Functional Anatomy of Prokaryotic and Eukaryotic Cells

Comparing Prokaryotic and Eukaryotic Cells: Overview

Prokaryotic and eukaryotic cells differ fundamentally in their structure and organization. Understanding these differences is essential for microbiology students, as it underpins microbial classification and physiology.

• Prokaryote: Derived from Greek for 'prenucleus'.

• Eukaryote: Derived from Greek for 'true nucleus'.

Size, Shape, & Arrangement of Bacterial Cells

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

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

• Example: E. coli is 1 μm × 2 μm

• Animal cell comparison: 10–100 μm

• Monomorphic: Most bacteria have a single shape

• Pleomorphic: Some bacteria can have multiple shapes

Common Shapes

• Bacillus: Rod-shaped

• Coccus: Spherical-shaped

• Spiral: Includes vibrio, spirillum, spirochete

• Star-shaped and Rectangular: Less common

Arrangements

• Pairs: diplococci, diplobacilli

• Clusters: staphylococci

• Chains: streptococci, streptobacilli

• Groups of four: tetrads

• Cubelike groups of eight: sarcinae

Structure of a Prokaryotic Cell

Prokaryotic cells possess specialized structures that contribute to their survival and pathogenicity.

• Capsule: Protective outer layer

• Cell wall: Provides structural support

• Plasma membrane: Regulates transport

• Cytoplasm: Contains cellular components

• Nucleoid: Region containing DNA

• Ribosomes: Sites of protein synthesis

• Flagella, fimbriae, pili: Appendages for movement and attachment

Glycocalyx (Sugar Coat)

The glycocalyx is an external layer that enhances bacterial survival and virulence.

• Types: Slime layer (loose) and Capsule (organized)

• Composition: Polysaccharide and/or polypeptide

• Functions: Prevents phagocytosis, protects from antibiotics and desiccation, aids in attachment, forms biofilms

• Examples: Bacillus anthracis, Streptococcus pneumoniae, Klebsiella pneumoniae

Flagella

Flagella are motility structures that enable bacteria to move in response to environmental stimuli.

• Structure: Filamentous appendages external to the cell

• Function: Motility via runs and tumbles (taxis)

• Antigenic properties: Flagella proteins are H antigens (e.g., E. coli O157:H7)

• Arrangements: Peritrichous, monotrichous, lophotrichous, amphitrichous

• Atrichous: Bacteria without flagella

Axial Filaments

Axial filaments, or endoflagella, are unique to spirochetes and facilitate corkscrew movement.

• Location: Anchored at one end of the cell

• Function: Rotation causes cell to move like a corkscrew

• Example: Spirochetes

Fimbriae and Pili

Fimbriae and pili are surface structures involved in attachment, motility, and genetic exchange.

• Fimbriae: Hairlike, enable attachment and biofilm formation (Neisseria gonorrhoeae, E. coli O157)

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

The Cell Wall

The cell wall is a rigid structure that maintains cell shape and prevents osmotic lysis.

• Composition: Peptidoglycan (polymer of N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM))

• Function: Contributes to pathogenicity

• Susceptibility: Damaged by lysozyme and penicillin

Peptidoglycan Structure

• Repeating disaccharide rows: NAG and NAM

• Rows linked by polypeptides

Gram-Positive vs. Gram-Negative Cell Walls

Gram Stain Mechanism

• Gram-positive: Alcohol dehydrates peptidoglycan, retains crystal violet-iodine complex

• Gram-negative: Alcohol dissolves outer membrane, leaves holes in peptidoglycan, crystal violet washes out, cells are colorless until counterstained with safranin

Atypical Cell Walls

• Acid-fast cell walls: Like gram-positive, but with waxy mycolic acid (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

The plasma membrane is a selectively permeable barrier composed of a phospholipid bilayer and proteins.

• Fluid mosaic model: Membrane is as viscous as olive oil; proteins and phospholipids move freely

• Functions: Selective permeability, enzyme activity, photosynthetic pigments (chromatophores)

Movement of Materials Across Membranes

• Passive processes: Move substances from high to low concentration; no energy required

• Active processes: Move substances from low to high concentration; require energy (ATP) and transporter proteins

Passive Processes

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

• Facilitated diffusion: Solute combines with transporter protein; transports ions and larger molecules

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

• Osmotic pressure: Pressure needed to stop water movement across the membrane

Osmosis Solutions

• 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

Active Processes

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

• Group translocation: Requires transporter protein and phosphoenolpyruvic acid (PEP); substance is chemically altered during transport

Additional info: These notes are based on textbook slides and cover the essential structural and functional differences between prokaryotic and eukaryotic cells, with emphasis on bacterial cell anatomy, cell wall composition, and membrane transport mechanisms.