Cell structure - EdexcelAnalysing biological specimens using microscopes

Investigating 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

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 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.

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 significant figures)

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).