Functional Anatomy of Prokaryotic and Eukaryotic Cells: Structure and Function

Functional Anatomy of Prokaryotic and Eukaryotic Cells

Overview: Prokaryotic vs. Eukaryotic Cells

Cells are the fundamental units of life, and in microbiology, they are classified as either prokaryotic or eukaryotic based on structural and functional differences. Understanding these differences is essential for studying microbial physiology, genetics, and taxonomy.

• Prokaryotes: Organisms whose cells lack a true nucleus and membrane-bound organelles. Includes Bacteria and Archaea.

• Eukaryotes: Organisms with cells containing a true nucleus and various organelles. Includes fungi, algae, protozoa, and helminths.

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

The Size, Shape, and Arrangement of Bacterial Cells

Size and Morphology

Bacteria exhibit a variety of shapes and arrangements, which are important for identification and classification.

• Average size: 0.2–2.0 μm in diameter, 2–8 μm in length

• Monomorphic: Most bacteria have a single, consistent shape

• Pleomorphic: Some bacteria can vary in shape

Common Shapes

• Bacillus: Rod-shaped

• Coccus: Spherical

• Spiral: Includes vibrio (comma-shaped), spirillum (rigid spiral), and spirochete (flexible spiral)

• Star-shaped and Rectangular: Rare morphologies

Arrangements

• Pairs: Diplococci, diplobacilli

• Chains: Streptococci, streptobacilli

• Clusters: Staphylococci

• Groups of four: Tetrads

• Cubelike groups of eight: Sarcinae

Structure of a Prokaryotic Cell

Major Components

Prokaryotic cells have a simple structure but contain all the necessary components for life.

• Cell wall: Provides shape and protection

• Plasma membrane: Regulates transport

• Cytoplasm: Contains enzymes, nutrients, and genetic material

• Nucleoid: Region containing the bacterial chromosome

• Ribosomes: Sites of protein synthesis

• Inclusions: Storage granules

• External structures: Glycocalyx, flagella, fimbriae, pili

Glycocalyx

Structure and Function

The glycocalyx is a viscous, gelatinous layer external to the cell wall, composed of polysaccharide and/or polypeptide. It exists as either a capsule (organized, firmly attached) or a slime layer (unorganized, loosely attached).

• Capsules prevent phagocytosis, contributing to virulence

• Extracellular polymeric substance (EPS) helps form biofilms

Flagella, Archaella, and Axial Filaments

Flagella

Flagella are long, filamentous appendages that provide motility to bacteria. They are composed of the protein flagellin and consist of three parts: filament, hook, and basal body.

• Arrangement: Peritrichous (all over), monotrichous (single, polar), lophotrichous (tuft at one end), amphitrichous (both ends)

• Function: Movement toward/away from stimuli (taxis), "run and tumble" motility

• H antigens: Flagellar proteins used for serotyping

Archaella

Archaella are motility structures found in Archaea, composed of glycoproteins called archaellins. They rotate like bacterial flagella but are structurally distinct.

Axial Filaments

Axial filaments, or endoflagella, are found in spirochetes. They are anchored at one end and cause the cell to move in a corkscrew motion.

Fimbriae and Pili

Fimbriae

Fimbriae are hairlike appendages that allow bacteria to adhere to surfaces and each other, playing a key role in colonization and biofilm formation.

Pili

Pili are longer than fimbriae and are involved in motility (gliding, twitching) and the transfer of DNA between cells (conjugation pili).

The Cell Wall

Structure and Function

The bacterial cell wall is a rigid structure that prevents osmotic lysis, maintains shape, and contributes to pathogenicity. It is primarily composed of peptidoglycan in bacteria.

• Peptidoglycan: Polymer of N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM) linked by polypeptides

Gram-Positive Cell Walls

• Thick peptidoglycan layer

• Teichoic acids (lipoteichoic and wall teichoic acids) provide rigidity and antigenic specificity

• Two rings in basal body of flagella

• Produce exotoxins; highly susceptible to penicillin; disrupted by lysozyme

Gram-Negative Cell Walls

• Thin peptidoglycan layer

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

• Four rings in basal body of flagella

• Produce endotoxins and exotoxins; low susceptibility to penicillin

• Porins form channels through the outer membrane

Gram Stain Mechanism

The Gram stain differentiates bacteria based on cell wall structure:

• Gram-positive: Alcohol dehydrates peptidoglycan, trapping crystal violet-iodine complex (cells appear purple)

• Gram-negative: Alcohol dissolves outer membrane, crystal violet-iodine complex washes out, safranin counterstain colors cells red

Atypical Cell Walls

• Acid-fast cell walls: Like Gram-positive but with waxy mycolic acid (e.g., Mycobacterium, Nocardia)

• Mycoplasmas: Lack cell walls; have sterols in plasma membrane

• Archaea: May lack cell walls or have walls of pseudomurein (lack NAM and D-amino acids)

Damage to the Cell Wall

• Lysozyme: Hydrolyzes bonds in peptidoglycan

• Penicillin: Inhibits peptide bridges in peptidoglycan

• Protoplast: Wall-less Gram-positive cell

• Spheroplast: Wall-less Gram-negative cell

• L forms: Wall-less cells that swell into irregular shapes

The Plasma (Cytoplasmic) Membrane

Structure

The plasma membrane is a phospholipid bilayer with embedded proteins, following the fluid mosaic model. It is selectively permeable and self-sealing.

Functions

• Selective permeability: Regulates passage of substances

• Contains enzymes for ATP production

• Photosynthetic pigments may be present on infoldings called chromatophores

Movement of Materials Across Membranes

Passive Processes

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

• Facilitated diffusion: Solute combines with transporter protein; moves with concentration gradient

• Osmosis: Movement of water across a selectively permeable membrane

• Osmotic pressure: Pressure needed to stop water movement

• Isotonic: Equal solute concentrations

• Hypotonic: Lower solute outside; water enters cell (risk of lysis)

• Hypertonic: Higher solute outside; water leaves cell (plasmolysis)

Active Processes

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

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

Cytoplasm and Internal Structures

Cytoplasm

The cytoplasm is the substance inside the plasma membrane, consisting of water, proteins, carbohydrates, lipids, ions, and a cytoskeleton.

Nucleoid

• Bacterial chromosome: Circular DNA containing genetic information

• Plasmids: Extrachromosomal DNA elements; may carry antibiotic resistance or toxin genes

Ribosomes

Ribosomes are the sites of protein synthesis, composed of protein and rRNA. Prokaryotic ribosomes are 70S (50S + 30S subunits).

Inclusions

• Metachromatic granules: Phosphate reserves

• Polysaccharide granules: Energy reserves

• Lipid inclusions: Energy reserves

• Sulfur granules: Energy reserves

• Carboxysomes: Contain RuBisCO for CO2 fixation

• Gas vacuoles: Maintain buoyancy

• Magnetosomes: Iron oxide inclusions; destroy H2O2

Endospores

Formation and Function

Endospores are highly resistant, dormant structures formed by certain bacteria (e.g., Bacillus, Clostridium) when nutrients are depleted. They are resistant to desiccation, heat, chemicals, and radiation.

• Sporulation: Process of endospore formation

• Germination: Endospore returns to vegetative state