Functional Anatomy of Prokaryotic and Eukaryotic Cells: Study Guide

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Cell Structure of Prokaryotes and Eukaryotes

Comparison of Prokaryotic and Eukaryotic Cells

Prokaryotic and eukaryotic cells differ fundamentally in their structural organization, genetic material, and cellular processes. Understanding these differences is essential for microbiology students.

Prokaryotic Cells: Lack a nucleus; DNA is not enclosed by a membrane, usually present as a single circular chromosome. No membrane-bound organelles (e.g., mitochondria, ER, Golgi). Cell wall typically contains peptidoglycan. Divide by binary fission.
Eukaryotic Cells: Possess a true nucleus with DNA enclosed in a nuclear membrane. Multiple linear chromosomes. Membrane-bound organelles are present. Cell walls, when present, are composed of polysaccharides. Divide by mitosis.

Main distinguishing feature: Prokaryotes lack a nucleus, while eukaryotes have a nucleus.

Diagram of a prokaryotic cell with labeled structures TEM image of prokaryotic cell showing nucleoid, ribosomes, plasma membrane, cell wall, and capsule

Shapes and Arrangements of Bacteria

Basic Bacterial Shapes and Arrangements

Bacteria exhibit three primary shapes: coccus (spherical), bacillus (rod-shaped), and spiral. Their arrangement is determined by their division patterns and whether cells remain attached after division.

Coccus: Spherical-shaped. Arrangements include diplococci (pairs), streptococci (chains), tetrads (groups of four), sarcinae (cubelike groups of eight), and staphylococci (clusters).
Bacillus: Rod-shaped. Arrangements include single bacillus, diplobacilli (pairs), streptobacilli (chains), and coccobacillus (short, oval rods).
Spiral: Includes vibrio (curved rod), spirillum (rigid spiral), and spirochete (flexible spiral).

Streptococci: Cocci arranged in chains, dividing in one plane and remaining attached.

SEM images of cocci arrangements: diplococci, streptococci, tetrad, sarcinae, staphylococci SEM images of single bacillus and coccobacillus SEM images of diplobacilli and streptobacilli SEM images of spiral bacteria: vibrio, spirillum, spirochete

Glycocalyx: Structure and Function

Bacterial Capsules and Slime Layers

The glycocalyx is an external, viscous, gelatinous layer composed of polysaccharide and/or polypeptide. It exists in two forms: capsule (organized, firmly attached) and slime layer (unorganized, loosely attached).

Capsule: Neatly organized, firmly attached. Contributes to virulence by preventing phagocytosis.
Slime Layer: Unorganized, loosely attached. Facilitates biofilm formation.
Medical Importance: Capsules enable bacteria to evade the immune system, increasing pathogenicity.

TEM image showing bacterial capsules

Motility and Surface Structures

Flagella, Axial Filaments, Fimbriae, and Pili

Bacteria possess various appendages for motility and attachment. These structures are critical for movement, colonization, and genetic exchange.

Flagella: Long, filamentous appendages for swimming via rotation. Enable taxis (movement toward/away from stimuli).
Axial Filaments: Also called endoflagella; found in spirochetes. Located between cell wall and outer sheath, causing corkscrew motion.
Fimbriae: Short, hairlike structures for attachment to surfaces or host cells. Important for colonization.
Pili: Longer than fimbriae; involved in twitching/gliding motility and conjugation (DNA transfer).

Diagram showing bacterial movement: run and tumble SEM image of spirochete with axial filament TEM image showing fimbriae on a bacterial cell

Bacterial Cell Walls

Gram-Positive, Gram-Negative, and Acid-Fast Bacteria

Bacterial cell walls provide structural support and protection. Their composition determines staining properties and antibiotic susceptibility.

Gram-Positive: Thick peptidoglycan, teichoic acids, two rings in flagellar basal body, high susceptibility to penicillin, disrupted by lysozyme.
Gram-Negative: Thin peptidoglycan, outer membrane with LPS, four rings in flagellar basal body, low susceptibility to penicillin, resistant to lysozyme.
Acid-Fast: Peptidoglycan with waxy mycolic acid, similar to gram-positive, resistant to staining.

