Observing Onion Epidermal Cells Under a Light Microscope

This experiment investigates the structure of plant cells by preparing and observing a temporary slide of onion epidermis under a compound light microscope.

It matters because the microscope is the main tool for studying cells, and the skills you practise here (preparing a thin specimen, focusing, observing, drawing and calculating magnification) are used throughout biology.

All plants and animals are made of cells. Plant cells have several structures that animal cells lack.

Chloroplasts contain chlorophyll and carry out photosynthesis. They are found in the green parts of plants that receive light, such as leaves. The cells of an onion bulb grow underground where there is no light, so the epidermis of the bulb has no chloroplasts.

Why a microscope works: light passes through a very thin specimen. The objective lens forms a magnified image, which the eyepiece lens magnifies again. Stains such as iodine absorb more light in some structures and so increase contrast.

Learn each part of the microscope and what it does. Questions often ask you to name a part from a diagram and state its function.

1. Place a drop of water on a clean slide. This keeps the specimen moist and helps it lie flat.
2. Peel a very thin layer of epidermis from the inner surface of a scale leaf. A thin, single layer lets light pass through. A thick piece appears dark and cells overlap.
3. Lay the layer flat on the water. Folds would hide the cells and confuse the image.
4. Add a drop of iodine solution. It stains the nucleus so it shows up clearly.
5. Lower the cover slip slowly at an angle using a mounted needle. This pushes air out and prevents bubbles, which look like dark-edged circles.
6. Blot excess liquid with filter paper. Excess liquid can get onto the objective lens or slide stage.
7. Focus on low power first. Use the coarse knob to find the cells on the 4x objective, then the fine knob to sharpen. Move up to 10x, then 40x, using only the fine knob on 40x.
8. Centre the part you want before changing objective. Higher power shows a much smaller field of view, so a cell that is not centred may be lost.

• Many cells arranged like bricks, roughly rectangular, fitting tightly together with almost no gaps.
• A distinct cell wall around every cell.
• A nucleus in each cell. After iodine staining it appears yellow-brown and is clearest at 40x. It often lies near one side of the cell.
• A thin layer of cytoplasm against the wall (difficult to see at school magnifications).
• No chloroplasts: the cells look pale, not green.

Without a stain the cells have very little contrast, and the nucleus is hard to see. At low power you see many cells but little detail. At high power you see few cells but more detail.

Worked example: a 10x eyepiece with a 40x objective gives 10 x 40 = 400x.

Worked example: a cell measures 30 mm on your drawing and the actual cell is 0.3 mm long. Magnification = 30 / 0.3 = 100x. Always convert to the same units first (1 mm = 1000 micrometres).

Using the field of view: if the field of view at 100x total magnification is 1800 micrometres across and exactly 6 identical cells span it, each cell is 1800 / 6 = 300 micrometres long.

A good conclusion links what you saw to what it shows.

• Observation: brick-shaped cells each with a wall and a nucleus. Conclusion: onion epidermis is made of plant cells with a cell wall, cytoplasm and a nucleus.
• Observation: no green chloroplasts. Conclusion: the bulb epidermal cells do not carry out photosynthesis, because the bulb grows underground without light.
• Observation: nucleus clearer after iodine. Conclusion: a stain increases contrast.

Do not claim more than the evidence supports. For example, a light microscope at these magnifications cannot show the cell membrane or mitochondria clearly, so you should not say you saw them.

Random errors vary unpredictably, such as differences between cells you measure. Systematic errors push results in one direction, such as a wrongly calibrated scale. Procedural mistakes are faults in method, such as folding the specimen. To improve reliability, observe several cells and several slides.

The warnings in the red box above apply here as well. In particular:

• Do not use scalpels, razor blades or iodine solution without trained supervision.
• Carry a microscope with one hand on the arm and the other under the base.
• Never use direct sunlight with a mirror.
• Wash your hands after the practical.

This list shows important practical skills linked to this experiment. It is not a claim about how often any question has appeared in past papers; check your own syllabus and past papers.

• Naming microscope parts and giving their functions.
• Describing the correct order for preparing a wet mount and why each step is needed.
• Calculating total magnification and the magnification of a drawing.
• Calculating actual size from a measurement and magnification.
• Identifying plant cell structures and giving reasons for the absence of chloroplasts.
• Drawing and labelling a few cells.
• Explaining the reasons for precautions (low power first, slow cover slip, thin layer).
• Suggesting sources of error and improvements.
• Writing a conclusion supported by observations.

• Drawings: use a sharp pencil, draw clear single lines (no shading or colouring), make the drawing large, draw only what you see, and label with ruled lines that do not cross. Add a title and the magnification.
• Observations: describe shape, arrangement and structures you can see. Do not list structures you did not observe.
• Calculations: write the formula, show substitution with units, then give the answer with units.
• Method questions: give the step and its purpose (for example, a slow cover slip to avoid air bubbles).
• Errors: name the error, its effect, and one specific improvement.
• Conclusions: link each conclusion to an observation.

Aim to understand the reason behind each step, so you can answer questions in different wording instead of memorising model answers.

• Medicine: doctors and laboratory scientists examine stained cells and tissue samples under microscopes to help identify diseases.
• Agriculture and plant science: plant scientists study cell structure to understand plant growth, disease and responses to the environment.
• Forensic and environmental science: microscopes are used to examine fibres, pollen and microorganisms in samples.
• Education and research: temporary mounts with simple stains are a basic technique for studying biological tissue.

These key points summarise the experiment.

1. Onion epidermis is a single layer of tightly packed, roughly rectangular plant cells.
2. Plant cells have a cell wall, cell membrane, cytoplasm, nucleus and a large central vacuole.
3. Onion bulb cells have no chloroplasts because the bulb grows underground in the dark.
4. Iodine solution stains the nucleus, improving contrast.
5. Always begin on the lowest-power objective and focus with the coarse knob there.
6. On high power use only the fine focus knob.
7. Total magnification = eyepiece magnification x objective magnification.
8. Field of view gets smaller as magnification increases, so centre the specimen before changing objective.
9. Lower the cover slip slowly at an angle to avoid air bubbles.
10. A thin single layer is used so that light can pass through and cells do not overlap.
11. Actual size = measured (image) size divided by magnification.
12. Biological drawings use a sharp pencil, clear single lines, ruled label lines, a title and the magnification.
13. Plant cells have a regular shape and a cell wall; animal cells do not have a cell wall.
14. Conclusions must be linked to observations, not to things you did not see.

Use these links to revise the ideas in this experiment and practise related skills.

• Lesson: Plant cells
• Lesson: Basic cell structures
• Lesson: Cell functions
• Lesson: Measuring accurately
• Glossary: Photosynthesis
• Experiment: Acid-Base Titration: Finding the Concentration of Hydrochloric Acid
• Experiment: Investigating Ohm's Law

Practise again with new questions