1.10 Chapter Summary

A. Temporary Preparations

• Sample Collection: Obtain cells from various sources (plant, animal, microbial).
• Fixation: Use chemical fixatives (e.g., formaldehyde) to preserve cell structure and prevent decomposition.
• Mounting: Place a drop of water or staining solution on a clean microscope slide, add the specimen, and gently cover with a cover slip to flatten the sample.
• Staining: Apply stains (e.g., methylene blue, iodine) to increase contrast by coloring specific cell components, making them more visible under the microscope.
• Dehydration: For some preparations, especially those viewed under higher magnifications, gently remove excess moisture to prevent distortion.

B. Key Techniques

• Smearing: Thinly spreading cells on the slide for even viewing.
• Heat Fixing: Passing the slide briefly through a flame to adhere cells to the slide and preserve structures.

A. Observation Skills

• Magnification Selection: Start with lower magnification to locate cells, then switch to higher magnifications for detailed observation.
• Focus Adjustment: Carefully adjust the focus to obtain a clear image before drawing.

B. Drawing Techniques

• Accuracy: Replicate the shape, size, and structure of cells and organelles as observed.
• Labels: Clearly label key components (e.g., nucleus, cell membrane, mitochondria) to demonstrate understanding.
• Photomicrographs Reference: Use microscope photographs as references to ensure precision in your drawings.

C. Tips for Effective Drawing

• Use Light Pencil Lines: Allows for easy corrections and additions.
• Include Scale: Indicate the scale or magnification used to provide context to your drawing.
• Highlight Details: Emphasize important structures using shading or additional lines.

A. Understanding Magnification

• Total Magnification: Product of the objective lens magnification and the eyepiece (ocular) lens magnification.
  • Formula: Total Magnification = Objective Magnification × Eyepiece Magnification
  • Example: 40× objective × 10× eyepiece = 400× total magnification

B. Calculating Actual Size

• From Image Size:
  • Formula: Actual Size = Image Size / Total Magnification
  • Units: Convert to millimetres (mm), micrometres (µm), or nanometres (nm) as appropriate.
• From Drawings and Micrographs:
  • Use scale bars provided in photomicrographs or measurements from drawings to determine actual sizes.

C. Electron Microscopy Calculations

• Scanning Electron Microscopy (SEM): Provides detailed surface images; calculate sizes similarly using provided scales.
• Transmission Electron Microscopy (TEM): Offers high-resolution internal images; actual size calculations require precise scale interpretation.

D. Practice Example

• Given: A cell appears 5 cm long in a photomicrograph taken at 1000× magnification.
• Calculate Actual Size:
• Actual Size = 5 cm / 1000 = 0.005 cm = 50 µm

A. Tools Overview

• Eyepiece Graticule: A transparent grid etched into the microscope’s eyepiece used for measuring specimen dimensions.
• Stage Micrometer: A slide with a precisely known scale (e.g., 1 mm divided into 100 parts of 0.01 mm) used for calibrating the graticule.

B. Calibration Process

1. Insert Stage Micrometer: Place the stage micrometer slide on the microscope stage.
2. Align Graticule: Focus on the stage micrometer and align the graticule lines with the micrometer scale.
3. Determine Scale: Calculate the real distance each graticule division represents.
   • Example: If 1 graticule division equals 0.02 mm on the stage micrometer, use this as your scale for measuring specimens.

C. Measuring Specimens

1. Replace with Specimen Slide: After calibration, remove the stage micrometer and place your specimen slide on the stage.
2. Measure: Use the graticule to count the number of divisions covering the specimen feature.
3. Calculate Actual Size: Multiply the number of divisions by the scale factor determined during calibration.

D. Units of Measurement

• Millimetre (mm): Suitable for larger cellular structures.
• Micrometre (µm): Commonly used for cell and organelle sizes.
• Nanometre (nm): Used in electron microscopy for sub-cellular components.

A. Magnification

• Definition: The process of enlarging the appearance of an object.
• Importance: Allows observation of small structures by making them appear larger.
• Limitations: High magnification alone does not improve image quality; without good resolution, images can be blurry.

B. Resolution

• Definition: The ability to distinguish two adjacent points as separate entities.
• Importance: Determines the clarity and detail of the image.
• Factors Affecting Resolution:
  • Wavelength of Light/Electrons: Shorter wavelengths (e.g., electrons in TEM) provide higher resolution.
  • Numerical Aperture of Lenses: Higher numerical aperture allows better resolution.
  • Quality of Optics: Superior lenses reduce aberrations and improve image clarity.

C. Differences Between Light and Electron Microscopy

• Light Microscopy:
  • Resolution: Limited to ~200 nm due to the wavelength of visible light.
  • Magnification: Typically up to ~1000×.
  • Advantages: Suitable for observing living cells and basic structures.
  • Limitations: Lower resolution, cannot visualize fine sub-cellular details.
• Electron Microscopy:
  • Resolution: Much higher, down to ~0.1 nm with TEM.
  • Magnification: Can exceed 1,000,000×.
  • Advantages: Detailed images of cell ultrastructure and molecular components.
  • Limitations: Requires fixed, non-living samples; more complex and expensive equipment.

