Required Practical: Microscopy (AQA GCSE Biology): Revision Note

Using a light microscope

• Aim: To use a light microscope to observe, draw and label a selection of plant and animal cells, including a magnification scale

• This practical involves:

  • using a light microscope to make observations of biological specimens and produce labelled scientific drawings

  • including a magnification scale

Preparing a microscope slide

• Specimens must be prepared on a microscope slide to be observed under a light microscope

• This must be done carefully to avoid damaging any biological specimen

• The most common specimens to observe under a light microscope are cheek cells (animal cells) and onion cells (plant cells)

• Stains are used to highlight structures within cells – methylene blue is used to stain cheek cells, iodine for onion cells

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Using a microscope

• Understanding the main features of a light microscope is essential in order to use it correctly

• Always hold the microscope by the arm when moving it around the lab, and always start an observation with the lowest-powered objective lens

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To use a microscope to look at a specimen:

1. Clip the slide carefully onto the stage.

2. Ensure the lowest-powered objective lens is over the slide.

3. Use the coarse adjustment knob to bring the stage up just below the lens.

4. Look down the eyepiece and gradually move the stage downwards using the coarse adjustment knob. Stop when the image is roughly in focus.

5. To bring the image into focus, adjust the fine-adjustment knob until a clear image is obtained.

6. To observe the image with a higher magnification, change the objective lens to a higher power and readjust the stage using the coarse and fine adjustment knobs.

Biological drawings

• Producing biological drawings of what can be seen under the microscope is a key skill

• The key is not to try to be too artistic with microscopy drawings – they are supposed to be scientific, so the rules must be followed

Rules for biological drawing

• Always draw what you see with a sharp pencil using clear, unbroken lines

• All structures drawn should be in proportion

• Label all features using straight, uncrossed lines

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To calculate the size of a single cell

• Clip a ruler or eyepiece graticule on top of the slide

• View the ruler and slide under the ×100 objective lens and adjust focus to obtain a clear image

• Line the cells along 1 mm and count the number of cells that fit across that length