Preparing a Temporary Slide (Wet Mount)

What a wet mount is

A wet mount is a temporary slide made by placing a thin specimen in a drop of liquid on a glass slide and covering it with a coverslip. It is used to view cells, such as onion epidermal cells or human cheek cells, under a light microscope.

The slide is called temporary because the specimen dries out or deteriorates after a short time and the slide cannot be stored.

Aim

To prepare a temporary wet-mount slide of plant cells (such as onion epidermis) and animal cells (such as human cheek cells) and to observe them under a light microscope, applying the manipulative skills of handling apparatus, handling specimens, and drawing specimens.

Materials and apparatus

• A clean glass slide and coverslip
• A dropper and distilled water
• Forceps and a mounting needle
• A scalpel or new razor blade for cutting a thin section
• Iodine solution (for plant cells) and methylene blue (for animal cells)
• A specimen, such as a piece of onion or a clean cotton bud for cheek cells
• Filter paper or paper towel
• A light microscope

Method

1. Clean a glass slide and coverslip with a soft cloth to remove dust or grease.
2. Place one or two drops of water in the centre of the slide, using a dropper.
3. Using forceps, place a thin section or peel of the specimen into the drop of water, spreading it flat.
4. Add a drop of stain, such as iodine solution for plant cells or methylene blue for animal cells, at the edge of the specimen.
5. Hold a coverslip at an angle of about 45 degrees with one edge touching the slide near the specimen, then lower it slowly and steadily using a mounting needle.
6. Remove any excess liquid at the edge of the coverslip with a piece of filter paper or paper towel.

Why stains and angled lowering are used

Many cell structures, such as the nucleus and cytoplasm, are almost colourless and difficult to see clearly under a microscope. A stain adds colour and contrast, making structures like the nucleus stand out.

Lowering the coverslip at an angle, rather than dropping it flat, allows air to be pushed out from one side as the coverslip settles, which prevents air bubbles from being trapped between the coverslip and the slide.

Expected observations

A correctly prepared slide gives a clear, evenly lit field with the cells spread in a single layer and no trapped air bubbles. After staining, the nucleus of each cell stands out against the paler cytoplasm.

Conclusion

A thin, well-spread specimen mounted in stain and covered without trapping air gives a field in which individual cells and their nuclei can be identified. Comparing the two specimens shows the main difference between plant and animal cells: the plant cell has a fixed, rectangular shape given by its cell wall, while the animal cell has no wall and takes an irregular, rounded shape.

Paper 3-style questions

Context. A student prepared a temporary slide of onion epidermal cells stained with iodine and viewed it under a light microscope.

Question 1 (observation and inference). State one observation of the onion cells and make one inference from it.

Model answer. Observation: the cells are rectangular and fit together with a clear boundary around each cell. Inference: plant cells have a cell wall that gives them a fixed, regular shape.

Question 2 (recording a drawing). Describe how to make a good labelled biological drawing of the cells.

Model answer. Use a sharp pencil and draw clear, continuous lines with no shading. Draw a few representative cells large enough to see clearly, then label the cell wall, cytoplasm and nucleus using straight, ruled label lines that do not cross, and add a title and the magnification.

Question 3 (tabulation). Design a table to compare an onion cell with a cheek cell.

Model answer. Draw a table with a first column for the feature (cell wall, overall shape, nucleus, chloroplasts) and two more columns headed onion cell and cheek cell, then enter present or absent, and the shape, for each feature.

Question 4 (inference from an error). Under the microscope the student sees dark circular outlines with thick black edges among the cells. Explain what these are and how to avoid them.

Model answer. They are air bubbles trapped under the coverslip, not cells. They are avoided by lowering the coverslip slowly at an angle so that air is pushed out to one side instead of being trapped.

Safety

• Glass slides and coverslips are thin and break easily, so hold them by the edges and put any broken glass in a sharps or broken-glass container.
• The scalpel or razor blade used to cut a section is very sharp; cut on a tile, away from your fingers.
• Iodine solution and methylene blue stain skin and clothing and are mild irritants, so avoid contact and rinse off any splashes.
• Use a clean cotton bud for your own cheek cells, do not share it, and dispose of it as instructed; wash your hands afterwards.

Common mistakes

Source:SRC-DSKP-EN

Frequently asked questions