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May 2014
BIOLOGY - PRATICAL 1
Topic 1.1.
Introduction to Cells
SKILLS
1) Use of a light microscope to investigate the structure of cells and
tissues, with drawing of cells.
2) Calculation of the magnification of drawings and the actual size of
structures and ultrastructures shown in drawings or micrographs.
SKILL 1 - Use of the light microscope
The light microscope (shown in figure 1), so called because it employs visible light to
detect small objects, is probably the most well-known and well-used research tool in
biology. It is used to observe microscopic organisms or cells and some of their
structures.
Figure 1 shows the microscope with its main parts and its respective functions.
Figure 1
Figure taken from: https://kleinsclasses.wikispaces.com/science_handbook_4.3
Internal Assessment Coversheets and Self-Assessment
Session:
Student Name:
Candidate Number: 0033-___
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May 2014
SKILL 2 - Magnification
The magnification shows the total increase in size from actual sample to the diagram or
viewed sample.
We can look at two types of magnification: the magnification power of the microscope and
the magnification of the drawing.

Magnification power of the microscope is the product of the magnifications of all the lenses
in the system, e.g. 10x ocular lens and 40x objective lens gives microscope magnification 10
x 40 = 400times. So in this example this means that the image that we see at the microscope
 is x 400 bigger that the actual size of the specimen. 

The magnification of the drawing is given by this simple formula:

Magnification = Measured Size of Diagram
Actual Size of Object
In this case you need to know - the actual size of the object you drew, and the actual size of
the drawing (or photo, etc.).
NOTE: Magnification of the drawing is NOT the same as the magnification power of the
microscope. If you need to mention magnification of the microscope, write "as viewed under
microscope at 400X".
The magnification can be indicated by a scale bar, shown below. This bar shows how large a 1
µm sample would appear in the diagram.

A scale bar: |------| = 1 µm 
How to draw a scale bar in a drawing of a picture that you see under the microscope?
1) First you need to do your drawing!
Read the support material about some tips of how to do biological drawings.
Internal Assessment Coversheets and Self-Assessment
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May 2014
2) Then, when your drawing is done, you need to estimate the actual
size of the specimen.
Calculate the field of view diameter (FOV) seen using the low
power using a clear metric ruler. Record the distance in mm and
µm. So for example in the picture the FOV is about 1.3mm or 1300
micrometres.
3) On low power, find an object to view. You could estimate its size
at this power. It may help you to imagine dividing the field into
half, then dividing a half into two quarters, then dividing a quarter
into two eighths, until you get to the size of the organism.
That fraction times the FIELD is the approximate length of the organism. Imagine
that the size of the object you want to see if about ¼ of the field of view. So, in this
example, the organism’s length will be ¼ of the field, that is:
¼ x1300 µm= 325 µm
Suppose you drew something like this
You have calculated the size of your organism and it is actually 325
µm long.
Your drawing of it is 6.2cm long.
On your drawing, therefore, 325µm is represented by
6.2cm=62mm=62 000µm.
The MAGNIFICATION OF THE DRAWING is therefore 62 000/325 = about
190 times (you can write something like "drawn at 190x actual size").

Suppose you want a scale bar to represent 500µm.
Actual size of drawing: 6.2 cm
Actual size of the specimen: 325 µm
6.2 cm--------------325
x
-------------500
x= 9.5 cm
The scale bar will measure 9.5 cm and will represent 500 µm.
Internal Assessment Coversheets and Self-Assessment
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In the exam you might be asked to work with the magnification magnification of drawings. Here are two
examples to help you understand how you need to work them out.
Example 1
If you have a drawing and you want to calculate the magnification or the actual size of the object you can
use this formula:
Example 1:
III
I
II
× 90 000
Question: Calculate the actual length of the cell, showing your working.
1. Choose an obvious length, for example the maximum diameter of a cell. Measure it on the drawing.
________________________________________
2. Use the formula of magnification to calculate the actual length of the cell.
Example 2
Calculate the magnification of the prokaryotic cell. Use the following steps to guide you.
1) Measure the size of the drawing.
2) Calculate the actual size of the cell using the bar scale.
3) Use the formula and calculate the magnification.
Internal Assessment Coversheets and Self-Assessment
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May 2014
PRACTICAL WORK
What are we going to observe at the microscope?
Activity 1 - Elodea
Elodea is a genus of aquatic plants often called the waterweeds. Elodea is native to
North America and is also widely used as aquarium vegetation. It lives in freshwater.
The introduction of some species of Elodea into waterways in parts of Europe,
Australia, Africa, Asia, and New Zealand has created a significant problem and it is
now considered a noxious weed in these areas. An older name for this genus is
Anacharis, which serves as a common name in North America. Silty sediments and
water rich in nutrients favor the growth of American water weed in nutrient-rich lakes.
However, the plants will grow in a wide range of conditions, from very shallow to deep
water, and in many sediment types.
http://en.wikipedia.org/wiki/Elodea
Prelab Questions
1. Name two structures found in plant cells but not animal cells.
2. Name three structures found in plant cells AND in animal cells.
3. What structure surrounds the cell membrane (in plants) and gives the cell support.
4. What is the function of chloroplasts?
Material

