Measuring cells

Syllabus reference:

This symbol in the corner of a slide indicates a picture, diagram or table taken from your text book

To accurately measure the size of cellular structures we need a suitable scale:

Ideally, we need a scale we can see directly alongside the cells we are observing:

Start by putting a ruler under the microscope:

Appearance of ruler at medium magnification

Appearance of tissue at medium magnification

Estimating cell size at medium magnification
1mm
5
1 ÷ 5 = 0.2mm
12
1 ÷ 12 = 0.083mm

Mean length of cells = 0.2 x 1000 = 200µm
1mm = 1000µm
Mean width of cells = 0.083 x 1000 = 83µm

The graticule
a more suitable ‘ruler’ for measuring cells
The slide graticule:
The eyepiece graticule:

The stage graticule shows true lengths
stage graticule

The eyepiece graticule has regular divisions.
These need to be calibrated for each magnification
stage graticule
eyepiece graticule
e.g. x100

stage graticule
eyepiece graticule
e.g. x400
The eyepiece graticule has regular divisions.
These need to be calibrated for each magnification

The eyepiece graticule
remains constant no matter
what magnification the cells
are viewed at.

The eyepiece graticule
remains constant no matter
what magnification the cells
are viewed at.

The eyepiece graticule
remains constant no matter
what magnification the cells
are viewed at.

Eyepiece & stage graticules
Low magnification
High magnification

Figure 4.3
Stage micrometer viewed at x100 magnification.
The total length of the micrometer is 1mm
Therefore, 1 division on the
eyepiece graticule represents
1000 ÷ 94 = 10.6 μm
at this magnification.

Figure 4.1
Cells of onion epidermis as viewed at x100 magnification with a graticule in the eyepiece of the microscope
We know that at this
magnification, 1 division
on the eyepiece graticule
represents 10.6 μm
Therefore the total length
of the eyepiece graticule
represents 10.6 x 100 = 1060μm
at this magnification
In the two columns covered
by the graticule there is an
average of five cells in the
length of the graticule
Therefore the average
length of one cell is
1060 ÷ 5 = 212μm

remember that each
division here is 10μm
Figure 4.4
Part of the stage micrometer viewed at x400 magnification
so the length shown
by the bracket is 240μm
Therefore, 1 division on the
eyepiece graticule represents
240 ÷ 90 = 2.67 μm
at this magnification.

Figure 4.2
Cells of onion epidermis as viewed at x400 magnification with the same graticule in the eyepiece
The length of the cell covered
by the graticule is 98 divisions,
therefore the length of this cell
is 2.67 x 98 = 262μm
We know that at this magnification,
each division of the eyepiece graticule
represents 2.67μm

We now have two measurements for the length of an onion cell; 212μm and 262 μm.
Which of these is the more accurate estimate of the length of onion epidermal cells?
The answer from Q. 2 [212 μm]
because this is a mean of several cells.
Only one cell was measured in Q.3, and this one may not be representative.

Estimating cell width. Figure 4.5.
Cells of the onion epidermis as viewed at x100 magnification with a graticule in the eyepiece of the microscope
Remember the total length
of the eyepiece graticule
represents 1060μm
at this magnification
There are approximately
thirteen cells in the
length of the graticule
Therefore the average
width of one cell is
1060 ÷ 13 = 81.5μm

Figure 4.6.
Cells of the onion epidermis as viewed at x400 magnification with the same graticule in the eyepiece of the microscope
Remember, we know that at this
magnification, each division of
the eyepiece graticule
represents 2.67μm
Here, two cells span 62 divisions
on the eyepiece graticule. This
represents 2.67 x 62 = 165.5 μm
Therefore the average
width of one cell is
165.5 ÷ 2 = 82.8μm

Comparison of estimates from Ws1 with those from Ws3
The answers on Ws1 are means of several samples
Whilst the answers on Ws3 are based on a single sample
The variety of onion used may be different
The samples may have come from onions at different stages of growth

Calculating actual size:

Calculating magnification & actual size:

Calculating actual size:

Calculating magnification:

Calculating magnification & actual size: