Cell Structure » Required Practical: Looking at Cells Under a Microscope

The aim of the practical

In this required practical you use a light microscope to observe, draw and label a selection of plant and animal cells. A common pair is onion epidermis (a plant tissue) and cheek cells (an animal cell). A magnification scale must be included on your drawing.

Plant cells show a cell wall and sometimes a permanent vacuole. Animal cells show a nucleus inside a thin cell membrane. You met these parts in Animal and Plant Cells. Here the focus is on the skill of looking at them properly.

Key terms:

• Epidermis: a thin layer of cells that covers the surface of a plant or animal.
• Stain: a coloured liquid that soaks into cells so that their parts are easier to see.
• Slide: a thin piece of glass that holds the specimen.
• Coverslip: a very thin square of glass or plastic placed over the specimen.

Parts of the light microscope

Total magnification is the eyepiece magnification multiplied by the objective magnification. An eyepiece of x10 with an x10 objective gives x100.

Preparing a slide

The specimen is the thing you are looking at. It must be thin enough for light to pass through it. If it is too thick, you see a dark blur.

Onion epidermis

1. Cut a small piece from the inside of an onion and peel off a very thin, see-through layer.
2. Place the layer flat on a clean slide.
3. Add a drop of iodine solution as the stain.
4. Hold the coverslip at an angle with one edge touching the liquid. Lower it slowly with a mounted needle.

Lowering the coverslip slowly stops air bubbles being trapped. Bubbles look like dark circles and hide the cells.

Cheek cells

1. Gently rub the inside of your cheek with a clean cotton bud.
2. Smear the cotton bud on a clean slide.
3. Add a drop of stain, for example methylene blue, then lower a coverslip over it.

Cheek cells are colourless and almost invisible without a stain. The stain makes the nucleus show up clearly.

Focusing the microscope

1. Turn the turret to the lowest power objective lens.
2. Place the slide on the stage and clip it in place.
3. Look from the side and use the coarse focus wheel to bring the objective lens close to the slide, without touching it.
4. Look through the eyepiece and turn the coarse focus wheel slowly the other way until the cells come into view.
5. Use the fine focus wheel to make the image sharp.
6. Move the slide so the cells you want are in the middle of the view.
7. Only then switch to a higher power objective lens and sharpen the image with the fine focus wheel only.

Always start on low power because the field of view (the circle of the slide you can see) is larger, so it is much easier to find the cells. Never use the coarse focus wheel on high power, because the lens could hit the slide and crack the coverslip.

Making a scientific drawing

A scientific drawing is a record of what you see. It is not a work of art. Follow these rules:

• Use a sharp pencil and a clean eraser. Draw single, clear lines with no sketching and no shading.
• Do not colour in the drawing.
• Make the drawing large. Use at least half the space on the page.
• Draw only what you see. Show a few cells, not the whole view, and keep the proportions correct.
• Use a ruler to draw label lines. They must touch the part and must not cross each other.
• Write labels horizontally, away from the drawing.
• Give the drawing a title that says what it is, and write the magnification beside it.

Magnification scale and relative size

Your drawing must show how much bigger it is than the real cell. Work out the magnification of the drawing and write it next to the title, for example "x300". You can also draw a scale bar, which is a short line labelled with a real length such as 50 µm.

The spec also asks you to use estimations to judge relative size. You do not need exact measurements. Compare one length with another, for example "one onion cell is about three times as long as it is wide". That relative size is the same on the drawing and in the real cell.

Use a ruler to record the length of the drawing, and the area of a cell if you are asked, in millimetres, and always write the units.

Exam-style question

A student looked at onion epidermis cells under a light microscope.

(a) Describe how the student should prepare the slide. [3 marks]

(b) Why is it important to start by focusing on low power? [1 mark]

(c) The student made a drawing 72 mm wide of a cell that is 0.12 mm wide. Calculate the magnification. [2 marks]