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Practical skill

INTRODUCTION:

Making and viewing slides is an extremely important skill in Life Sciences. We make slides to

view under microscopes so that we can get up close and personal to microscopic structures,

which is beneficial to understanding life at a cellular level.

Some specimens can be viewed directly underneath a microscope, however many look a lot

better with a drop of liquid on the microscope slide. This is called a wet mount.

The liquid is usually water but can also be a stain. The liquid helps support the specimen, it fills

the space between the cover slip and the slide allowing light to pass through easily, it prevents

specimens from drying out and if it is a stain that is being used it makes some cellular structures

(such as cell walls and nuclei) become more visible.

INSTRUCTIONS:

You are going to get the opportunity to make a wet mount slide using onion epidermal

cells and then to view these cells underneath a light microscope.

As you will be using expensive equipment, make sure that you know how to handle and

use microscopes otherwise let your teacher know so that they can help you.

AIM: To make a wet mount using onion epidermal cells and then viewing these slides

underneath a light microscope.

MATERIALS:

Onion

Scalpel or knife

Dissecting needle

Forceps

Microscope slide

Cover slip

Dropper or straw

Tissue paper / filter paper

Water

Iodine

Light microscope

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METHOD:

Step 1 Cut the onion into quarters. Using forceps, remove and discard the first few inner layers

of one of the quarters.

Step 2 Pull and cut off a small piece of membrane-like epidermis lining the inner layer of the

onion.

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Step 3 Complete the following steps quickly or the onion epidermis will dry out. Using a

dropper, place a droplet of water on the microscope slide. You could also place a drop

of iodine solution on the microscope slide.

Step 4 Using forceps, place the piece of onion epidermis in the droplet of water/iodine. Take

care that it remains flat.

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Step 5 Gently lower a cover slip at an angle onto the droplet using a dissection needle. This

helps to prevent air bubbles. Avoid leaving fingerprints on the cover slip.

Step 6 If necessary, press gently in the middle of the cover slip to get rid of any air bubbles. Do

not use bare fingertips.

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Step 7 Place tissue paper or filter paper along the edge of the cover slip to draw off any excess

water.

Step 8 Examine the wet mount under the microscope.

Step 9 When you have finished, wash and dry the slide and cover slip.

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RESULTS:

Fig. 1 A micrograph taken of onion epidermal cells prepared as a wet mount using iodine solution.

Use the space below to do a biological drawing of onion epidermal cells as seen under your

light microscope. (Try taking a picture of your slide through the microscope using your cell

phone.)

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