Prepare Slides of Animal and Plant Cells

Aim

This experiment prepares temporary mounts of a plant cell (onion epidermis) and an animal cell (human cheek epithelial cell) for observation under a light microscope, so that their structures can be compared directly.

Variables

• Manipulated variable, the type of cell examined (plant cell from onion epidermis, or animal cell from cheek epithelium).
• Responding variable, the structures observed in each cell, such as the presence or absence of a cell wall and chloroplasts, the shape of the cell, and the size of the vacuole.
• Controlled variables, the staining technique used for each cell type, the magnification of the microscope used for observation, and the intensity of light passing through the specimen.

Materials and Apparatus

• a fresh onion
• a clean cotton bud (for collecting cheek cells)
• methylene blue stain
• iodine solution
• glass microscope slides and cover slips
• forceps
• a mounted needle
• a light microscope
• blotting paper

Procedure

1. Using forceps, peel a thin, transparent layer of epidermis from the inner surface of an onion scale.
2. Place the epidermis flat in a drop of iodine solution on a clean glass slide.
3. Lower a cover slip onto the specimen at an angle using a mounted needle, to avoid trapping air bubbles.
4. Remove any excess iodine solution with blotting paper, then observe the slide under low power and then high power of the light microscope.
5. Gently scrape the inside of the cheek with a clean cotton bud to collect a small sample of epithelial cells.
6. Smear the sample thinly onto a clean glass slide.
7. Add a drop of methylene blue stain onto the smear.
8. Lower a cover slip onto the specimen at an angle, avoiding air bubbles, and observe under low power and then high power.

Expected Results

Observation under the microscope shows clear differences between the plant cell and the animal cell, which can be recorded in a comparison table.

Conclusion

The onion epidermis cells show a regular, brick-like shape with a clear cell wall and a large central vacuole, confirming the typical structure of a plant cell. The cheek cells show an irregular, rounded shape with no visible cell wall and little or no visible vacuole, confirming the typical structure of an animal cell.

This supports the aim of comparing plant and animal cell structure using temporary mounts.

Paper 3-style questions

These original questions practise the science process skills examined with slide preparation and the microscope.

Question 1 (variables). State the manipulated, responding and one controlled variable for this comparison. Model answer. Manipulated: the type of cell (onion epidermis or cheek epithelium).

Responding: the structures seen, such as the presence of a cell wall and the shape of the cell. Controlled: the magnification used, kept the same so the two cells are compared fairly.

Question 2 (inference). Under the microscope a cell has a regular box shape, a clear outer wall and one large central space. Infer which type of cell it is and give a reason.

Model answer. It is a plant cell, because the cell wall gives it a fixed regular shape and the large central space is the vacuole; an animal cell has neither.

Question 3 (measuring and using numbers). A cheek cell measures 40 mm across in a drawing made at a magnification of ×400. Calculate its real width.

Model answer. Real width = image size ÷ magnification = 40 mm ÷ 400 = 0.1 mm = 100 µm.

Question 4 (communicating). Describe two rules for making a good biological drawing of the cells observed. Model answer. Use clean, continuous lines with no shading, draw what is actually seen in the correct proportion, and add label lines that do not cross, each ending on the structure it names, with the magnification stated.

Safety

• Handle glass slides and cover slips by their edges, because they are thin and break easily, and clear away any broken glass safely.
• Use a clean cotton bud for the cheek sample, use it only once, and place it in the bin provided rather than reusing it.
• Keep iodine solution and methylene blue off the skin and clothes, as they stain, and wipe up any spills at once.
• Wash your hands after collecting cheek cells and after handling the stains and specimens.

Common Mistakes

Source:SRC-DSKP-EN

Frequently asked questions