Biol 20: Chapter 2

Chapter Two: How We See the Invisible World

Video 1:

Introduction to Light Microscopy

  • Overview of light microscopy fundamentals.

  • Discussion of different types of microscopes and their uses.

  • Importance of staining in microscopy.

Types of Microscopes

  • Bright Field Microscopy:

    • Field is bright while specimens appear darker.

    • Requires the use of stains to visualize organisms.

  • Dark Field Microscopy:

    • Field appears dark; organisms are seen as bright against the dark background.

    • Does not require staining for visualization.

Light and Microscopy

  • Light travels in waveforms, like vibrations.

  • Wavelength: Length between two peaks or troughs; inversely related to frequency.

  • Frequency: Number of peaks in a unit of time.

    • Low-frequency waves have longer wavelengths; high-frequency waves have shorter wavelengths.

  • Amplitude: Height of the wave; indicates the energy level.

Properties of Materials with Light Waves

  • Transparent vs. Opaque:

    • Transparent materials allow light to pass through (e.g., petri dish).

    • Opaque materials absorb light and do not allow it to pass (e.g., metal).

  • Light Interaction with Materials:

    • Reflection: Light bounces off a surface.

    • Absorbance: Material captures energy of a light wave.

    • Transmission: Light travels through a material.

    • Refraction: Bending of light when transitioning between media (e.g., air to water).

    • Refractive Index: Measure of light-bending ability of a medium.

Examples of Light Behavior

  • Optical Illusion: A straight pole looks bent in water due to refraction.

  • Prism and Color Spectrum:

    • Light passing through a prism is dispersed into seven colors of the rainbow due to its high refractive index.

Visible Spectrum and Energy of Light

  • Visible Light: Portion of the electromagnetic spectrum that is perceivable by the human eye.

    • Higher frequency (shorter wavelength) correlates with higher energy (e.g., gamma rays).

    • Lower frequency (longer wavelength) correlates with lower energy (e.g., infrared).

Historical Figures in Microscopy

  • Anthony van Leeuwenhoek:

    • First observed microorganisms with a primitive microscope; coined the term "animalcules."

  • Robert Hooke:

    • First described cells in cork in 1665; important in cell biology.

  • Securius and Hans Janssen:

    • Credited with inventing the compound microscope; lesser known compared to Leeuwenhoek and Hooke.

  • Galileo:

    • Invented the telescope but also contributed to optical science.

Parts of a Bright Field Microscope

  • Eyepiece (Ocular Lens): Magnification typically 10x.

  • Arm: Used for holding the microscope.

  • Nosepiece: Holds different objective lenses.

  • Objective Lenses: Include scanning, low power, high power, and oil immersion lenses.

  • Focusing Knobs:

    • Coarse Focus: Used with lower magnifications.

    • Fine Focus: Used with higher magnifications.

  • Stage: Platform for placing slides.

  • Light Source: Provides illumination from below.

  • Condenser Lens and Diaphragm: Focuses light onto the specimen; adjusts light intensity.

Understanding Magnification and Resolution

  • Magnification: Ability to enlarge an object’s image compared to its actual size.

    • Low power: 10x; High power: 40x; Oil immersion: 100x.

  • Resolution: Ability to distinguish two separate points.

    • Higher resolution images appear sharper and clearer.

    • Improved by using shorter wavelengths and high numerical apertures.

Oil Immersion Technique

  • Importance of oil between the lens and slide for minimizing refraction, which improves resolution.

  • Oil has a refractive index similar to glass, aiding in clarity and focus.

Conclusion

  • The chapter encapsulates the principles of microscopy and importance of various techniques for visualizing microscopic life.

  • Follow-up discussions to explore more microscopy types and staining techniques.

Video 2:

Overview of Microscopy

  • Microscopy is the study of small objects using various types of instruments.

  • Micrograph: A picture or photograph generated by a microscope.

  • Focus on light microscopes for practical applications, though awareness of various types is beneficial.

Types of Light Microscopes

  • Commonly used light microscopes include:

    • Bright Field

    • Dark Field

    • Phase Contrast

    • Differential Interference Contrast (DIC)

    • Fluorescence

    • Confocal

    • Two-Photon Microscopes

Dark Field Microscopy

  • Converts bright field to dark field using an opaque stopper.

  • Blocks light from illuminator to objective lens.

  • Allows only light reflected off the specimen to reach the viewer.

  • Advantage: No stains needed; contrast is achieved with a dark background.

  • Example: Unstained Treponema pallidum appears as light on a dark background.