D. Key Points to Remember

• Resolution > Magnification: High magnification without adequate resolution results in poor image quality.
• Complementary Tools: Use both magnification and resolution to achieve clear, detailed observations.
• Application-Based Understanding: Choose the appropriate microscopy type based on the detail and type of structures you need to study.

a. Cell Surface Membrane

• Structure: Phospholipid bilayer with embedded proteins, cholesterol, and carbohydrates.
• Function: Regulates entry and exit of substances, cell communication, and maintains homeostasis.

b. Nucleus, Nuclear Envelope, and Nucleolus

• Nucleus:
  • Structure: Largest organelle, contains DNA.
  • Function: Controls cellular activities and genetic information.
• Nuclear Envelope:
  • Structure: Double membrane with nuclear pores.
  • Function: Protects DNA and regulates transport between nucleus and cytoplasm.
• Nucleolus:
  • Structure: Dense region within the nucleus.
  • Function: Synthesizes ribosomal RNA (rRNA) and assembles ribosomes.

c. Rough Endoplasmic Reticulum (RER)

• Structure: Studded with ribosomes.
• Function: Protein synthesis and processing, lipid synthesis.

d. Smooth Endoplasmic Reticulum (SER)

• Structure: Lacks ribosomes.
• Function: Lipid synthesis, detoxification, calcium storage.

e. Golgi Body (Golgi Apparatus/Complex)

• Structure: Stacked flattened membrane sacs.
• Function: Modifies, sorts, and packages proteins and lipids for storage or transport.

f. Mitochondria

• Structure: Double membrane with cristae; contains small circular DNA.
• Function: Powerhouse of the cell; ATP production through respiration.

g. Ribosomes

• Types:
  • 80S Ribosomes: Found in cytoplasm.
  • 70S Ribosomes: Found in chloroplasts and mitochondria.
• Function: Protein synthesis.

h. Lysosomes

• Structure: Membrane-bound vesicles containing digestive enzymes.
• Function: Break down waste materials and cellular debris.

i. Centrioles and Microtubules

• Centrioles:
  • Structure: Cylindrical structures made of microtubules.
  • Function: Aid in cell division (mitosis).
• Microtubules:
  • Structure: Hollow tubes made of tubulin.
  • Function: Maintain cell shape, enable intracellular transport, form the spindle fibers during mitosis.

j. Cilia

• Structure: Hair-like projections on the cell surface.
• Function: Movement of the cell or movement of substances over the cell surface.

k. Microvilli

• Structure: Small, finger-like projections.
• Function: Increase surface area for absorption.

l. Chloroplasts

• Structure: Double membrane with internal thylakoid membranes; contains small circular DNA.
• Function: Photosynthesis in plant cells.

m. Cell Wall

• Structure: Rigid layer outside the cell membrane (in plant cells).
• Function: Provides structural support and protection.

n. Plasmodesmata

• Structure: Channels between plant cell walls.
• Function: Allow transport and communication between plant cells.

o. Large Permanent Vacuole and Tonoplast (Plant Cells)

• Large Permanent Vacuole:
  • Structure: Large central vacuole filled with cell sap.
  • Function: Maintains cell rigidity, stores nutrients and waste products.
• Tonoplast:
  • Structure: Membrane surrounding the vacuole.
  • Function: Regulates movement of ions and molecules in and out of the vacuole.

a. Photomicrographs and Electron Micrographs

• Photomicrographs: Images captured using light microscopes; useful for viewing cell structures like the nucleus, chloroplasts, and cell membrane.
• Electron Micrographs: Higher resolution images using electron microscopes; essential for detailed views of organelles like ribosomes, mitochondria, and the endoplasmic reticulum.

b. Cell Drawings

• Skill: Ability to accurately draw and label plant and animal cells, indicating key organelles and structures.

Similarities:

• Both are eukaryotic.
• Share organelles: nucleus, mitochondria, RER, SER, Golgi apparatus, ribosomes, lysosomes, centrioles, microtubules, cilia, microvilli.

Differences:

• Plant Cells:
  • Have cell walls, chloroplasts, large central vacuole, plasmodesmata.
• Animal Cells:
  • Lack cell walls and chloroplasts, have smaller vacuoles, possess centrioles.

Concept: Cells utilize ATP (adenosine triphosphate) produced during cellular respiration to power energy-requiring processes such as active transport, muscle contraction, and biosynthesis.

Key Features:

• Unicellular: Single-celled organisms.
• Size: Generally 1–5 µm in diameter.
• Cell Wall: Composed of peptidoglycan.
• DNA: Circular, not enclosed in a nucleus.
• Ribosomes: 70S type (smaller than eukaryotic ribosomes).
• Organelles: Lack membrane-bound organelles; no nucleus, mitochondria, or chloroplasts.

Prokaryotic Cells (Bacteria) vs. Eukaryotic Cells (Plants and Animals)

Key Characteristics:

• Structure:
  • Core: Nucleic acid (DNA or RNA).
  • Capsid: Protein shell surrounding the nucleic acid.
  • Envelope (optional): Some viruses have an outer lipid envelope derived from the host cell’s membrane.

Function:

• Replication: Require host cells to replicate.
• Non-living: Do not carry out metabolic processes independently.