draw cells from microscope slides and photomicrographs

The Microscope in Cell Studies

Learning Objective

Students will be able to:

• Identify the main parts of a light microscope and their functions.
• Prepare a wet mount slide of plant and animal cells.
• Observe cells at appropriate magnifications and record observations.
• Produce accurate sketches and interpret photomicrographs.

1. Types of Microscopes Used in A‑Level Biology

The curriculum focuses on the compound light microscope, but it is useful to compare it with other common types.

2. Parts of the Compound Light Microscope

Understanding each component helps in troubleshooting and achieving optimal image quality.

3. Magnification and Resolution

The total magnification is the product of the ocular and objective magnifications:

\$\text{Total Magnification} = \text{Ocular Magnification} \times \text{Objective Magnification}\$

Resolution is limited by the wavelength of light (λ) and the numerical aperture (NA) of the objective:

\$d = \frac{0.61 \lambda}{\text{NA}}\$

Where d is the smallest resolvable distance. Typical visible light (λ ≈ 550 nm) gives a practical resolution limit of about 0.2 µm.

4. Preparing a Wet‑Mount Slide

1. Place a clean glass slide on a flat surface.
2. Using a pipette, add a drop of water (or appropriate mounting medium) to the centre of the slide.
3. Transfer a small fragment of the specimen (e.g., onion epidermis, cheek cells) onto the drop.
4. Gently place a cover slip at an angle to avoid air bubbles, then lower it slowly.
5. Secure the edges with slide clips if needed.
6. Label the slide with the specimen name, date, and magnification to be used.

5. Observing Cells – Step‑by‑Step Procedure

1. Start with the lowest power objective (4×) and centre the specimen using the mechanical stage.
2. Adjust the diaphragm to obtain a bright, even field of illumination.
3. Bring the specimen into focus with the coarse focus knob.
4. Switch to a higher power objective (10×, then 40×) and fine‑focus each time.
5. Record the total magnification for each observation.
6. Note key features: cell shape, size, nucleus, cell wall (plants), vacuoles, chloroplasts, etc.

6. Sketching Cells from the Microscope

Accurate sketches are essential for assessment. Follow these guidelines:

• Use a ruler to draw a scale bar; calculate the length represented using the magnification and the eyepiece reticle (if available).
• Label all visible structures clearly (e.g., nucleus (N), cell wall (CW), cytoplasm (Cyt), chloroplasts (Chl)).
• Indicate the direction of light (usually from the left).
• Use shading or hatching to show dense regions (e.g., nucleolus).
• Include a brief caption with specimen name, magnification, and any staining used.

7. Interpreting Photomicrographs

Photomicrographs are digital images captured through the microscope. When analysing them:

1. Check the scale bar; convert pixel measurements to micrometres if required.
2. Identify the same structures you would sketch – compare with your hand‑drawn diagram.
3. Note any artefacts (e.g., air bubbles, staining precipitates) that may affect interpretation.
4. Use the image to discuss cell specialization (e.g., guard cells vs. pavement cells).

8. Safety and Care of the Microscope

• Never force the coarse focus knob when using high‑power objectives.
• Always use oil immersion objectives with a drop of immersion oil; clean oil off the lens with lens paper after use.
• Keep the microscope covered when not in use to prevent dust accumulation.
• Handle slides by the edges; dispose of broken glass safely.
• Turn off the light source when the microscope is not being used.

9. Summary Checklist

• Identify microscope parts and their functions.
• Calculate total magnification correctly.
• Prepare a clean wet‑mount slide.
• Adjust illumination and focus for each objective.
• Produce a labelled sketch with a scale bar.
• Analyse a photomicrograph and compare with the sketch.

10. Sample Assessment Questions

1. Explain why the resolution of a light microscope is limited to about 0.2 µm.
2. Calculate the total magnification when using a 10× ocular and a 40× objective.
3. Describe three common sources of error when preparing a wet‑mount slide and how to minimise them.
4. Given a photomicrograph with a 50 µm scale bar representing 200 pixels, determine the size of a cell that measures 80 pixels in the image.