Core Practical: Drawing specimen from microscope (Edexcel GCSE Biology Combined Science): Revision Notes

Core Practical: Drawing specimen from microscope

Tools Needed

Method

Resolution- how well a microscope distinguishes 2 points close together

Variables

Independent Variable: The type of specimen (plant or animal cells) being observed under the microscope.

Dependent Variable: The size of the cells observed.

Control Variables:

• The magnification used during observation.
• The staining method applied to the cells.
• The microscope settings, such as the brightness of the light source.

Drawing the Image

Determine the Magnification:

• Check the magnification of the objective lens and the eyepiece. Multiply the two to get the total magnification (e.g., 10x eyepiece and 40x objective lens = 400x total magnification).
• This helps you understand the scale of the specimen in relation to real life.

1. Low-Power Magnification:

• Use low magnification (e.g., 10x) to get a broad view of the tissue. This allows you to observe the general arrangement of cells but not in fine detail.
• Draw a diagram that shows the regions of the tissue and its overall structure.

2. High-Power Magnification:

• Switch to a higher magnification (e.g., 40x or 100x) to observe the individual cells in detail.
• Draw a detailed diagram of the specimen at this magnification, focusing on the shapes and structures of the cells. Make sure to label important parts of the cell, such as the nucleus, cytoplasm, and cell membrane (or cell wall in plant cells).

3. Drawing Guidelines:

• Use sharp pencils for clear diagrams.
• Draw smooth lines without shading.
• Label all significant features of the cell or tissue, and indicate magnification.

4. Draw What You See, Not What You Expect:

• Focus on the main structures that are visible. Label organelles or features such as the nucleus, cell membrane, or chloroplasts (if viewing plant cells).
• Avoid artistic interpretation – draw what is actually visible under the microscope, even if it's unclear or incomplete.

5. Label Your Drawing:

• Include the magnification used, the scale (e.g., 1 cm = 50 µm), and any important features (e.g., cell wall, nucleus, flagella).
• Adding a scale bar on your drawing helps others understand the size of the specimen without needing to refer back to the microscope's magnification.

Measuring Cell Size

1. Eyepiece Graticule:

• This is a small glass or plastic disc with a scale marked on it that is fitted into the microscope's eyepiece. It allows you to measure the size of cells.

2. Calibrating the Graticule:

• To accurately measure cells, you must calibrate the eyepiece graticule using a stage micrometre.
• A stage micrometre is a glass slide with a finely etched scale. Place the stage micrometre on the microscope stage and align it with the divisions on the eyepiece graticule.

3. Steps to Measure Cell Size:

• Align one division of the eyepiece graticule with a fixed point on the stage micrometre.
• Count the number of graticule divisions that correspond with a set length on the stage micrometre.
• Calculate the length of one division of the eyepiece graticule (in micrometres, µm).
• Once the eyepiece graticule is calibrated, use it to measure the size of the cells in the specimen. Multiply the number of divisions by the calibrated value to get the cell size.

$$\text{Cell Size} = \text{Number of Divisions on Graticule} \times \text{Calibration Value} \, (\mu m)$$

4. Example:

• If one division of the graticule equals 10 µm, and the cell spans 4 divisions, then the size of the cell is:

$$4 \, \text{divisions} \times 10 \, \mu m/\text{division} = 40 \, \mu m$$