How do you do the microscopy required practical?

What does the microscopy practical investigate?

There is no hypothesis to test here. The practical is about technique: you use a light microscope to look at plant and animal cells, then draw and label what you see, with the magnification or a scale bar on the drawing. Onion skin is the usual plant sample. For animal cells you might use a prepared slide, or cells from inside your cheek if your school does that version.

It is one of AQA's required practicals in both GCSE Biology and Combined Science: Trilogy. It belongs to the cell biology topic, next to the parts of a cell and the reason an electron microscope shows far more detail than a light microscope.

What equipment do you need?

You need a light microscope and the kit to make a slide of onion cells. Iodine solution stains the cells, so the nucleus and the edges of each cell are easier to see.

• A light microscope with a lamp (or a mirror)
• Microscope slides and coverslips
• A piece of onion
• Forceps, to peel off a thin layer of onion skin
• A mounted needle, for lowering the coverslip
• Iodine solution and a dropping pipette
• Paper towel, to soak up extra liquid
• A sharp pencil, plain paper and a ruler for the drawing
• Eye protection

What is the method?

In short: make a thin, stained slide of onion cells, find them with the lowest-power lens, then switch to a higher power and draw a few cells with the magnification. These steps follow the usual school method.

• Put a drop of water in the middle of a clean microscope slide.
• Peel a thin layer of skin from the inside of a piece of onion with forceps. It needs to be one cell thick, so light can pass through it.
• Lay the onion skin flat on the water without folds, then add a drop or two of iodine solution to stain it.
• Rest one edge of a coverslip on the slide and lower it slowly with a mounted needle, so air bubbles are pushed out rather than trapped. Soak up any extra liquid with paper towel.
• Clip the slide onto the stage and turn to the lowest-power objective lens.
• Looking from the side, use the coarse focusing knob to bring the lens close to the slide without touching it.
• Now look through the eyepiece and turn the coarse focus slowly the other way until the cells appear. Sharpen the image with the fine focus.
• For more detail, switch to a higher-power objective and refocus using only the fine focus.
• Draw a few cells in pencil, label the parts you can see, and write the magnification or add a scale bar.

What are the variables?

Strictly, there are none: you are observing cells, not testing whether one thing affects another. If a question turns it into a comparison, such as onion cells against cheek cells, the variables look like this.

How do you work out and present the results?

Your result is the drawing, so there is no graph. Use a sharp pencil and one clear line for each edge, with no shading. Draw what is on your slide, not a textbook cell. Label with ruled lines that don't cross, and include the magnification or a scale bar.

The calculation is magnification = image size ÷ real size, where the image is your drawing or a photo. Put both sizes in the same unit first: 1 mm = 1000 µm.

Worked example: a cell in your drawing is 50 mm long and the real cell is 0.25 mm long, so the magnification is 50 ÷ 0.25 = ×200. Rearranged, real size = image size ÷ magnification. A cheek cell 20 mm long in a photo taken at ×400 is really 20 ÷ 400 = 0.05 mm long, which is 50 µm.

For the microscope itself, multiply the magnifications of the two lenses: a ×10 eyepiece with a ×40 objective lens gives a total magnification of ×400.

How do you make it accurate and safe?

Most of the quality comes from the slide. A layer one cell thick lets light through, and the stain adds contrast so the nucleus stands out. Air bubbles are easy to mistake for cells, which is why the coverslip goes on at an angle. Always find the cells on low power first, where you can see a wider area, then move up.

Iodine solution can irritate your eyes and stains skin, so wear eye protection. Slides and coverslips are thin glass: hold them by the edges and tell your teacher about any breakage. If your microscope has a mirror instead of a lamp, never point it at direct sunlight, which could damage your eyes. If you sample cheek cells, use a fresh cotton bud and put it straight into disinfectant afterwards.

What do exam questions ask about it?

Microscopy questions lean on the reasons behind each step: why the sample must be thin, why you add a stain, why the coverslip goes on at an angle, why you start on low power. Calculations use the magnification formula, rearranged whichever way the question needs, often with a switch between mm and µm along the way. An improvement question might ask how to see finer detail. The answer is a higher-power lens or, for the smallest parts of a cell, an electron microscope. A longer question may ask you to describe how to prepare a slide and view it, with the steps in order.

An exam-style question we wrote: "A student's drawing of an onion cell is 45 mm long. The real cell is 0.15 mm long. Calculate the magnification of the drawing." The answer is 45 ÷ 0.15 = ×300. Write the division down as well as the answer, because the working can still earn a mark if the final number has a slip in it.

Common mistakes

• Mixing units in the magnification formula. Convert first: 1 mm = 1000 µm.
• Adding the lens powers instead of multiplying them. A ×10 eyepiece with a ×40 objective is ×400, not ×50.
• Drawing chloroplasts in onion cells. Cells from an onion bulb have none, so draw only what is on your slide.
• Shading, or sketching with lots of short strokes. Use one clean pencil line for each edge.
• Using the coarse focus on high power, which can push the lens into the slide. Use the fine focus only.