Biology Practicals

How to Prepare a Wet Mount Specimen/Microscopic Slide

Step One - Obtain a clean microscope slide.

Step Two - Place a drop of liquid on the slide. This is the “wet” part of the wet mount. The liquid used depends on the type of cell being viewed:

•  If examining a plant cell, tap water can be used. 

• If examining an animal cell, physiological saline (or contact lens solution) must be used, because if plain water is used, the cell will explode from osmotic pressure. Unlike plant cells and bacteria, animal cells have no cell wall to structurally support them.

Step Three - Obtain the specimen to be used. Some introductory biology classics for viewing include:

• Skin of an onion bulb: In order to view the cells, a very thin layer of skin must be obtained. Take a single layer of onion and bend it towards the shiny side. After it snaps, pull gently, and a transparent layer of skin, similar to Scotch tape, will appear.

• Elodea leaf: Elodea leaves are two cell layers thick. The cells in one layer are smaller than the cells in the other, so elodea leaves can be used to better understand a microscope's depth of field.

• Cheek cells: Human epithelial cells can be obtained by gently rubbing a toothpick on the inside of the mouth, and then swirling the toothpick in the physiological saline on the slide.

• Pond water: Obtaining some water from a pond makes wet mount preparation a breeze, since the water and the specimens are both included.

Step Four - Place the specimen into the liquid on the slide.

Step Five - If the specimen is transparent, such as onion skin or check cells, stain should be added to increase contrast. A drop of iodine or methylene blue is commonly used. Do not use stain if viewing photosynthetic cells (which already appear green due to chlorophyll), or living organisms, such as protozoans in pond water (stains will kill them).

Step Six - Place a cover slip over the specimen. This will sandwich the specimen between the slide and the cover slip. To avoid trapping air bubbles, set one edge of the cover slip on the slide, and then let the rest of the cover slip drop. As it drops from one side to the other, air will be pushed out, and this will reduce the number of bubbles in the wet mount. 

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