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

  • 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

  • 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.

Method

Setting up the light microscope

Before looking at your own slides of cell preparations, you will need to set up your microscope.

1Many 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.

2Place a prepared slide on the centre of the stage of the microscope. Fix it in place using the clips.

3Turn 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.

4If 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.

5Before removing the slide, rotate the nosepiece again until the low power objective lens clicks into place again.

6If 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.

1Take a cotton bud from a newly-opened pack.

2Move the cotton bud over the inside of the cheek on one side of the mouth and along the outer lower side of the gum.

3Smear the cotton bud over a small area of a clean microscope slide.

4Place the used cotton bud immediately into a small volume of disinfectant in a suitable container.

5Add 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.

6If 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.

7Place 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.

8Make a drawing of one of the cheek cells and label it as fully as you can.

9Dispose 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.

1Make 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 5mm 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.

2Sprouting 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.

3Place 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

1Why is it important to avoid trapping air bubbles under the cover slip?

2Describe the structures which you can distinguish easily.

3Why does the use of the methylene blue stain and the dilute iodine solution make the cells easier to see?

4Make a list of the similarities and differences between the different cells that you have looked at.

5What structures are difficult to see? Why are they difficult to see?

6Why 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

This resource sheet may have been changed from the original.

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