Prokaryotic Structure: Cell Membranes & Walls (Bacteria and Archaea)

Prokaryotic Structure: Cell Membranes & Walls

Overview

This section explores the structural features of prokaryotic cells, focusing on the membranes and cell walls of bacteria and archaea. Understanding these structures is essential for classifying microbes, understanding their physiology, and targeting them with antibiotics.

Bacterial Morphology (Shape)

Common Shapes of Bacteria

• Coccus: Spherical-shaped bacteria (e.g., Staphylococcus aureus).

• Rod (Bacillus): Cylindrical-shaped bacteria (e.g., Bacillus megaterium).

• Spirochete: Spiral-shaped bacteria (e.g., Borrelia burgdorferi).

• Comma-shaped: Curved rods (e.g., Vibrio cholerae).

• Filamentous: Long, thread-like bacteria (e.g., Actinomyces).

• Budding & Appendaged: Bacteria with stalks or hyphae (e.g., Hyphomicrobium).

Example: The diversity in bacterial shapes is linked to their ecological niches and modes of motility.

Microbial Size Variation

Relative Sizes of Microbes

• Eukaryotic microbes: Protozoa, algae, fungi (10–100 μm)

• Prokaryotes: Bacteria, archaea (typically 0.4–10 μm)

• Viruses: 0.03–0.3 μm

Key Point: Smaller cell size increases surface area-to-volume ratio, enhancing nutrient uptake and growth rates.

Prokaryotic Cell Structure

General Features

• Capsule: Protective outer layer.

• Cell wall: Provides shape and protection.

• Plasma membrane: Selective barrier for transport.

• Nucleoid: Region containing highly condensed chromosome (no true nucleus).

• Plasmids: Small, circular DNA molecules.

• Ribosomes: Sites of protein synthesis.

• Flagella, fimbriae: Structures for motility and attachment.

Nucleoid

Genetic Organization

• Highly condensed chromosome located in the nucleoid region.

• No nuclear membrane (unlike eukaryotes).

Example: Prokaryotic DNA is typically a single, circular chromosome.

Plasmids

Types and Functions

• Extrachromosomal: Small, double-stranded, circular DNA molecules with their own origin of replication.

• Not essential for survival, but confer selective advantages such as antibiotic resistance.

• Types:

  • Conjugative (mating)

  • R plasmids (antibiotic resistance)

  • Virulence

  • Metabolic

Bacterial Cytoskeleton

Key Proteins

• MreB (actin homolog): Forms coils inside rod-shaped cells, maintains cell shape.

• FtsZ (tubulin homolog): Forms "Z ring" essential for cell division (septation).

• CreS (Crescentin) (intermediate filament homolog): Curves inner side of crescent-shaped bacteria, maintains cell shape.

Bacterial Cell Membrane

Structure and Composition

• Phospholipid bilayer: Main structural component.

• Peripheral and integral membrane proteins: Involved in transport and signaling.

• Non-polar lipids (hopanoids): Function similarly to cholesterol in eukaryotes, stabilizing the membrane.

• Carbohydrates: Often attached to proteins or lipids, involved in cell recognition.

Membrane Lipid Adaptation

• Physical state depends on fatty acid composition:

  • Longer hydrophobic chains = higher melting point

  • More saturated (no double bonds) = higher melting point

• Adaptation to temperature:

  • Higher temperature: More saturated, longer chains

  • Lower temperature: More unsaturated, shorter chains

  • Cyclic fatty acids (e.g., cyclopropane) increase membrane rigidity

Archaeal Membranes

Unique Features

• Ether-linked lipids (vs. ester-linked in bacteria/eukaryotes)

• Glycerol diethers and diglycerol tetraethers: Increase stability in extreme environments

• Cyclopentane rings: Further stabilize membrane structure

Bacterial Cell Wall

Peptidoglycan Structure and Function

• Peptidoglycan (murein): Strong, mesh-like polymer of sugars and amino acids

• Sugar chains (glycan) wrapped around cell, linked by short peptide chains

• Functions:

  • Maintains cell shape

  • Protects against osmotic lysis

  • Barrier to toxic substances

  • Can contribute to pathogenicity

Comparison: Archaea have pseudopeptidoglycan; plants have cellulose cell walls.

Peptidoglycan Structure (Zoomed In)

• Alternating N-acetylmuramic acid and N-acetylglucosamine sugars

• Peptide chains with alternating D- and L-amino acids

• Pentaglycine interbridges (in Gram-positive bacteria)

Gram Positive vs. Gram Negative Cell Walls

Key Differences

Feature	Gram Positive	Gram Negative
Peptidoglycan Layer	Thick	Thin
Teichoic Acids	Present	Absent
Outer Membrane	Absent	Present
Lipopolysaccharide (LPS)	Absent	Present
Periplasmic Space	Small	Large

Gram Stain: Differentiates bacteria based on cell wall structure using a dye that reacts with thick peptidoglycan.

Peptide Cross Bridges and Antibiotic Targeting

Transpeptidases

• Transpeptidase (penicillin-binding protein, PBP): Enzyme that crosslinks peptide chains in peptidoglycan.

• Targeted by β-lactam antibiotics (e.g., penicillin).

Antibiotic Resistance: Bacteria can develop resistance by altering PBPs or producing β-lactamases.

Lysozyme Action

Cleavage of Peptidoglycan

• Lysozyme: Enzyme that cleaves the β(1→4) glycosidic bond between N-acetylglucosamine and N-acetylmuramic acid.

• Destroys bacterial cell wall, leading to cell lysis.

Gram Positive Cell Walls

Teichoic Acids

• Glycerol teichoic acid: Polymers exclusive to Gram-positive bacteria.

• Contribute to cell wall rigidity and ion regulation.

Gram Negative Cell Walls

Lipopolysaccharide (LPS) and Porins

• LPS: Major component of the outer membrane; acts as an endotoxin.

• Porins: Proteins that form channels, increasing permeability of the outer membrane.

Lipopolysaccharide (LPS) as Endotoxin

Biological Significance

• Endotoxin: Cell component harmless when intact, but triggers strong immune response (septic shock) upon bacterial lysis.

Mycobacterial Cell Walls

Unique Features

• Waxy, hydrophobic cell wall containing mycolic acids.

• Resistant to acid decolorization (acid-fast staining diagnostic).

• Advantages: Protection from desiccation and antibiotics.

• Disadvantages: Slow nutrient uptake and growth.

Archaeal Cell Walls

Pseudopeptidoglycan Structure

• Pseudopeptidoglycan (pseudomurein): Polymer of alternating N-acetyltalosaminuronic acid and N-acetylglucosamine.

• Sugars linked by β(1→3) bonds (not cleaved by lysozyme).

• Peptide cross bridges differ from bacterial peptidoglycan (not susceptible to penicillin).

Additional info: Archaeal cell walls provide resistance to extreme environmental conditions.