Functional Anatomy of Prokaryotic and Eukaryotic Cells: Structure, Composition, and Transport Mechanisms

Comparing Prokaryotic and Eukaryotic Cells

Overview and Definitions

Prokaryotic and eukaryotic cells are the two fundamental cell types in microbiology, distinguished by their structural and genetic organization.

• Prokaryote: Derived from Greek meaning 'prenucleus'; lacks a true nucleus.

• Eukaryote: Derived from Greek meaning 'true nucleus'; possesses a membrane-bound nucleus.

Key Differences

Feature	Prokaryote	Eukaryote
Chromosomes	One circular chromosome, not in a membrane	Paired chromosomes, in nuclear membrane
Histones	Absent	Present
Organelles	Absent	Present
Cell Wall	Peptidoglycan (Bacteria), Pseudomurein (Archaea)	Polysaccharide (when present)
Division	Binary fission	Mitosis

Size, Shape, & Arrangement of Bacterial Cells

General Characteristics

Bacterial cells exhibit diversity in size, shape, and arrangement, which aids in their identification and classification.

• Average size: 0.2 to 2.0 μm diameter × 2 to 8 μm length

• Example: E. coli is typically 1 μm × 2 μm

• Animal cell comparison: 10–100 μm

• Monomorphic: Most bacteria have a single, consistent shape

• Pleomorphic: Some bacteria can vary in shape

Shapes

• Bacillus: Rod-shaped

• Coccus: Spherical-shaped

• Spiral: Includes vibrio, spirillum, and spirochete forms

• Other: Star-shaped, rectangular

Arrangements

• Pairs: Diplococci, diplobacilli

• Clusters: Staphylococci

• Chains: Streptococci, streptobacilli

• Groups of four: Tetrads

• Cubelike groups of eight: Sarcinae

Structure of a Prokaryotic Cell

Major Components

• Capsule: Protective outer layer

• Cell wall: Provides shape and protection

• Plasma membrane: Regulates transport

• Cytoplasm: Contains cellular contents

• Nucleoid: Region containing DNA

• Ribosomes: Sites of protein synthesis

• Flagella, fimbriae, pili: Structures for motility and attachment

Glycocalyx (Sugar Coat)

Structure and Function

The glycocalyx is a viscous, gelatinous layer external to the cell wall, composed of polysaccharide and/or polypeptide.

• Types:

  • Slime layer: Loosely organized

  • Capsule: Well-organized, firmly attached

• Functions:

  • Contributes to virulence by preventing phagocytosis

  • Protection from antibiotics, chemicals, and desiccation

  • Aids in attachment to surfaces and biofilm formation

• Examples: Bacillus anthracis, Streptococcus pneumoniae, Klebsiella pneumoniae, Streptococcus mutans, Vibrio cholerae

Flagella and Axial Filaments

Flagella

• Filamentous appendages for motility

• Enable movement via runs and tumbles (taxis)

• Flagella proteins are H antigens (serovar identification)

• Bacteria without flagella are termed atrichous

Arrangements of Flagella

Type	Description
Peritrichous	Flagella distributed over entire cell
Monotrichous	Single flagellum at one pole
Lophotrichous	Tuft of flagella at one pole
Amphitrichous	Flagella at both poles

Axial Filaments (Endoflagella)

• Found in spirochetes

• Anchored at one end; rotation causes corkscrew movement

Fimbriae and Pili

Functions

• Fimbriae: Hairlike structures for attachment and biofilm formation; enable adherence to surfaces (e.g., Neisseria gonorrhoeae, E. coli O157)

• Pili: Involved in motility (gliding, twitching) and DNA transfer (conjugation pili)

The Cell Wall

Composition and Function

• Prevents osmotic lysis and protects the cell membrane

• Composed of peptidoglycan (in bacteria)

• Contributes to pathogenicity

• Targeted by lysozyme and penicillin

Peptidoglycan Structure

• Polymer of repeating disaccharides: N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM)

• Rows linked by polypeptides

Gram-Positive vs. Gram-Negative Cell Walls

Gram-Positive Cell Walls

• Thick peptidoglycan layer

• Teichoic acids (lipoteichoic and wall teichoic acids)

• Carry negative charge, regulate cation movement

• High susceptibility to penicillin, disrupted by lysozyme

Gram-Negative Cell Walls

• Thin peptidoglycan layer

• Periplasmic space between outer and plasma membranes

• Outer membrane contains lipopolysaccharide (LPS), lipoproteins, phospholipids

• Porins form channels

• Low susceptibility to penicillin

Outer Membrane Functions

• Protects from phagocytes, complement, antibiotics

• LPS:

  • O polysaccharide: antigenic function

  • Lipid A: endotoxin

• More susceptible to mechanical breakage than Gram-positive cells

Gram Stain Mechanism

• Crystal violet-iodine crystals form inside cell

• Gram-positive: Alcohol dehydrates peptidoglycan, crystals remain

• Gram-negative: Alcohol dissolves outer membrane, crystals wash out, cells are colorless; safranin stains cells

Atypical Cell Walls

• Acid-fast cell walls: Waxy lipid (mycolic acid) bound to peptidoglycan (Mycobacterium, Nocardia)

• Mycoplasmas: Lack cell walls, sterols in plasma membrane

• Archaea: Wall-less or walls of pseudomurein (lack NAM and D-amino acids)

The Plasma (Cytoplasmic) Membrane

Structure

• Phospholipid bilayer enclosing cytoplasm

• Peripheral, integral, and transmembrane proteins

• Fluid mosaic model: Membrane is dynamic, proteins and lipids move freely

Functions

• Selective permeability

• Enzymatic activity (ATP production)

• Photosynthetic pigments in some membranes (chromatophores)

• Damage by alcohols, detergents, antibiotics can cause leakage

Movement of Materials Across Membranes

Passive Processes

• Simple diffusion: Movement from high to low concentration until equilibrium is reached

• Facilitated diffusion: Transport via specific or nonspecific transporter proteins

• Osmosis: Net movement of water across a selectively permeable membrane; aquaporins facilitate water movement

• Osmotic pressure: Pressure needed to stop water movement

Active Processes

• Active transport: Requires transporter protein and ATP; moves substances against concentration gradient

• Group translocation: Substance is chemically altered during transport (requires PEP)

Osmosis and Solutions

Solution Type	Description
Isotonic	Solute concentrations equal inside and outside; water at equilibrium
Hypotonic	Lower solute outside; water moves into cell
Hypertonic	Higher solute outside; water moves out of cell

Key Equations

• Osmotic Pressure: $ext{Osmotic Pressure} = iMRT$ Where i is the van 't Hoff factor, M is molarity, R is the gas constant, and T is temperature in Kelvin.

Summary Table: Gram-Positive vs. Gram-Negative Cell Walls

Feature	Gram-Positive	Gram-Negative
Peptidoglycan	Thick	Thin
Teichoic acids	Present	Absent
Outer membrane	Absent	Present
Periplasmic space	Absent	Present
Porins	Absent	Present
Penicillin susceptibility	High	Low
Lysozyme sensitivity	High	Low

Additional info: These notes expand upon the provided slides and images, integrating textbook-level explanations and relevant examples for microbiology students.