Gram-negative bacteria are harder to treat: Their outer membrane blocks antibiotics, porins limit drug entry, and LPS provides extra protection.

Type	Wall Structure	Antibiotic Susceptibility
Gram-Positive	Thick peptidoglycan, teichoic acid	High
Gram-Negative	Thin peptidoglycan, LPS, outer membrane	Low
Acid-Fast	Peptidoglycan + mycolic acid (waxy)	Variable

Diagram of gram-positive bacterial cell wall

Prokaryotic Plasma Membrane

Structure, Chemistry, and Functions

The prokaryotic plasma membrane is a phospholipid bilayer with embedded proteins, following the fluid mosaic model. It is essential for selective permeability, transport, enzymatic activity, and energy production.

Structure: Phospholipid bilayer, peripheral and integral proteins, self-sealing.
Chemistry: Composed of phospholipids and proteins; proteins act as channels, carriers, and enzymes.
Functions: Selective permeability, ATP production, photosynthesis (chromatophores in some bacteria).
Agents causing injury: Alcohols, quaternary ammonium, polymyxin antibiotics, polypeptide antibiotics.

Diagram and TEM image of plasma membrane in Vibrio cholerae Diagram of lipid bilayer and membrane proteins

Nucleoid and Ribosomes

Functions and Location in Prokaryotes

The nucleoid contains the bacterial chromosome and stores genetic information, controlling cell structure, metabolism, and reproduction. Ribosomes are the site of protein synthesis, translating mRNA into proteins.

Nucleoid: Single, circular, double-stranded DNA; not surrounded by a nuclear membrane; attached to plasma membrane.
Ribosomes: 70S ribosomes (50S + 30S subunits); site of protein synthesis; essential for enzyme production and metabolism.

TEM image of prokaryotic cell showing nucleoid and ribosomes Diagram of prokaryotic ribosome subunits

Endospores, Sporulation, and Germination

Survival Structures in Bacteria

Endospores are highly resistant survival structures formed by certain bacteria (e.g., Bacillus, Clostridium) under harsh conditions. Sporulation is the process of endospore formation, while germination is the return to a vegetative cell when conditions improve.

Endospores: Resist desiccation, heat, chemicals, radiation; not reproductive structures.
Sporulation: Occurs when nutrients are depleted; cell forms a thick protective structure around DNA.
Germination: Endospore absorbs water and resumes normal metabolism when conditions become favorable.

TEM image of Bacillus subtilis endospore

Ribosome Structure and Antibiotic Action

Comparison of Prokaryotic and Eukaryotic Ribosomes

Prokaryotic ribosomes are 70S (30S + 50S), while eukaryotic ribosomes are 80S (40S + 60S). Antibiotics like erythromycin target the 50S subunit of prokaryotic ribosomes, inhibiting protein synthesis and bacterial growth, but have minimal effect on eukaryotic ribosomes.

Prokaryotic Ribosomes: 70S, found in cytoplasm.
Eukaryotic Ribosomes: 80S, found in cytoplasm and rough ER.
Erythromycin: Binds 50S subunit, inhibits protein synthesis in bacteria; generally safe for human cells, but may affect mitochondria (which contain 70S ribosomes).

Diagram of prokaryotic ribosome subunits

Endosymbiotic Theory of Eukaryotic Evolution

Evidence for Prokaryotic Origins of Mitochondria

The endosymbiotic theory proposes that eukaryotic cells evolved from prokaryotic cells through symbiosis. Mitochondria share several features with bacteria, supporting this theory.

Size and Shape: Mitochondria are similar to bacteria.
DNA: Mitochondria contain circular DNA, like bacteria.
Replication: Mitochondria replicate independently by binary fission.
Ribosomes: Mitochondria have 70S ribosomes, similar to prokaryotes.
Protein Synthesis: Mitochondrial protein synthesis resembles bacterial processes; antibiotics affecting bacterial ribosomes also affect mitochondria.

Diagram of prokaryotic ribosome subunits (relevant to mitochondria)