Prokaryotic and Eukaryotic Cell Structure: Cell Walls, Membranes, and Surface Structures

Cell Types and Basic Characteristics

Prokaryotes vs. Eukaryotes

Cells are the fundamental units of life and can be classified as prokaryotic or eukaryotic. These two cell types differ in size, complexity, and internal organization.

• Prokaryotes: Typically 1–10 μm in diameter. Lack a true nucleus and membrane-bound organelles. Domains include Bacteria and Archaea.

• Eukaryotes: Typically 10–100 μm in diameter. Possess a true nucleus surrounded by a nuclear envelope and contain membrane-bound organelles. Domain Eukarya includes protozoa, algae, fungi, animals, and plants.

ATP (Adenosine Triphosphate): The primary energy currency of the cell, storing energy in its chemical bonds.

Reproduction: Can occur asexually (e.g., binary fission) or sexually (involving gametes).

Cell Surface Structures

Glycocalyx

The glycocalyx is a gelatinous, sticky substance composed of polypeptides or polysaccharides that surrounds the outside of some cells. It serves several functions:

• Protects cells from desiccation (drying out).

• Enables cells to adhere to surfaces, forming biofilms (e.g., dental plaque).

• Can contribute to pathogenicity by helping bacteria evade the immune system.

The glycocalyx can be organized as a capsule (firmly attached) or a slime layer (loosely attached).

Flagella

Flagella are long, whip-like appendages that provide motility to many prokaryotic cells. They are composed of three main parts:

• Filament: A long, hollow shaft made of flagellin protein.

• Hook: A curved structure that connects the filament to the basal body.

• Basal body: Anchors the flagellum to the cell wall and plasma membrane.

Flagella arrangements include:

• Monotrichous: Single flagellum at one end.

• Lophotrichous: Tuft of flagella at one end.

• Amphitrichous: Flagella at both ends.

• Peritrichous: Flagella all over the cell surface.

Flagella rotate at speeds up to 100,000 rpm, propelling bacteria at rates up to 670 miles per hour. Movement patterns include "runs" (straight movement) and "tumbles" (random changes in direction).

Endoflagella (axial filaments) are found in spirochetes, allowing corkscrew motion through viscous environments.

Fimbriae and Pili

Fimbriae are short, bristle-like proteinaceous projections that help bacteria adhere to surfaces and to each other, playing a key role in biofilm formation and pathogenicity.

Pili (singular: pilus) are longer than fimbriae and are involved in the transfer of DNA between cells via conjugation. Pili are hollow tubes that can be used to transfer genetic material from one cell to another.

Cell Wall Structure

Function and Composition

The cell wall provides structural support, maintains cell shape, and protects against osmotic pressure. In bacteria, the cell wall is primarily composed of peptidoglycan, a mesh-like polymer of sugars and amino acids.

• N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM) are the alternating sugar units in peptidoglycan.

• Chains of NAG and NAM are cross-linked by short peptide bridges (tetrapeptides).

Gram-Positive vs. Gram-Negative Bacteria

Bacterial cell walls are classified based on their response to the Gram stain:

Feature	Gram-Positive	Gram-Negative
Peptidoglycan Layer	Thick	Thin
Teichoic Acids	Present (linked to peptidoglycan and plasma membrane)	Absent
Outer Membrane	Absent	Present (contains lipopolysaccharide, LPS)
Periplasmic Space	Usually absent or small	Present (between outer and plasma membranes)
Sensitivity to Penicillin	Generally sensitive	Generally less sensitive
Gram Stain Color	Purple	Pink/Red

Lipopolysaccharide (LPS) in Gram-negative bacteria contains Lipid A, which can act as an endotoxin, causing fever and shock in humans.

Cell Membrane Structure and Function

Phospholipid Bilayer

The cytoplasmic (plasma) membrane is a selectively permeable barrier composed of a phospholipid bilayer with embedded proteins. The fluid mosaic model describes the dynamic arrangement of proteins and lipids within the membrane.

• Hydrophilic heads face outward toward water.

• Hydrophobic tails face inward, away from water.

• Proteins serve as channels, carriers, receptors, and enzymes.

• Some membranes contain sterol-like molecules (e.g., hopanoids) for stability.

Transport Across Membranes

Substances move across the membrane by passive or active processes:

• Passive transport: Does not require energy. Includes diffusion, facilitated diffusion, and osmosis.

• Active transport: Requires energy (usually ATP) to move substances against their concentration gradient.

The periplasmic space in Gram-negative bacteria contains digestive enzymes and transport proteins, aiding in nutrient acquisition and defense.

Key Terms and Definitions

• Biofilm: A community of microorganisms attached to a surface and embedded in a self-produced matrix.

• Endotoxin: A toxin present inside bacterial cells, especially Lipid A in Gram-negative bacteria, released when cells disintegrate.

• Teichoic acids: Polymers in Gram-positive cell walls that provide rigidity and can play a role in ion transport.

• Periplasm: The space between the cytoplasmic and outer membranes in Gram-negative bacteria, containing enzymes and transport proteins.

Example: Gram Staining

Gram staining is a differential staining technique that distinguishes bacteria based on cell wall structure:

1. Crystal violet stains all cells.

2. Iodine forms a complex with the dye.

3. Alcohol decolorizes Gram-negative cells (thin peptidoglycan), but not Gram-positive cells (thick peptidoglycan).

4. Safranin counterstains Gram-negative cells pink/red.

Application: Gram staining is essential for bacterial identification and guides antibiotic therapy.

Summary Table: Prokaryotic Cell Surface Structures

Structure	Composition	Function
Glycocalyx	Polysaccharides/polypeptides	Protection, adhesion, biofilm formation
Flagella	Flagellin protein	Motility
Fimbriae	Protein	Attachment to surfaces, biofilms
Pili	Protein	DNA transfer (conjugation)
Cell wall	Peptidoglycan	Shape, protection, osmotic stability
Plasma membrane	Phospholipid bilayer, proteins	Selective permeability, transport

Additional info:

• Some details, such as the specific arrangement of flagella and the role of teichoic acids, were expanded for clarity.

• Definitions and examples were added to ensure the notes are self-contained and suitable for exam preparation.