2.2.1 Methods of studying cells

Light Microscopes:

1. How they work:

• Use a pair of convex glass lenses to magnify specimens.
• Rely on visible light, limiting their resolution to about 0.2 μm.

2. Key Features:

• Magnification: Enlarges the image.
• Formula:

$$\text{Magnification} = \frac{\text{Size of Image}}{\text{Size of Real Object}}$$

• Resolution: The ability to distinguish between two objects as separate. Defined as the minimum distance at which two objects can still be seen as distinct.
• The wavelength of light limits resolution, which cannot resolve structures smaller than 0.2 μm.

3. Uses:

• Suitable for observing live specimens.
• Used for general cellular observations.

Electron Microscopes:

1. How they work:

• Use a beam of electrons focused by electromagnets.
• Operate in a vacuum to prevent air particles from scattering the electrons.
• Can resolve objects as small as 0.1 nm, providing a much greater resolution than light microscopes.

2. Types:

• Transmission Electron Microscope (TEM):
• A beam of electrons passes through a thin specimen.
• Produces a detailed 2D image of the internal structure.
• Areas absorbing more electrons appear darker.
• Scanning Electron Microscope (SEM):
• A beam of electrons scans the surface of the specimen.
• Produces a detailed 3D image of the specimen's surface.

3. Advantages:

• TEM: Extremely high resolution for internal details.
• SEM: Produces 3D images, useful for understanding surface structures.

4. Limitations:

• Require a vacuum, so living specimens cannot be observed.
• A complex staining process is necessary, which may introduce artefacts (false structures not present in the specimen).
• Specimens must be thin (especially for TEM).
• SEM has a lower resolving power than TEM, but both are far superior to light microscopes.

Comparison of Light and Electron Microscopes:

Feature	Light Microscope	Electron Microscope
Source of illumination	Visible light	Electrons
Resolution	0.2 μm (limited by light wavelength)	0.1 nm (higher due to electrons)
Magnification	Up to x1500	Up to x2,000,000
Specimen preparation	Simple, can observe live samples	Complex, requires vacuum, no live samples
Image produced	2D	2D (TEM) or 3D (SEM)