Required Practical 01: Microscopy

Question 1

Describe how you would prepare a microscope slide using onion tissue (5).

Answer 1

1. Peel off thin sheet of epidermal layer using tweezers.
2. Place onion sample on a drop of water on microscope slide.
3. Add few drops of iodine solution to the onion sample.
4. Slowly lower a cover slip onto sample using mounting needle to ensure no air bubble are trapped.
5. Dab excess liquid from the edge using paper towels.

Question 1

Why is a thin sample of tissue required?

Answer 1

Allows light to pass through so internal structures can be observed.

Question 2

Why is water added to the slide before the sample is mounted? (2)

Answer 2

• Allows the sample to be suspended between the slide and the cover slip.
• Ensures the cover slip stays in place.

suspended: when the sample is floating in a liquid

Question 3

Why is iodine solution added to stain the cells? (2)

Answer 3

• Adds contrast - reacts with starch in plant cells and turns blue-black.
• Allows internal structures (e.g. nucleus) to become more visible.

Question 4

How would you measure the length of a cell viewed under the microscope?

Answer 4

Measurement (µm) = graticule divisions × magnification factor

Question 5

How would you calculate the magnification of an image?

Answer 5

Magnification = image cell length ÷ actual cell length

Question 6

What safety precautions should be taken during this experiment? (2)

Answer 6

• Wear safety goggles when handling iodine solution.
• Take care when using iodine solution to avoid staining and ingestion.

Question 7

Describe how to use a microscope to look at a specimen in a prepared slide (7).

Answer 7

1. Clip the slide carefully onto the stage.
2. Ensure the lowest-powered objective lens is over the slide.
3. Use the coarse focussing dial to bring the stage up just below the lens.
4. Look down the eyepiece.
5. Gradually move the stage downwards using the coarse focussing dial until the image is roughly in focus.
6. Adjust the fine focussing dial until a clear image of the specimen is obtained.
7. Make a labelled drawing of a few of the cells you can see, including any features (eg. cell wall, nucleus) and write down the magnification.

Question 8

State the rules for biological drawing. (5)

Answer 8

• Always draw what you see with a sharp pencil using clear, unbroken lines.
• All structures drawn should be in proportion.
• Clearly label all features using straight, uncrossed lines.
• Have a clear title and a scale of magnification.
• Make the drawing as large as possible - aim for about half the page with drawings.

Question 9

How would you calculate the size of a single cell when looking through a microscope? (4)

Answer 9

1. Clip a ruler or eyepiece graticule on top of the slide.
2. View the ruler and image under the x100 objective lens and adjust the focus to obtain a clear image.
3. Line the cells along 1mm and count the number of cells that fit inside that length.
4. As 1mm = 1000µm, divide 1000µm by the number of cells that fit inside that length (1mm) to obtain the size of a single cell.