Lesson 1, Topic 1
In Progress

More of Microscopy

It’s all very well known what microscopes do – you also have to know how to actually use one.

You need to prepare your slide

If you want to look at a specimen (e.g. plant or animal cells) under a light microscope, you need to put it on a microscope slide first. A slide is a strip of clear glass or plastic onto which the specimen is mounted.

Here’s how to prepare a slide to view onion cells.

  1. Add a drop of water to the middle of a clean slide.
  2. Cut up an onion and separate it out into layers. Use tweezers to peel off some epidermal tissue form the bottom of one of the layers.
  3. Using the tweezers, place the epidermal tissue into the water on the slide.
  4. Add a drop of iodine solution. Iodine solution is a stain. Stains are used to highlight objects in a cell by adding colour to them.
  5. Place a cover slip (a square of thin, transparent plastic or glass) on top. To do this, stand the cover slip upright on the slide, next to the water droplet. Then carefully tilt and lower so it covers the specimen. Try not to get any air bubbles under there – they’ll obstruct your view of specimen.

Use a light microscope to look at your slide

To look at your prepared slides, you need to know how to use a light microscope:

  1. Clip the slide you’ve prepared onto the stage.
  2. Select the lowest-powered objective lens (i.e. the one that produces the lowest magnification).
  3. Use the coarse adjustment knob to move the stage up to just below the objective lens.
  4. Look down the eyepiece. Use the coarse adjustment knob to move the stage downwards until the image is roughly in focus.
  5. Adjust the focus with the fine adjustment knob, until you get a clear image of what’s on the slide.
  6. If you need to see the slide with greater magnification, swap to higher- powered objective lens and refocus.

Draw your observation neatly with a pencil

  1. Draw what you see under the microscope using a pencil with a sharp point.
  2. Make sure your drawing takes up at least half of the space available and that it is drawn with clear, unbroken lines.
  3. Your drawing should not include any colouring or shading.
  4. If you are drawing cells, the subcellular structures should be drawn in proportion.
  5. Remember to include a title of what you were observing and write down the magnification that it was observed under.
  6. Label the important features of your drawing (e.g. nucleus, chloroplasts), using straight, uncrossed lines.

You can work out the real size of a cell by counting the number of cells in 1 mm of the sample. You can work out the magnification of your drawing using the formula: magnification = length of drawing of cell ÷ real length of cell. So here magnification = 33 mm ÷ 0.3 mm = x 110.

Practical work

Looking at cells

Plant cells – preparing a slide of onion epidermis cells

The onion provides a very useful source of epidermal plant tissue which is one cell thick, making it relatively easy to set up as a temporary slide. The onion is made up of fleshy leaves. On the incurve of each leaf there is an epidermal layer which can be peeled of Figure (a).

  • Using forceps, peel a piece of epidermal tissue from the incurve of an onion bulb leaf.
  • Place the epidermal tissue on a glass microscope slide.
  • Using a scalpel, cut out a 1 cm square of tissue (discarding the rest) and arrange it in the centre of the slide.
  • Add two to three drops of iodine solution. (This will stain any starch in the cells and provides a contrast between different components of the cells.)
  • Using forceps, a mounted needle or a wooden splint, support a coverslip with one edge resting near to the onion tissue at an angle of about 45° Figure (b).
  • Gently lower the coverslip over the onion tissue, trying to avoid trapping any air bubbles. (Air bubbles will reflect light when viewing under the light microscope, obscuring the features you are trying to observe.)
  • Leave the slide for about 5 minutes to allow the iodine stain to react with the specimen. The iodine will stain the cell nuclei pale yellow and the starch grains blue.
  • Place the slide on to the microscope stage, select the lowest power objective lens and focus on the specimen. Increase the magnification using the other objective lenses.
  • Make a large drawing of one cell and label the following parts: cell wall, cell membrane, cytoplasm, nucleus.

An alternative tissue is rhubarb epidermis Figure(c). This can be stripped off from the surface of a stalk and treated in the same way as the onion tissue. If red epidermis from rhubarb stalk is used, you will see the red cell sap in the vacuoles.

(a) peel the epidermis from the inside of an onion bulb leaf
(b) place the epidermis on to the slide, adding 2-3 drops of iodine solution and carefully lowering a coverslip on to it.
(c) alternatively, peel a strip of red epidermis from a piece of rhubarb skin