Aim of the microscopy required practical

Use a light microscope to observe, draw and label plant and animal cells, and include a magnification scale (AQA topic 1).

Why peel a thin layer of onion epidermis?

It's one cell thick, so light passes through it and you see separate cells instead of a dark blur.

What does iodine solution do on an onion slide?

It stains the cells so the nucleus and cell wall show up more clearly.

Why lower the coverslip at an angle?

To push air out so no air bubbles get trapped; bubbles can be mistaken for cells and block the view.

Which objective lens do you start with?

The lowest power one. It has the widest field of view, so the cells are easiest to find.

Coarse vs fine adjustment knob

The coarse knob moves the stage or lens a long way to find the image; the fine knob makes small changes to sharpen it.

Total magnification of a light microscope

Eyepiece lens magnification × objective lens magnification. A ×10 eyepiece and ×40 objective give ×400.

1 mm = 1000 µm. Convert both sizes to the same unit before you divide.

Rules for a biological drawing

Sharp pencil, clear continuous lines, no shading, labels with ruled lines that don't cross, a title and the magnification.

A line on a drawing or image labelled with the real length it represents, such as 50 µm.

Why can't a light microscope show ribosomes?

Its resolution is too low. Ribosomes are seen with an electron microscope, which has a much higher resolution.

GCSE Biology Required Practical: Microscopy flashcards

In 1665 Robert Hooke published Micrographia, a book of drawings made through his microscope. One of them shows a thin slice of cork, full of tiny boxes. Hooke called them "cells", comparing them to the cells of a honeycomb (Latin cella means a small room). He was looking at the empty walls of dead plant cells, not living ones, but the name stuck. The microscopy required practical asks you to do what Hooke did: prepare a thin specimen, focus on it, and make an accurate drawing.

This lesson covers the practical for AQA GCSE Biology Topic 1 (Cell biology). It's also useful for Combined Science.

What the practical asks you to do

Use a light microscope to observe, draw and label a selection of plant and animal cells. Your drawing must include a magnification scale. Onion epidermis is the usual plant specimen. For animal cells you may use cheek cells or a prepared slide.

Preparing an onion slide

Put a drop of water on a clean glass slide.
Peel a thin layer of epidermis from the inside of an onion piece with forceps. It should be one cell thick, so light can pass through it.
Lay it flat in the water without folds.
Add a drop of iodine solution. The stain makes the nucleus and cell wall easier to see.
Lower a coverslip at an angle, using a mounted needle, so air is pushed out. Trapped air bubbles look like dark-ringed circles and are easy to mistake for cells.

Wear eye protection: iodine solution is an irritant and stains skin and clothes. Follow your teacher's safety instructions.

Using the microscope

Clip the slide onto the stage.
Select the lowest power objective lens. It shows the widest area, so the cells are easiest to find.
Use the coarse adjustment knob to bring the image into view, then the fine adjustment knob to make it sharp.
Switch to a higher power objective, then refocus with the fine adjustment knob only.

Total magnification = eyepiece lens × objective lens. A ×10 eyepiece with a ×40 objective gives ×400.

Drawing what you see

A biological drawing records what is there, not what you expect to be there.

Use a sharp pencil and clear, continuous lines.
Don't shade or colour.
Draw cells in proportion, and big enough to show detail.
Label structures with ruled lines that don't cross.
Add a title and the magnification.

Working out magnification

Magnification = size of image ÷ size of real object.

Units trip people up. Convert both sizes to the same unit first: 1 mm = 1000 µm (micrometres). If a cell is 0.05 mm across and your drawing of it is 25 mm across, the magnification is 25 ÷ 0.05 = ×500.

You can rearrange the equation to find the real size: real size = image size ÷ magnification. A scale bar does the same job visually: a line labelled with the real length it represents.

Light and electron microscopes

Magnification is how much bigger the image is. Resolution is how close two points can be and still be told apart. Electron microscopes have much higher magnification and resolution than light microscopes. That's why the detail of mitochondria and ribosomes was only seen once electron microscopes were in use.

Common exam mistakes

Dividing the wrong way round. Image size goes on top. A magnification smaller than 1 is a sign you've flipped it.
Mixing units. Dividing millimetres by micrometres gives an answer 1000 times out.
Adding the lens magnifications. ×10 and ×40 make ×400, not ×50.
Shading drawings. Outlines only.
Expecting chloroplasts in onion cells. Onion epidermis grows out of the light, so it has none. Its cells do have a cell wall, which animal cells lack.

Frequently asked questions

Why do you use iodine in the microscopy practical?

Iodine solution stains the cells, so the nucleus and cell wall stand out against the paler cytoplasm.

Why do you start on the lowest magnification?

The lowest power objective shows the widest field of view, so it's easiest to find the cells before zooming in.

How do you stop air bubbles under the coverslip?

Lower the coverslip at an angle with a mounted needle, so the water pushes the air out as it spreads.

What is the formula for magnification?

Magnification = size of image ÷ size of real object, with both sizes in the same unit.

Try it

Say each answer before you flip the card. Then put yourself in Hooke's place in 1665: you've seen empty boxes in dead cork. What would you have tested next?

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