B1 More on Microscopy (page 13)

Question 1

If you want to look at a specimen (e.g. plant or animal cells) under light microscope, what do you do?

Answer 1

You need to put it on a microscope slide first. A slide is a strip of clear glass or plastic onto which the specimen is mounted.

Question 2

How do you prepare a slide to view onion cells?

Answer 2

1) Add a drop of water to the middle of the clean slide

2) Cut up an onion and separate it out into layers. Use tweezers to peel off some epidermal tissue from the bottom of one of the layers.

3) Using the tweezers, place the epidermal tissue into the water on the slide.

4) Add a drop of Iodine solution. (Iodine is a stain - stains are used to highlight objects in a cell by adding colour to them).

5) Place a cover slip (a square of thin, transparent plastic or glass) on top. To do this, stand the cover slip upright on the slide, next to the water droplet. Then carefully tilt and lower it so it covers the specimen. Try not to get any air bubbles under there as they will obstruct your view of the specimen.

(see picture on page 13)

Question 3

How do you look at your prepared slide under the light microscope?

Answer 3

1) Clip the slide you’ve prepared onto the stage.
2) Select the lowest-powered objective lens (i.e. the one that produces the lowest magnification).
3) Use the coarse adjustment knob to move the stage up to just below the objective lens.
4) Look down the eyepiece. Use the coarse adjustment knob to move the stage downwards until the image is roughly in focus.
5) Adjust the focus with the fine adjustment knob, until you get a clear image of what’s on the slide.
6) If you need the slide with greater magnification, swap to a higher-powered objective lens and refocus.

Question 4

List what is on a Light Microscope?

see diagram on page 13, (you need to know the list and where they are on the diagram for your exam)

Answer 4

Eyepiece
Coarse adjustment knob
Fine adjustment knob
High and low power objective lenses
Stage
Light

(picture is on page 13, where all pieces are)

Question 5

What is the next stage regarding observations of cells under a microscope?

Answer 5

Draw what you see under the microscope using a pencil with a sharp point.

Make sure your drawing takes up at least half of the space available and that it is drawn with clear, unbroken lines.

Your drawing should not include any colouring or shading.

If you are drawing cells, the subcellular structures should be drawn in proportion.

Remember to include a title of what you were observing and write down the magnifcation that it was observed under.

Lable the important features of your drawing (e.g. nucleus, chloroplasts), using straight, uncrossed lines.

see onion cells diagram of a drawing example on page 13.

Question 6

How can you work out the real size of a cell?

Answer 6

by counting the number of cells you can see along 1mm (see page 234). You can work out the magnification of your drawing using this formula:

Magnification = length of drawing of cell ÷ real lengh of cell. So here, magnification = 33mm ÷ 0.3mm = x 110 (see page 13 for onion cell diagram).

Question 7

Why might you add stain to the sample on a microscope slide? (1 mark)

Answer 7

To highlight objects within the sample by adding colour to them (1 mark)