B2, Topic 1.1
Name ……………………………………………………
Class …………
Date ………………….
Observation of cells under a microscope
Aims
Most human, animal and plant cells have many features in common: a nucleus, a cell
membrane, cytoplasm, mitochondria and ribosomes. Plant and algal 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. In this
activity, you will use a light microscope to observe the features of different animal, plant
and algal cells.
Safety
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Take care with sharp scalpels
Dispose of used cotton buds and prepared cheek cell slides in disinfectant provided
Take care with cover slips as they are very sharp especially if they break
Wash your hands if you get any chemicals on them.
Equipment and materials
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light microscope with low and high power objective lenses
binocular microscope
various prepared slides of different cells
transparent plastic ruler
micrometer eye piece or graticule
microscope slides
cover slips
methylene blue stain
dilute iodine solution
sterile cotton bud
mounted needle
forceps
scissors
scalpel
dropping pipettes
blotting paper or filter paper
beaker of disinfectant
onion, green alga, rhubarb, sprouting mung beans, etc.
© Oxford University Press 2014 http://www.oxfordsecondary.co.uk/acknowledgements
This resource sheet may have been changed from the original.
1
B2, Topic 1.1
Name ……………………………………………………
Class …………
Date ………………….
Method
Setting up the light microscope
Before looking at your own slides of cell preparations, you will need to set up your
microscope.
1 Many microscopes have a built-in light source, but if the one you are using does not
then you need to arrange the mirror so that light is directed through the lens systems.
2 Place a prepared slide on the centre of the stage of the microscope. Fix it in place
using the clips.
3 Turn the rotating nosepiece until the low power objective lens clicks into place. Slowly
rotate the coarse adjustment until the object on the slide comes into focus. You may
need to use the fine adjustment to sharpen the focus.
4 If you wish to view the object at greater magnification, rotate the nosepiece until the
high power objective lens clicks into place. Any adjustment needed to focus the object
should only be done using the fine adjustment. Take care not to allow the objective to
touch the slide as this might damage the lens and break the slide.
5 Before removing the slide, rotate the nosepiece again until the low power objective
lens clicks into place again.
6 If at any point you are unsure of the procedure, check with your teacher. Microscopes
are expensive pieces of equipment and can be damaged if treated roughly. Try to
avoid touching the surfaces of the lenses as these can be scratched easily.
Looking at cheek cells
Your teacher will give you a prepared slide of some human cheek cells or you may be
asked to make your own slide, following the instructions given below.
1 Take a cotton bud from a newly-opened pack.
2 Move the cotton bud over the inside of the cheek on one side of the mouth and along
the outer lower side of the gum.
3 Smear the cotton bud over a small area of a clean microscope slide.
4 Place the used cotton bud immediately into a small volume of disinfectant in a
suitable container.
5 Add methylene blue stain from a dropper pipette onto the smear and cover with a
cover slip. The best way of doing this is to hold the cover slip in your left hand so that
one side touches the slide and then, resting the opposite side on a mounted needle
held in your right hand, lower it gently until it is flat. This should avoid trapping too
many air bubbles under the cover slip. The scrapings and the stain will spread out
under the cover slip.
6 If the stain is unevenly distributed or too dark, place a drop of water at one side of the
cover slip and hold a small piece of blotting paper or filter paper on the opposite side
of the cover slip. The stain can then be drawn through, making the cells easier to
observe.
7 Place the slide on the stage of the microscope, making sure the bottom of the slide is
dry. Use the low power objective to locate some cells and then turn to the high power
objective to look at one cell in detail.
© Oxford University Press 2014 http://www.oxfordsecondary.co.uk/acknowledgements
This resource sheet may have been changed from the original.
2
B2, Topic 1.1
Name ……………………………………………………
Class …………
Date ………………….
8 Make a drawing of one of the cheek cells and label it as fully as you can.
9 Dispose of your slide in the disinfectant provided.
Looking at plant and algal cells
You are provided with one of the layers of an onion bulb, some sprouting mung beans
and some strands of a filamentous green alga.
1 Make a slide using the inner epidermis of the onion bulb leaf by pulling away a strip
using a pair of forceps. Cut a small piece of epidermis, about 5 mm across, and place
it on a clean microscope slide. Add a drop of dilute iodine solution and place a cover
slip on top, using the method described above. Look at the slide under the
microscope, using low power to locate the cells and then the high power. Make a
drawing of a small group of cells, labelling the structures.
2 Sprouting mung beans can first be viewed using a binocular microscope and the
presence of root hair cells observed. Using a scalpel, carefully scrape a few cells from
the root hair and mount them on a clean microscope slide as described above. You
can see the structure of the root hair cells using the high power of the light
microscope. Make a drawing of one of the cells and label the structures you can see.
3 Place a few strands of the filamentous green alga on to a clean microscope slide, add
a drop of water and cover with a cover slip. There is no need to add dilute iodine
solution to this as the chloroplasts and cell walls will show up clearly under the light
microscope.
Measuring the size of cells
It may be possible to measure the dimensions of the cells you are looking at if your light
microscope has a micrometer in the eyepiece. This is a scale on a glass which is
inserted into the eyepiece lens. You can measure the length and breadth of cells in
eyepiece units or your teacher may tell you what each division on the scale represents if
the eyepiece has been calibrated.
Alternatively, you can place a transparent ruler across the microscope stage directly
underneath the low power objective lens. Focus on one of the lines and then count how
many millimetre divisions there are across the field of view (this is the diameter of the
field of view). You can work out the magnifying power of your microscope by multiplying
the magnifying power of the eyepiece lens by the magnifying power of the objective lens.
Results
You can record the observations you make on the drawings of the cells. Use a sharp
pencil and make clear lines, avoiding any shading. If possible, add the dimensions of the
cells, the magnification of your drawing or a scale.
Questions
1 Why is it important to avoid trapping air bubbles under the cover slip?
2 Describe the structures which you can distinguish easily.
3 Why does the use of the methylene blue stain and the dilute iodine solution make the
cells easier to see?
© Oxford University Press 2014 http://www.oxfordsecondary.co.uk/acknowledgements
This resource sheet may have been changed from the original.
3
B2, Topic 1.1
Name ……………………………………………………
Class …………
Date ………………….
4 Make a list of the similarities and differences between the different cells that you have
looked at.
5 What structures are difficult to see? Why are they difficult to see?
6 Why is it important to add a scale or magnification to a drawing or a photograph?
Discussion
Discuss the advantages and disadvantages of using stains to show up the features of
plant and animal cells.
© Oxford University Press 2014 http://www.oxfordsecondary.co.uk/acknowledgements
This resource sheet may have been changed from the original.
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