Phase Contrast Microscopy

  • Utilizes a phase plate to create contrasts through different light paths.

  • Enhances image quality without staining.

  • Commonly available in university labs.

Differential Interference Contrast (DIC) Microscope

  • Produces a three-dimensional image by differing optical paths.

  • Useful for enhancing contrast without stains.

Fluorescence Microscopy

  • Requires special dyes known as fluorochromes to visualize specimens.

  • Often used to tag specific cell components or proteins.

Electron Microscopes

  • Main difference: Uses a beam of electrons instead of light, resulting in high resolution.

  • Types of electron microscopes:

    • Transmission Electron Microscope (TEM): Provides detail of internal structures.

    • Scanning Electron Microscope (SEM): Provides a three-dimensional view of surface structures.

Key Differences Between Light and Electron Microscopes

  • Energy Source: Light microscopy uses visible light, while electron microscopy uses electrons.

  • Lens Type: Glass lenses in light microscopes; electromagnets in electron microscopes.

  • Medium: Light microscopes use air; electron microscopes operate in a vacuum.

  • Viewing Method: Light microscopes viewed directly through ocular lenses; images from electron microscopes are seen on a screen.

Scanning Probe Microscopy

  • Produces high magnification images through sharp probes interacting with specimens.

  • Types include:

    • Scanning Tunneling Microscope (STM)

    • Atomic Force Microscope (AFM)

  • Ability to visualize molecular structures.

Summary of Unique Features in Microscopy

  • The content highlights critical features and differences among various types of microscopy, including light, electron, and scanning probe techniques.

  • Importance of understanding applications of different microscopes for specific scientific investigations.

Conclusion

  • Focus on knowing the parts and functions of light microscopes for practical lab work.

  • Be familiar with the distinctions between light and electron microscopes, and probe microscopy.

  • Next discussion will center on stains utilized in microscopy

Video 3:

Specimen Preparation for Microscopy

  • Proper sample preparation is crucial for successful microscopy.

  • Involves creating a smear of bacteria on a slide.

    • Smear Preparation: A thin film is prepared on a slide for staining.

    • Fixation Methods: Two main methods are heat fixation and chemical fixation.

      • Heat Fixation: Passing the slide over a flame to kill and adhere bacteria.

      • Chemical Fixation: Not covered in the class; only heat fixation will be utilized.

  • Purpose of fixing samples:

    • To kill live bacteria and prepare them for safe viewing under the microscope.

Types of Stains

Two Main Types of Stains

  • Basic Stains: Positively charged chromophore.

    • Attracts to the negatively charged cell surface, resulting in colored cells.

    • Also called positive stains; cells appear colored against a clear background.

  • Acidic Stains: Negatively charged chromophore.

    • Repels from negatively charged cells, leaving the cells colorless but staining the background.

    • Also referred to as negative stains.

Staining Technologies

Staining Techniques

  1. Simple Staining

    • Uses a single type of stain to color cells for direct observation.

  2. Differential Staining

    • Employs multiple stains to differentiate between organisms.

    • Example: Gram Staining

      • Primary stain: Crystal Violet (dark purple).

      • Mordant: Iodine (helps retain the primary stain).

      • Decolorizer: Alcohol (different effects on Gram-positive vs. Gram-negative organisms).

      • Counterstain: Saffron (red color for decolorized Gram-negative bacteria).

      • Result: Gram-positive bacteria retain purple; Gram-negative bacteria appear red/pink.

    • Significance: Helps in clinical diagnosis to prescribe appropriate antibiotics.

  3. Special Staining Techniques

    • Used for specific structures (e.g., endospores, flagella).

    • Examples:

      • Endospore Staining: Two color combinations (e.g., green and red like a Christmas tree).

      • Flagella Staining: Highlights flagella for viewing flagella presence and structure.

Clinical Importance of Staining Techniques

  • Acid Fast Staining: Used for diagnosing tuberculosis (TB).

    • Primary stain: Carbolfuchsin (pink color).

    • Decolorizer: Acid alcohol (removes stain from non-acid-fast bacteria).

    • Secondary stain: Methylene Blue (colors non-acid-fast bacteria).

    • Acid-fast organisms retain pink color, while non-acid-fast cells turn blue.

Overview of Staining Use in Microscopy

  • Important for observing cell structures and types.

  • Different microscopy techniques (e.g., dark-field, bright-field, scanning electron microscopy) demonstrate clarity and details of specimens.

  • Familiarity with microscope parts and functions is essential for lab work.