitochondrion | 1-2 micrometres | Micrometre scale |
| Red blood cell | 7-8 micrometres | Micrometre scale |
| Onion cell | 70-100 micrometres | Micrometre scale |
| Human egg cell | About 120 micrometres | Micrometre scale |
| Problem | Likely cause | Solution |
|---|---|---|
| Cannot see cells | Specimen folded or too thick | Flatten the sample and use a thinner tissue layer. |
| Blurry image | Fine focus not used or lens dirty | Refocus carefully and clean lenses with lens tissue only. |
| No scale visible | Graticule misaligned or not inserted | Check the eyepiece graticule and rotate the eyepiece. |
| Measurements inconsistent | Wrong magnification or calibration factor | Recalibrate at the magnification being used. |
| Air bubbles mistaken for cells | Coverslip lowered too quickly | Lower coverslip at an angle and demonstrate bubble appearance. |
Higher magnification always means higher resolution.
Correction: magnification enlarges the image; resolution determines how much detail can be distinguished.
Electron microscopes can observe living cells.
Correction: vacuum conditions and specimen preparation usually kill living specimens.
Conclusion: Cells can be measured accurately by calibrating an eyepiece graticule against a stage micrometer. Repeated measurements improve reliability, while careful calibration and correct unit conversions are essential for accurate biological data.
Introduction: 10 mins | Calibration demonstration: 15 mins | Slide preparation: 10 mins | Measuring cells: 20 mins | Evaluation and plenary: 10 mins
Overall risk: low. Schools should complete their own risk assessment according to local procedures. Eye protection is recommended when using stains, and students should handle glass slides, coverslips and microscopes carefully.
| Hazard | Risk | Control measures |
|---|---|---|
| Glass microscope slides | Cuts from broken slides or sharp edges | Handle carefully, check slides for cracks and dispose of broken glass in a sharps container. |
| Glass coverslips | Cuts from breakage during handling | Handle by the edges and lower gently using forceps or a mounted needle. |
| Iodine solution | Skin or eye irritation and staining | Wear eye protection, use small quantities, avoid contact and clean spills immediately. |
| Methylene blue stain | Irritation and staining of skin or clothing | Wear eye protection, use small volumes and wash hands after use. |
| Human cheek cells, if used | Low-level biological contamination risk | Use individual sterile swabs, students sample only themselves, dispose of swabs appropriately and disinfect benches. |
| Microscope electrical supply | Electric shock from damaged cables or liquid spills | Inspect equipment before use, keep liquids away and report damage immediately. |
| Microscope handling | Dropping equipment causing injury or damage | Carry with two hands, one on the arm and one supporting the base, and keep away from bench edges. |
| Eye strain | Temporary discomfort or headache | Encourage breaks, correct seating posture and appropriate eyepiece adjustment. |
| Skill assessed | Example evidence |
|---|---|
| AO2 application | Uses the microscope safely and correctly. |
| AO3 analysis | Processes measurements and calculates means accurately. |
| AO3 evaluation | Identifies errors and suggests improvements. |
| Maths skills | Converts between mm, micrometres and nm correctly. |
| Practical technique | Prepares slides carefully with minimal air bubbles. |
The graticule has divisions but no fixed unit value. It must be compared with a stage micrometer to find the real size represented by each division.
Changing objective lens changes the apparent size of the image, so the value of each eyepiece division also changes.
Fine focus sharpens the image precisely and reduces the risk of pushing the objective lens into the slide.
Cells vary naturally in size. Measuring several cells and calculating a mean improves reliability and reduces the impact of anomalies.
Explore more A Level Biology practical guides, equipment support and laboratory planning resources from Better Equipped.
This guide was written and reviewed by Better Equipped's technical team, drawing on experience supplying practical science equipment to schools, colleges, laboratories and science departments throughout the UK. Our technical team include ex-school laboratory technicians and are here to support schools, colleges and laboratories across the UK. If you have feedback on this guide, please contact us.
Last reviewed and updated: July 2026
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