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Glass microscope slides 
Cover glass 
Paper towels or tissues 
Elodea 
Distilled Water 
Method
Preparing the slide - Elodea
1. Pick off an entire healthy looking Elodea leaf, with small scissors and place it on the
microscope slide. The tip is the best part to observe.
2. Add a drop of water and a cover glass and observe to the microscope.
Cover glass
Viewing the slide
Slide
1. Make sure that the microscope is plugged in, and turn on the microscope.
2. Make sure that the low power objective is over the hole on the stage to begin.
3. Place your slide on the stage.
Internal Assessment Coversheets and Self-Assessment
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4. Look at you slide and find a cell (s) to focus on. Make sure that you place it in the centre of
the viewing are. Use the coarse adjustment to bring it in as clearly as possible, then use the
fine adjustment to make it as clear as possible. Bring the specimen into focus.
Note: If you lose sight of your specimen at any time just start over on the low power.
5. Repeat the procedure in stage 4 for the medium power objective lens and the higher
power objective lens. Do not use coarse adjustment at high power. Use only fine
adjustment to bring the picture in clearly, and then draw, inside the circle, and label
(outside the circle) what you see.
Post Lab Questions
1. Describe the shape and the location of chloroplasts.
2. As the slide warms from the light of the microscope, you may see the chloroplasts
moving, a process called cytoplasmic streaming. Explain the reason for the cytoplasmic
streaming.
3. Estimate the actual size of the cells viewed through the microscope.
How big are Elodea cells?
1. Measure the field of view diameter (FOV) seen using the low
power using a clear metric ruler. Record the distance in mm
and µm.
2. On low power, find an object to view. You could estimate its size
at this power.
3. Estimate how many of the objects viewed with this lens can fit
across the circular area at its widest.
4. Divide the FOV Diameter for low power by the number that you
estimated in 3.
Internal Assessment Coversheets and Self-Assessment
Session:
Student Name:
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May 2014
Activity 2 – Onion Cells
In this task you have instructions to prepare a slide of a piece of onion epidermis. You will then
estimate the dimensions of the cells and do a statistical analysis to assess the variation of the
dimensions. You will compare the dimension recorded for the outside layer and a layer further
inside.
Material
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Microscope
Onion
Microscope slide
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Coverslip
Mounting needle
Scalpel
Ruler
How to make the onion epidermis slide
1. Cut onion into quarters. Peel off and discard brown skin.
2. Fold the layer from inside to outside. This means the concave side will split and the
convex side will become wrinkled. Tear the pieces apart so the outer epidermis
comes off. (The inner, concave side separates more easily BUT it pulls several layers
and is unsuitable).
3. Cut a piece of the skin measuring about 2x4 mm and put on the microscope slide.
4. Put a drop of water on the specimen and apply a coverslip. Lower the coverslip
carefully using a mounting needle and make sure there are no bubbles.
5. Use paper tissue to remove excess water.
How to estimate the size of the epidermal cells
1. Put the slide under the microscope and get the cells in focus under
low & medium power.
2. Replace the slide with a clear, plastic ruler.
3. The gridlines of 1mm should be in view.
4. Estimate the diameter of the field of view.
5. Remove the ruler and replace the slide.
6. Count the number of cells that fit the field of view lengthways and width-ways in 5
areas.
7. Record the results in a table and process the mean length and width of the cells using
suitable units for each trial/area.
8. Collect the estimated lengths and widths of epidermal cells from your colleagues and
record them in your table.
9. Repeat for an inner layer that is decided together in class.
10. Present the data in a suitable format to show the variation and carry out a statistical
test to find out if there is a significant difference in the dimensions of cells from the
inner and outer shells.
Biological drawings with a scale
1.
2.
3.
4.
View an onion slide at high power (about x400)
Draw 3 or 4 cells on paper.
Measure the length of the cells in your drawing and calculate the mean length.
Calculate the magnification shown in your drawing. (i.e. how many times would the
real mean length fit into the drawn mean length?