MIKE MILNER: Hello, and welcome to this core practical video. My name is Mike Milner, and
I'm a biology teacher at Reddam House school in Berkshire. I'm going to take you through
the core practical looking at cells so you can see the apparatus, perform the magnification
calculations, and produce appropriate labelled diagrams from your observations.
In the first part of this experiment, we're going to calculate the diameter of the field
of view and the diameter of some plant root cells. To carry out this experiment, you will
need the following-- a microscope with a light source. This could be built in, or the microscope
may have an external lamp and mirror-- some prepared slides, a transparent ruler.
Switch on the microscope and make sure that it is set to the low power lens, which is
the smallest lens. We're going to look at the ruler under the microscope first to demonstrate
how wide the field of view is. Look through the eyepiece lens, and focus the image using
the coarse adjustment wheel, which is the bigger of the two. You can see that the field
of view on the low power or the 4 times lens is 4.5 millimeters.
You can adjust the focus now with the fine focusing wheel, which is the smaller of the
two. We can look at the ruler with the medium power or times 10 lens. We can say that the
field of view under this lens is 1.8 millimeters. For the high power, or the 40 times lens of
this microscope, we can use this calculation to show that the field of view is 0.45 millimeters
across.
We are now going to look at some cells on preprepared slides of a plant root. Focus
on the slide using the coarse focus and then the fine focus wheel. View these cells on
the highest magnification. You are now looking at plant root cortex cells under the high
power of this microscope. You can make out the following features-- the cellulose cell
wall, possibly the cytoplasm, and the vacuole.
We can see that there are approximately eight whole cells across this field of view. The
field of view on high power, remember, is 0.45 millimeters across. From our field of
view measurements, we can see that the cells are therefore 0.056 millimeters, or 56 micrometers,
in diameter.
Using a piece of paper and a pencil, you can draw the cells. Practice looking down the
eyepiece lens with one eye and looking at the paper with the other. The drawing should
be done with a sharp HB pencil, clean lines to be drawn carefully, and no shading. Draw
only what you see.
Key features can be identified with label lines, and a scale bar can be added to show
the actual size of the cell. You can work out the magnification of your drawing by dividing
the diameter of your cell drawing by the actual diameter of the cell. In this case, my cell
drawing diameter is 62 millimeters, and the actual diameter is 0.056 millimeters. This
makes the magnification of my image times 1,100 to two significant figures. Make sure
that the units that are drawn and the actual cells are the same, in this case, both in
millimeters.
In this second part of the experiment, we are going to look at cells from the inside
of our cheek. For this part of the experiment, you will need the microscope, disinfectant
solution, methylene blue solution, cotton bud, slide, a cover slip, pipette, distilled
water, and a toothpick. As this involves putting a cotton bud inside our mouth and then using
the tip on the slide, we need to be careful to wear gloves and dispose of the cotton hygienically
and promptly in the strong disinfectant solution at the end of the experiment.
Use the cotton bud to gently stroke the inside of your cheek. You only want to collect loose
cells, so do not scratch the inside of your mouth too hard. Carefully rub the cotton bird
in a droplet of water on the slide, and then add a drop of methylene blue solution. This
will stain the cells to make them easier to see.
Place a cover slip at the edge of your cell sample on the slide 45 degrees. Lower the
cover slip gently using a toothpick. Try to avoid trapping air bubbles. Look at these
cells under medium power and then high power. This is what the cells look like on high power.
You can clearly see the nucleus stains dark
Blue. We can use the method outlined earlier to estimate the size of the cells and the
magnification of any drawing we make of them. These cells appear similar in size to the
root cells, so are also approximately 56 micrometers in diameter.
Change the microscope to low power. Carefully remove the slide from the microscope stage
and place immediately in the disinfectant solution.
In the third part of this experiment, we are going to look at epidermal cells from an onion.
We will need a microscope, an onion, cutting tile, a scalpel, a toothpick, iodine solution,
a slide and cover slip, and some soapy water.
Carefully peel off a layer of epidermal cells. You can use your nail to gently lift this
layer of cells. Place the layer of cells under the microscope slide. Use the scalpel to trim
the onion sample to size if necessary, and then add a couple of drops of iodine solution.
Place the cover sleep at the edge of your cell sample on the slide at 45 degrees. Lower
the cover gently using a toothpick. Again, try to avoid trapping air bubbles. Remove
any excess solution with a tissue, which should be placed immediately in a bin allocated for
disposal. Look at the cells under the low power and then high power. This is what the
cells look like on high power. You can see the cells joined together by their cell walls.
We can use the method outlined earlier to estimate the size of the cells and the magnification
of any drawing we make of them. There are approximately 1.8 cell lengths across the
high power field of view, so they can be calculated to be 250 micrometers in length. Change the
microscope to low power, carefully remove the slide from the microscope stage, and place
immediately in the wash solution provided.