Sketch - diagram of onion cells as seen under a
A scale bar has been marked on the drawing,
allowing the size of a cell to be estimated.
I've just labelled the cell wall, nucleus and
I've added a scale showing a length of 500
µm (500 micrometres,
µ means 10-6 in standard form).
The average size of an onion cell is ~200
µm, and the average size of the onion cell nucleus is ~6
The size of each cell and its nucleus do vary a bit from cell to cell.
How to work out a scale bar for your drawing by measuring the
average real size of a cell
Note: In calculations I've used the symbol
≡ to indicate equivalence.
You fix a clear plastic ruler on the microscope slide
and clip both of them onto the stage.
Select a magnification of at least X100 and refocus
the microscope to obtain another clear image of the cells.
Adjust the slide so that the cells line up with the
scale and count the number of cells along a line of 1 mm.
A micrometre is 1 millionth of a metre, 1.0 x 10-6
m (in standard form).
≡ 1000 micrometres
≡ 1/1000th of a metre
Therefore 1 mm
≡ 10-3/10-6 = 1000
≡ 10-3 m
Suppose the length of 5 onion cells = 1 mm, average
length 0.2 mm.
µm, so the average
length of one onion cell = 1000/5 =
Once you know the average length of one cell you can
use it to calculate the length of the scale bar on you diagram using the
formula below. Suppose you want a 500
µm scale bar on your drawing.
scale bar length (for 500
µm) = 500 x drawing length of cell (µm)
actual length of cell (µm)
e.g suppose each cell you have drawn was on average 2.0 cm long on paper.
This is 20 mm, which equals 20 x 1000 = 20,000
µm and the actual real average length is 200
scale bar length = 500 x 20,000
200 = 50,000
1000 = 50, so the scale bar on my drawing would be
50 mm or 5 cm long.
AND you can calculate the actual magnification of
your drawing compared to the specimen's real size.
The magnification of your drawing = average length of
drawn cell (µm)
average real length of cell (µm)
The drawn cell is 20 mm long
20 x 1000 = 20,000
µm, real length of cell is 200
magnification of drawing = 20,000
200 = 100X
For more on estimating cell size see
magnification & measuring the size of a cell using a graticule & stage
e.g. above is a typical plant cell x
200 to x 500 magnification.
Note that is a simple black and white
Introduction to plant and animal cell structure and
function gcse biology revision notes
Before examining root tip cells, you need to
squash the root tip to break down the cellulose to free the cells and it
is possible to observe mitosis in root tip cells where new growth is
Other examples of what you can see
with a powerful light microscope
Cell division - mitosis
You can stain chromosomes
so that they are made visible under a microscope using a 'squashed'
layer of cells under the cover slip.
This enables you to actually visually follow the
In the school laboratory you can
use cells from a plant root tip (e.g. garlic cloves) to
observe the various stages of the cell cycle.
A few drops of the stain (dye
molecule solution) is added to the plant tissue sample and the
mixture squashed on the microscope slide so that you can see the
chromosomes more easily.
You can clearly observe the
chromosomes being pulled apart in a cell and the subsequent
formation of two cells as the mitosis cell division proceeds -
Observing the stomata, guard cells,
xylem and phloem in the leaves of a plant
It is possible to observe stomata and guard cells
on the leaf surface of a plant using a light microscope.
You can also do a comparison of the stomata density
of the upper and lower surfaces of a leaf.
Both surfaces of the leaf are coated with a layer of
clear nail varnish, allowing the leaf to dry after each coating.
Place a piece of clear cellophane tape over the top
of the coated leaf and then slowly and carefully peel away the tape
and varnish layer.
An impression of the leaf's surface is left on the
Stick the cellophane tape onto a microscope slide
and view under the light microscope.
You can sketch what you see and count the number of
stomata and guard cells in a given area from the cellophane
You should find there are more stomata on the
underside of the leaf than on the upper side.
To explain this see
Transport systems and gas exchange
With a light microscope, it is also possible to observe
the structures of xylem and phloem cellular structure in very
thin sections of a plant's stem.
You mount a short section of stem vertically in a
beaker solution of eosin dye - which diffuses up the stem staining
the xylem red.
You carefully a cut a very thin cross-section of the
stem, mounted on a slide and viewed under a light microscope.
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