Prokaryotic vs. Eukaryotic Cells

Comparison and Contrast

Understanding the differences and similarities between prokaryotic and eukaryotic cells is fundamental in microbiology. These two cell types differ in complexity, structure, and function.

• Prokaryotic Cells: Lack a true nucleus and membrane-bound organelles. Genetic material is found in the nucleoid region. Examples include Bacteria and Archaea.

• Eukaryotic Cells: Possess a true nucleus enclosed by a nuclear membrane and various membrane-bound organelles (e.g., mitochondria, endoplasmic reticulum). Examples include Fungi, Protozoa, Plants, and Animals.

• Similarities: Both have a plasma membrane, cytoplasm, ribosomes, and genetic material (DNA).

Example: Escherichia coli (prokaryote) vs. Saccharomyces cerevisiae (eukaryote).

Bacterial Shapes and Arrangements

Common Bacterial Shapes

Bacteria exhibit characteristic shapes that aid in identification and classification.

Bacterial Arrangements

Bacterial cells can be arranged in specific patterns based on their division and grouping.

Cell Surface Structures

Glycocalyx: Structure and Function

The glycocalyx is a gelatinous, sticky substance surrounding the outside of some bacterial cells. It can be a capsule (organized, firmly attached) or a slime layer (loose, unorganized).

• Function: Protects cells from desiccation, helps in adherence to surfaces, and provides resistance to phagocytosis.

• Example: Streptococcus pneumoniae uses its capsule to evade the host immune system.

Role in Disease

• Contribution to Disease: The glycocalyx enhances bacterial virulence by preventing phagocytosis and aiding in biofilm formation.

• Example: Dental plaque formation by Streptococcus mutans.

Flagella and Motility

• Function: Flagella are whip-like structures that provide motility to bacteria.

• Arrangements: Monotrichous (single flagellum), Lophotrichous (tuft at one end), Amphitrichous (flagella at both ends), Peritrichous (flagella all over).

Axial Filaments

• Definition: Axial filaments are internal flagella found in spirochetes, enabling corkscrew movement.

• Difference from Flagella: Located within the periplasmic space, not external.

Fimbriae and Pili

• Fimbriae: Short, hair-like structures for attachment to surfaces.

• Pili: Longer, used for conjugation (transfer of DNA between bacteria).

Bacterial Cell Wall Structure

Gram-Positive vs. Gram-Negative Bacteria

• Gram-Positive: Thick peptidoglycan layer, teichoic acids, stains purple in Gram stain.

• Gram-Negative: Thin peptidoglycan layer, outer membrane with lipopolysaccharides, stains pink/red.

Example: Staphylococcus aureus (Gram-positive) vs. Escherichia coli (Gram-negative).

Drug Targeting of Cell Wall

• Mechanism: Antibiotics like penicillin inhibit peptidoglycan synthesis, leading to cell lysis.

Plasma Membrane Structure and Function

Structure

• Phospholipid Bilayer: Composed of hydrophilic heads and hydrophobic tails.

• Proteins: Integral and peripheral proteins involved in transport and signaling.

Function

• Selective permeability, transport of nutrients and waste, energy generation.

Transport Mechanisms

Active vs. Passive Processes

• Passive Transport: Movement of substances down their concentration gradient without energy input (e.g., diffusion, osmosis).

• Active Transport: Movement against the concentration gradient, requires energy (ATP).

Equation for Diffusion Rate:

Where J is the flux, D is the diffusion coefficient, and \frac{dC}{dx} is the concentration gradient.

Diffusion and Osmosis

• Diffusion: Movement of molecules from high to low concentration.

• Osmosis: Diffusion of water across a semipermeable membrane.

Types of Solutions

Internal Cell Structures

Major Structures, Cell Types, and Functions

Additional info: Some entries in the tables are inferred based on standard microbiology knowledge.