Microscopy

Key Concepts:
- Overview of cell structures like the cell membrane and nucleus.
- Example: Human cheek cell with bacteria.

Introduction to Microscopy:
- Most prokaryotic and eukaryotic cells are too small to be seen without the aid of microscopy.
- In laboratory settings, a light microscope is employed capable of magnifying up to 1,000x.
- Electron microscopes, detailed in the textbook, can magnify over 100,000x.

Light Path:
- In compound light microscopy, the path of light is as follows:
- Light source → Specimen → Magnifying lenses → Observer’s eye.
Components include:
- Ocular lens:
- The lens through which the viewer looks, magnifying the specimen typically at 10x.
- Objective lenses:
- Primary lenses that further magnify the specimen (choices of 4x, 10x, 40x, and 100x).
- Condenser lenses:
- Focuses light through the specimen.
- Diaphragm:
- Controls the amount of light entering the condenser.
- Illuminator:
- Light source.

To achieve total magnification:
- Formula: $ext{Total Magnification} = ext{Ocular Magnification} imes ext{Objective Magnification}$
- Example Calculation:
- When using a 4x objective lens: (10 ext{x} 4 = 40\text{x} \text{ total magnification})

Definition of Resolution:
- The ability to distinguish between two points that are very close together.
Importance:
- Megapixels of visible detail in cell structures.

Usage of Immersion Oil:
- The 100x objective lens necessitates immersion oil to maximize resolution.
- Effects of Refraction:
- Refraction causes light rays to bend and potentially miss the opening of the lens, resulting in fuzzy images.
- Benefit of Oil:
- Immersion oil nearly matches the refractive index of glass, minimizing refraction issues.

Definition of Contrast:
- Difference in color intensity between the specimen and the background.
Significance of Color Contrast:
- Colorless organisms can be transparent against a bright background, making them difficult to observe.
Staining Specimens:
- Staining kills cells but enhances visibility by increasing color contrast.

Initial Steps of Staining:
- A liquid containing bacteria is placed on a slide and dried, forming a smear.
- The slide is then heated to attach the cells to the slide (heat fixation).

Simple Stains:
- Use of a single dye (basic dye, positively charged) that stains cells due to attraction to negatively charged cellular components.
- Examples of Basic Dyes:
- Methylene blue, crystal violet, safranin, malachite green.
Acidic Dyes:
- Negatively charged dyes do not bind to cells but color the background; examples include capsule stains (where cells are repelled).

Common Stains:
- Differential Stains:
- Utilize multiple dyes to differentiate bacterial groups, e.g., Gram stain.

Purpose:
- Differentiates bacteria mainly into two groups: Gram-positive and Gram-negative via a four-step process:
1. Primary Stain:
  - Cells are soaked in crystal violet, turning all cells purple.
2. Mordant Application:
  - Soaked in Gram's iodine, which forms a larger molecule with crystal violet, thus helping retention.
3. Decolorization:
  - Smear is treated with 95% ethanol; crystal violet-iodine complex is washed away from Gram-negative cells only.
4. Counterstaining:
  - Application of safranin to visualize now colorless Gram-negative bacteria, which appear pink.
Outcome Summary for Each Step:
- Crystal Violet: Cells become purple.
- Iodine: Cells remain purple.
- Alcohol:
- Gram-positive: Purple.
- Gram-negative: Colorless.
- Safranin:
- Gram-positive: Remain purple.
- Gram-negative: Appear pink.

Introduction to Electron Microscopy:
- Magnifies over 100,000x by utilizing electrons instead of light.
- Electrons possess a wavelength approximately 1,000 times shorter than that of visible light, resulting in enhanced resolving power—able to observe internal cell structures and viruses.

Transmission Electron Microscopy (TEM):
- Electrons pass through the specimen, producing a flat image.
Scanning Electron Microscopy (SEM):
- Electrons scan the specimen's surface, yielding a three-dimensional image.