Sample Notes: The Microscope In Cell Studies

AS Level Biology – The Microscope in Cell Studies

1. Overview of Microscopy in Cell Studies

• Microscopy is essential for studying cells, their structures, and functions.
• Two primary types: light microscope and electron microscope.
• Core measurements: magnification and resolution.

2. Preparing Temporary Slides for Light Microscopy

• Temporary preparation refers to mounting a specimen on a slide for short-term viewing.
• Materials Needed:
  • Glass slide and cover slip
  • Staining reagent (e.g., iodine, methylene blue)
  • Water dropper
  • Mounted needle and forceps

Steps:

1. Place the specimen (e.g., onion epidermis) on the glass slide.
2. Add a drop of stain to enhance visibility.
3. Use a dropper to add water if needed.
4. Carefully place the cover slip at a 45° angle using a mounted needle to avoid air bubbles.
5. Blot any excess liquid.

3. Drawing Cells from Microscope Slides and Photomicrographs

• Scientific drawing guidelines:
  • Use pencil only.
  • Do not shade; use clear, labeled lines.
  • Include title and magnification.
  • Label organelles correctly (nucleus, cell wall, cytoplasm, etc.).
• Maintain proportions relative to actual observation.
• Draw only what is visible—no artistic interpretation.

4. Calculating Magnification and Actual Size

4.1 Definitions:

• Magnification = How much larger the image is than the actual specimen.
• Actual Size = Real-life size of the object.
• Image Size = Size of the object in the drawing or photomicrograph.

4.2 Formula:

Magnification = Image Size / Actual Size

• Rearranged:
  • Actual Size = Image Size / Magnification
  • Image Size = Actual Size × Magnification

4.3 Units Used:

• 1 mm = 1000 µm (micrometres)
• 1 µm = 1000 nm (nanometres)

4.4 Example:

• If the image size = 50 mm and magnification = ×500:
  • Actual size = 50 mm ÷ 500 = 0.1 mm = 100 µm

5. Eyepiece Graticule and Stage Micrometer

5.1 Eyepiece Graticule:

• Transparent scale fitted inside the eyepiece.
• Units are arbitrary (e.g., 1–100) and must be calibrated.

5.2 Stage Micrometer:

• Microscope slide with a scale of known lengths (e.g., 1 mm divided into 100 divisions = 0.01 mm or 10 µm per division).

5.3 Calibration Process:

1. Align eyepiece graticule with stage micrometer.
2. Count how many eyepiece units align with known stage micrometer divisions.
3. Calculate the value of 1 eyepiece unit.

Example:

• 50 eyepiece units align with 0.5 mm on stage micrometer:
  • 0.5 mm = 500 µm
  • 1 eyepiece unit = 500 µm ÷ 50 = 10 µm

6. Resolution vs Magnification

6.1 Magnification:

• Increases the apparent size of an object.
• Does not improve clarity.

6.2 Resolution:

• Ability to distinguish two close objects as separate.
• Higher resolution = greater detail.

6.3 Differences:

7. Light Microscopes vs Electron Microscopes

8. Types of Electron Microscopy

8.1 Transmission Electron Microscope (TEM):

• Electrons pass through ultra-thin specimens.
• Produces 2D images with high resolution.
• Shows internal structures in great detail.

8.2 Scanning Electron Microscope (SEM):

• Electrons bounce off specimen surface.
• Produces 3D images of the surface.
• Less resolution than TEM.

9. Practical Applications in Biology

• Measuring cell organelles (e.g., mitochondria ~1–10 µm, ribosomes ~20 nm)
• Distinguishing between prokaryotic and eukaryotic cells
• Observing mitosis and meiosis stages
• Visualizing viruses and subcellular structures

10. Units and Conversions Table

11. Checklist for Microscopy Competency

• Can prepare a temporary slide
• Can calculate magnification from images
• Can use and calibrate an eyepiece graticule
• Understands difference between resolution and magnification
• Knows the features of light vs electron microscopes
• Can explain how SEM and TEM differ