Measuring cell size

Cell size can be measured using an eyepiece graticule. The graticule has a scale ruled on it.

You must find out the distance measured for each division of the graticule. You can then use the graticule to measure cells. The distance will be different for each objective.

To do this, you will use a stage micrometer. You will use this to calibrate the eyepiece graticule. Once it's calibrated, you can use the eyepiece graticule every time you use the microscope.

1. Place a stage micrometer on the stage of the microscope.
2. Line up one of the divisions on the eyepiece graticule with a fixed point on the stage micrometer.
3. Count the number of divisions on the eyepiece graticule that correspond with a set measurement on the stage micrometer.
4. Calculate the distance in micrometres of one division on the eyepiece graticule.

The distance of 250 μm on the stage micrometer lines up against two divisions at 10 and 61 on the eyepiece graticule.

61 − 10 = 51 divisions on the eyepiece graticule are equivalent to 250 μm on the stage micrometer.

One division on the eyepiece graticule is equivalent to \(\frac{250}{51} μm\) on the stage micrometer: = 4.9 μm (to two significant figures).

Therefore one division is equal to 4.9 μm.

Using the same calibrated eyepiece graticule to measure a cell:

The width of the cell highlighted = 52 – 40 = 12 eyepiece graticule divisions.

The real width of the cell is 12 × 4.9 μm = 59 μm (to two significant figures).

Question

What would be the length of a plant cell, to two significant figures, that was 35 divisions on this graticule?

Reveal answer

170 μm

Each eyepiece graticule division is 4.9 μm

The real width of the cell is 35 × 4.9 μm = 171.5 μm

To two significant figures, this is 170 μm

Question

How many graticule divisions would a single celled organism that was 240 μm take up?

Reveal answer

49

Each graticule division is 4.9 μm

An organism that measured 240 μm would take up \(\frac{240}{4.9}\) divisions = 49 divisions