3️⃣PAG 1: Microscopy

Question 1

Magnification formula

Answer 1

Magnification =

Observed cell length / actual cell length

Question 2

Microscopy practical safety (4)

Answer 2

• Take care when handling glass slides as they are very fragile.
• Avoid getting iodine solution on your skin.
• Wear eye protection.
• Take care not to break the slide by moving the objective lens too far downwards.

Question 3

Equipment

Answer 3

• light microscope with low and high power objective lenses
• microscope slide and cover slip
• acetate grid
• selection of samples: onion, Elodea, filamentous algae
• dilute iodine solution
• dropper pipette
• scalpel, scissors, forceps
• mounted needle
• blotting paper or filter paper
• range of prepared animal cells including cheek cells and red blood cells
• range of prepared plant cells including onion epidermal cells and leaf palisade cells

Question 4

Setting the scene

Most animal and plant cells have many features in common: (5)

Plant cells also have __ __ __ and many have __ and a __ __ filled with __.

__ and __ are difficult to observe using a light microscope and are often not seen. In this activity, you will use a light microscope to observe the features of different plant and cells.

Answer 4

Setting the scene

Most animal and plant cells have many features in common: a nucleus, a cell membrane, cytoplasm, mitochondria, and ribosomes.

Plant cells also have cellulose cell walls and many have chloroplasts and a permanent vacuole filled with sap.

Mitochondria and ribosomes are difficult to observe using a light microscope and are often not seen. In this activity, you will use a light microscope to observe the features of different plant and cells.

Question 5

Method - part 1, preparing your slide

6 steps

Answer 5

1 Collect a sample of the cell you want to observe.
2 Remove the inner skin of a layer of onion using forceps, or a thin layer or Elodea or filamentous algae using the scalpel.
3 Place the thin slice onto a clean glass slide. Use your forceps to keep the onion skin flat on the glass slide.
4 Using a pipette, add one or two drops of dilute iodine solution on top of the onion skin or slice of algae or plant.
5 Hold the coverslip by its side and lay one edge of the cover slip onto the microscope slide near the specimen.
6 Lower the cover slip slowly so that the liquid spreads out.

Question 6

Method part 2 - setting up microscope

Before you can look at the cells on the slide, you will need to set up your microscope.

Most microscopes have a built-in light source, but if the one you are using does not then you need to (what?)

Answer 6

Most microscopes have a built-in light source, but if the one you are using does not then you need to arrange the mirror found underneath the stage so that light is directed through the lens system.

Question 7

Method part 2 - setting up microscope

6 steps

Answer 7

1 Move the stage to its lowest position.
2 Place a prepared slide on the centre of the stage and fix it in place using the clips.
3 Select the objective lens with the lowest magnification.
4 Look through the eyepiece and turn the coarse focus adjustment until the cells on the slide come into view.
5 Turn the fine focus adjustment to sharpen the focus so the cells can be clearly seen.
6 If you wish to view the object at greater magnification to see more detail, repeat the above steps using a higher magnification lens.

Question 8

How to calculate the total magnification?

The magnification of the microscope is calculated using the formula:

Answer 8

Total magnification = Eyepiece lens magnification x Objective lens magnification

Question 9

Method part 3 - how to calculate cell size (6 steps)

+ alternative method

Answer 9

How to calculate cell size
7. At a low magnification, place a transparent ruler across the microscope stage.
8 Measure the width of the field of view using the ruler markings.
9 Place the slide to be viewed into position. Increase the magnification until individual cells can be viewed.
10 Calculate the new width of the field of view at this magnification, using the formula:
Field of view=(Original magnification )/(New magnification) x Original field of view
11 Count the number of cells visible across the field of view.
Calculate the length of a single cell using the following formula:
Length of cell= (Field of view)/(Number of cells)

Alternatively, you can use scale grids to calculate cell size. These are grids that have been produced by photocopying graph paper onto acetate. The higher the magnification you use, the smaller the grid size you need as the field of view is smaller.
1 Place a sheet of acetate over your microscope slide. (Your teacher will tell you the distance between the grid lines.)
2 Measure the magnified length of one of the cells by lining it up with the grid lines.

Question 10

Results (3)

Answer 10

For each slide you observe you will need to produce a labelled diagram of the cells you see.
You will need to note on your diagrams the total magnification you were using to observe the cells and any measurements you have taken of the cells.
Estimate the area of any cells you have observed.

Question 11

Risk assessment (2)

Answer 11

Methylene blue is an irritant. Avoid contact with skin. Broken or cracked slides constitute a broken glass hazard.