Core practical 1 - analysing biological specimens using microscopes

Aims

• To use a light microscope to examine animal and/or plant cells.
• To make observations and draw scale diagrams of cells.

Method

Rotate the objectives so that the low power objective lens, eg ×10, is in line with the stage.

Risks

• Care must be taken when looking down the microscope that the illumination is not too bright as eye damage could be caused.
• Care when using microscope stains as some are harmful.
• Care when handling coverslips and microscope slides as broken glass could cut skin.

Drawing the image

A low power magnification is used to show the arrangement of cells in a tissue. This includes regions of the tissue but not individual cells.

A diagram of the slide under high power will be a detailed image of a part of the specimen which shows individual cells.

Measuring cell size

Cell size can be measured using an eyepiece graticule, which is a glass or plastic disc fitted into the eyepiece. 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. This is a glass slide etched with an accurate scale on it. You will use this to 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.

In the diagram:

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

1 division on the eyepiece graticule is equivalent to 250/51 μm on the stage micrometer:

= 4.9 μm (to two )

Using the calibrated eyepiece graticule to measure a cell:

The width of the cell shown = 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?

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?

49

Each graticule division is 4.9 μm

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

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