Required Practical: Microscopy (OCR GCSE Biology A, Combined (Gateway Science Suite)): Revision Notes

Aim: To use a light microscope to observe, draw, and label the structure of plant and animal cells. The practical involves learning how to prepare microscope slides, use the microscope correctly, and calculate magnification.

Method:

1. Preparing the Slide:

• For plant cells (e.g., onion epidermis):
• Peel a thin layer of onion skin using forceps.
• Place the onion skin flat on a glass slide.
• Add a drop of iodine solution to stain the cells and make them more visible.
• Gently place a coverslip over the onion skin, avoiding air bubbles.
• For animal cells (e.g., cheek cells):
• Gently scrape the inside of your cheek with a clean cotton swab.
• Smear the cells onto a glass slide.
• Add a drop of methylene blue stain to highlight the cell structures.
• Place a coverslip over the smear.

2. Using the Microscope:

• Start with the lowest magnification (e.g., 4x objective lens) and place the slide on the stage.
• Use the coarse focus knob to bring the image into general focus.
• Adjust the fine focus knob to sharpen the image.
• Switch to a higher magnification (e.g., 10x or 40x) if needed and adjust the focus again.

3. Observations:

• Look for the main features of the cells:
• Plant cells: Identify the cell wall, cell membrane, cytoplasm, nucleus, and chloroplasts (if present).
• Animal cells: Identify the cell membrane, cytoplasm, and nucleus.
• Draw clear, labelled diagrams of what you observe under the microscope.
• Include the magnification used in your drawings.

4. Calculating Magnification:

• Use the formula: Magnification = Size of Image / Real Size of Object.
• If you know the magnification of the objective lens and the eyepiece lens, multiply them to find the total magnification.

5. Recording Results:

• Record the observations and the magnification for each slide.
• Compare the size and structure of plant and animal cells.

Data Analysis:

• Plant Cells: Should show a regular shape, with a visible cell wall, chloroplasts, and a large central vacuole.
• Animal Cells: Should appear more irregular in shape, with no cell wall or chloroplasts, and a smaller or absent vacuole.

Conclusion:

• The practical demonstrates the differences between plant and animal cells and helps in understanding cell structure and function.
• The use of stains (iodine and methylene blue) enhances the visibility of cell structures under the microscope.