Cell Structure, Microscopy, and Cell Specialization: Study Notes for Microbiology

Cell Structure and Microscopy

Limits on Cell Surface Area and Volume

Cells are limited in size due to the relationship between their surface area and volume. As a cell grows, its volume increases faster than its surface area, which restricts the rate at which materials can enter or leave the cell.

• Surface Area-to-Volume Ratio: A high ratio allows efficient exchange of materials; as cells grow, this ratio decreases, limiting cell size.

• Example: Small cells like Escherichia coli have a high surface area-to-volume ratio, facilitating rapid nutrient uptake and waste removal.

Compound Light Microscope

A compound light microscope uses multiple lenses to magnify small objects, allowing visualization of cells and some organelles.

• How it Works: Light passes through the specimen and is magnified by objective and ocular lenses.

• Magnification: Total magnification is the product of the objective and ocular lens magnifications.

• Resolution: The ability to distinguish two close points as separate; higher resolution provides clearer images.

• Formula: $\text{Total Magnification} = \text{Objective Magnification} \times \text{Ocular Magnification}$

Light vs. Electron Microscopes

Light microscopes use visible light, while electron microscopes use electron beams for much higher resolution.

• Light Microscopes: Suitable for living cells, general cell structure, and larger organelles.

• Electron Microscopes: Used for detailed ultrastructure (e.g., membranes, ribosomes); cannot be used on living specimens.

• When to Use: Use light microscopes for routine cell observation; electron microscopes for detailed internal or surface structures.

Magnification and Resolution

• Magnification: The process of enlarging the appearance of an object.

• Resolution: The minimum distance at which two points can be distinguished as separate.

• Calculation: $\text{Resolution} = \frac{0.61 \lambda}{\text{Numerical Aperture}}$ where $\lambda$ is the wavelength of light used.

Interpreting Micrographs

• Light Micrograph: Image produced by a light microscope.

• Electron Micrograph: Image produced by an electron microscope, showing much finer detail.

Staining Techniques

• Purpose: Enhance contrast in light microscopy to visualize structures.

• Common Stains: Gram stain, crystal violet, safranin.

Prokaryotic vs. Eukaryotic Cells

Major Differences

• Prokaryotic Cells: Lack a nucleus and membrane-bound organelles; DNA is in the nucleoid region.

• Eukaryotic Cells: Have a true nucleus and membrane-bound organelles.

• Size: Eukaryotic cells are generally larger than prokaryotic cells.

Common Structures

• Both cell types have: Plasma membrane, cytoplasm, ribosomes, and genetic material (DNA).

Prokaryotic Cell Features

• Nucleoid: Region containing circular DNA; not membrane-bound.

• Cell Wall: Provides structure and protection; composition differs between Gram-positive and Gram-negative bacteria.

• Gram-Positive vs. Gram-Negative:

  • Gram-positive: Thick peptidoglycan layer, stains purple.

  • Gram-negative: Thin peptidoglycan layer, outer membrane, stains pink.

• Shapes of Bacteria: Bacilli (rod-shaped), cocci (spherical), spirilla (spiral-shaped).

• Flagella: Used for movement; prokaryotic flagella rotate, while eukaryotic flagella have a whip-like motion.

• Pili: Hair-like structures for attachment and conjugation.

• Capsule: Protective outer layer; helps evade immune response.

Antibiotics and Selective Toxicity

• Selective Toxicity: Antibiotics target bacterial structures (e.g., cell wall, ribosomes) not found in human cells, minimizing harm to the host.

Eukaryotic Cell Features

• Nucleus: Contains DNA; surrounded by a nuclear envelope.

• Ribosomes: Sites of protein synthesis; can be free or bound to the endoplasmic reticulum.

• Endomembrane System: Includes the nuclear envelope, endoplasmic reticulum (ER), Golgi apparatus, lysosomes, vacuoles, and vesicles.

• Endoplasmic Reticulum (ER):

  • Rough ER: Studded with ribosomes; synthesizes proteins.

  • Smooth ER: Lacks ribosomes; synthesizes lipids and detoxifies chemicals.

• Golgi Apparatus: Modifies, sorts, and packages proteins and lipids for secretion or delivery to other organelles.

• Lysosomes: Contain digestive enzymes; break down waste and cellular debris. Three functions: Digestion of macromolecules, recycling of cellular components, and defense against pathogens.

• Vacuoles: Storage organelles; large central vacuole in plant cells stores water and maintains turgor pressure.

• Peroxisomes: Break down fatty acids and detoxify harmful substances.

Mitochondria and Chloroplasts

• Mitochondria: Site of cellular respiration; produces ATP.

• Chloroplasts: Site of photosynthesis in plants and algae; converts solar energy to chemical energy.

• Producers vs. Consumers: Producers (plants, algae) have chloroplasts; consumers (animals) do not.

Endosymbiotic Theory

• Definition: Proposes that mitochondria and chloroplasts originated as free-living prokaryotes engulfed by ancestral eukaryotic cells.

• Evidence: Double membranes, own DNA, and ribosomes similar to bacteria.

Cytoskeleton

The cytoskeleton provides structural support, maintains cell shape, and enables movement.

• Microfilaments: Composed of actin; involved in cell movement and shape.

• Intermediate Filaments: Provide mechanical support; maintain cell integrity.

• Microtubules: Hollow tubes; involved in cell division, intracellular transport, and movement of cilia and flagella.

• Centrioles: Organize microtubules during cell division in animal cells.

• Cilia and Flagella: Motile structures for movement; cilia are short and numerous, flagella are longer and fewer.

Extracellular Matrix and Cell Junctions

• Extracellular Matrix (ECM): Network of proteins and carbohydrates outside animal cells; provides support and regulates cell behavior.

• Plant Cell Walls: Rigid structure made of cellulose; provides support and protection.

• Junctions:

  • Tight Junctions: Seal cells together, preventing leakage.

  • Adhering Junctions: Anchor cells to each other.

  • Gap Junctions: Allow communication between animal cells.

  • Plasmodesmata: Channels between plant cells for transport and communication; functionally similar to gap junctions in animals.

Specialized Structures in Plant and Animal Cells

• Plant Cells: Unique structures include cell wall, chloroplasts, and large central vacuole.

• Animal Cells: Unique structures include centrioles and lysosomes.

Cell Specialization

• Definition: The process by which cells develop specific structures and functions.

• Example: Muscle cells have abundant mitochondria for energy; nerve cells have long extensions for signal transmission.

• Structure-Function Relationship: A cell’s structure is closely related to its specialized function.

Feature	Prokaryotic Cells	Eukaryotic Cells
Nucleus	No	Yes
Membrane-bound Organelles	No	Yes
Cell Wall	Yes (peptidoglycan)	Plants (cellulose), Fungi (chitin), Animals (no)
Size	1-10 μm	10-100 μm
Examples	Escherichia coli	Plant, animal, fungal cells

Additional info: Some explanations and examples were expanded for clarity and completeness based on standard microbiology curriculum.