Comparing Prokaryotic and Eukaryotic Cells

Overview of Cell Types

Prokaryotic and eukaryotic cells represent two fundamental types of cellular organization in biology. Understanding their structural and functional differences is essential for microbiology students.

• Prokaryotes: Cells lacking a membrane-bound nucleus and organelles. Includes bacteria and archaea.

• Eukaryotes: Cells with a true nucleus and membrane-bound organelles. Includes plants, animals, fungi, and protists.

• Key Differences: Chromosome structure, presence of histones, cell wall composition, and division mechanisms.

Example: Bacteria (prokaryotes) have a single circular chromosome, while human cells (eukaryotes) have paired chromosomes within a nuclear membrane.

The Size, Shape, and Arrangement of Bacterial Cells

Bacterial Morphology

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

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

• Monomorphic: Most bacteria have a single, consistent shape.

• Pleomorphic: Some bacteria can vary in shape.

• Basic Shapes:

  • Bacillus: Rod-shaped

  • Coccus: Spherical-shaped

  • Spiral: Includes vibrio (curved), spirillum (corkscrew), and spirochete (helical)

  • Other Shapes: Star-shaped, rectangular

Example: Vibrio cholerae is a vibrio-shaped bacterium.

Arrangement of Cocci and Bacilli

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

• Cocci Arrangements:

  • Pairs: diplococci

  • Chains: streptococci

  • Clusters: staphylococci

  • Groups of four: tetrads

  • Cubelike groups of eight: sarcinae

• Bacilli Arrangements:

  • Single bacillus

  • Pairs: diplobacilli

  • Chains: streptobacilli

  • Coccobacillus: short, oval rods

Structure of a Prokaryotic Cell

Cell Components

Prokaryotic cells contain several key structures, each with specific functions.

• Capsule: Protective layer outside the cell wall

• Cell Wall: Provides structural support and shape

• Plasma Membrane: Controls entry and exit of substances

• Cytoplasm: Site of metabolic activity

• Nucleoid: Region containing DNA

• Plasmid: Small, circular DNA molecules

• Fimbriae: Attachment structures

• Flagella: Motility structures

• Inclusions: Storage granules

Glycocalyx

Structure and Function

The glycocalyx is an external layer found in many bacteria, contributing to their survival and pathogenicity.

• Composition: Polysaccharide and/or polypeptide

• Types:

  • Capsule: Organized, firmly attached

  • Slime layer: Unorganized, loose

• Functions:

  • Virulence: Capsules prevent phagocytosis and aid in adherence

  • Biofilm formation: Extracellular polymeric substance helps attach to surfaces

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

Flagella, Axial Filaments, Fimbriae, and Pili

Flagella

Flagella are filamentous appendages that provide motility to bacteria.

• Structure: Composed of filament, hook, and basal body

• Function: Propel bacteria by rotation

• Powered by: Proton gradient

Flagella Arrangements

Bacteria can have different flagellar arrangements, which affect their motility.

• Peritrichous: Flagella all over the cell

• Monotrichous: Single flagellum at one end

• Lophotrichous: Multiple flagella at one end

• Amphitrichous: Flagella at both ends

Flagellar Movement

Flagella rotate to produce "runs" and "tumbles," allowing bacteria to move toward or away from stimuli (taxis).

• Run: Flagella rotate together for straight movement

• Tumble: Flagella rotate independently, causing random movement

Axial Filaments

Axial filaments, or endoflagella, are found in spirochetes and enable corkscrew-like movement.

• Structure: Anchored at one end, wrap around the cell

• Function: Rotation causes cell to move in a spiral

Fimbriae and Pili

Fimbriae and pili are hairlike appendages with distinct functions.

• Fimbriae: Enable attachment to surfaces and biofilm formation

• Pili: Involved in motility and DNA transfer (conjugation)

The Cell Wall

Structure and Function

The bacterial cell wall is a critical structure for maintaining cell shape, protecting against osmotic lysis, and contributing to pathogenicity.

• Composition: Peptidoglycan (in bacteria)

• Function: Structural support, protection, antibiotic target

Peptidoglycan Structure

Peptidoglycan is a polymer of repeating disaccharides (N-acetylglucosamine and N-acetylmuramic acid) linked by polypeptides, forming a lattice.

• Penicillin: Interferes with peptide cross-bridges, weakening the cell wall

Gram-Positive Cell Walls

Gram-positive bacteria have thick peptidoglycan layers and teichoic acids, which regulate cation movement and provide antigenic specificity.

• Teichoic acids: Lipoteichoic acid links cell wall to plasma membrane; wall teichoic acid links to peptidoglycan

• Susceptibility: High susceptibility to penicillin; disrupted by lysozyme

Gram-Negative Cell Walls

Gram-negative bacteria have thin peptidoglycan, an outer membrane with lipopolysaccharide (LPS), and a periplasmic space.

• LPS: Contains O polysaccharide (antigen) and lipid A (endotoxin)

• Porins: Proteins forming channels in the membrane

• Susceptibility: Low susceptibility to penicillin; produce endotoxins and exotoxins

Gram Stain Mechanism

The Gram stain differentiates bacteria based on cell wall structure.

• Gram-positive: Retain crystal violet stain due to thick peptidoglycan

• Gram-negative: Lose crystal violet stain, appear colorless until counterstained with safranin

Atypical Cell Walls

Some bacteria have atypical cell walls, such as acid-fast bacteria and mycoplasmas.

• Acid-fast: Thick peptidoglycan with waxy mycolic acid; stain with carbolfuchsin

• Mycoplasmas: Lack cell walls; sterols in plasma membrane

• Archaea: Wall-less or walls of pseudomurein

The Plasma (Cytoplasmic) Membrane

Structure

The plasma membrane is a phospholipid bilayer with embedded proteins, forming a fluid mosaic model.

• Phospholipid bilayer: Hydrophilic heads, hydrophobic tails

• Proteins: Peripheral, integral, and transmembrane

• Carbohydrates: Glycoproteins and glycolipids

Function

The plasma membrane controls selective permeability, contains enzymes for ATP production, and may have photosynthetic pigments (chromatophores).

• Selective permeability: Allows passage of some molecules

• ATP production: Enzymes embedded in membrane

• Chromatophores: Membrane foldings with pigments

Movement of Materials Across Membranes

Passive Processes

Passive transport does not require energy and moves substances from high to low concentration.

• Simple diffusion: Movement of solutes until equilibrium

• Facilitated diffusion: Uses transporter proteins for ions and larger molecules

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

Active Processes

Active transport requires energy (ATP) to move substances against the concentration gradient.

• Active transport: Uses transporter proteins and ATP

• Group translocation: Substance is chemically altered as it crosses the membrane

Cytoplasm, Nucleoid, Ribosomes, and Inclusions

Cytoplasm

The cytoplasm is a thick, aqueous substance inside the plasma membrane, containing DNA, ribosomes, and inclusions.

• Cytoskeleton: Fibers for cell division, shape, and DNA movement

Nucleoid

The nucleoid contains the bacterial chromosome (circular, double-stranded DNA) and plasmids (extrachromosomal DNA).

• Plasmids: Carry non-crucial genes, replicate independently, may transfer between cells

Ribosomes

Ribosomes are the sites of protein synthesis, composed of protein and ribosomal RNA.

• Prokaryotic ribosomes: 70S (50S + 30S subunits)

• Antibiotics: Streptomycin, gentamicin, erythromycin, chloramphenicol target prokaryotic ribosomes

Inclusions

Inclusions are reserve deposits of nutrients within the cytoplasm.

• Types: Metachromatic granules (phosphate), polysaccharide granules, lipid inclusions, sulfur granules, carboxysomes, gas vacuoles, magnetosomes

Endospores

Formation and Function

Endospores are highly resistant, dormant structures formed by certain bacteria under nutrient depletion.

• Resistant to: Desiccation, heat, chemicals, radiation

• Produced by: Bacillus and Clostridium

• Sporulation: Endospore formation

• Germination: Return to vegetative state

Comparing Eukaryotic and Prokaryotic Structures

Flagella and Cilia

Eukaryotic flagella and cilia are projections used for locomotion or moving substances along the cell surface.

• Flagella: Long, few in number

• Cilia: Short, numerous

• Structure: Microtubules arranged in a 9+2 array

• Movement: Wavelike motion

Cell Wall and Glycocalyx

Eukaryotic cell walls are found in plants, algae, and fungi, composed of carbohydrates. Glycocalyx is found in animal cells, strengthening the cell surface and aiding in cell recognition.

Plasma Membrane

Eukaryotic plasma membranes are similar to prokaryotic membranes but contain sterols and carbohydrates for attachment and recognition. Functions include selective permeability and endocytosis.

Cytoplasm

Eukaryotic cytoplasm contains cytosol, cytoskeleton (microfilaments, intermediate filaments, microtubules), and exhibits cytoplasmic streaming.

Ribosomes

Eukaryotic ribosomes are 80S (60S + 40S subunits), found free in cytoplasm or bound to endoplasmic reticulum. 70S ribosomes are present in mitochondria and chloroplasts.

The Evolution of Eukaryotes

Endosymbiotic Theory

The endosymbiotic theory explains the origin of eukaryotes, proposing that larger bacterial cells engulfed smaller ones, leading to the development of mitochondria and chloroplasts.

• Evidence: Mitochondria and chloroplasts resemble bacteria, have circular DNA, reproduce independently, have bacterial-like ribosomes, and double membranes.

Example: Sequenced genomes of mitochondria and chloroplasts are similar to known bacteria (Cyanobacteria and Rickettsia).