Microbial Cell Structure and Function: Study Notes

Microbial Cell Structure and Function

Overview of Microbial Cell Structures and Functions

This section introduces the major structural components of microbial cells, highlighting their roles in survival, adaptation, and interaction with the environment.

• Cell Envelope/Membrane: Composed of phospholipid bilayers, forms the boundary of the cell.

• Cell Walls: Made of peptidoglycan; thick in Gram-positive (G+) and thin in Gram-negative (G-) bacteria.

• Capsules: Polysaccharide layers that act as virulence factors by helping bacteria evade phagocytosis.

• Fimbriae: Structures for attachment to surfaces.

• Pili: Involved in conjugation (genetic exchange).

• Flagella: Responsible for movement.

• Inclusions: Serve as storage and energy reserves.

• Endospores: Enable survival under harsh conditions.

The Cell Envelope

Structure and Components

The cell envelope is a series of layered structures that surround the cytoplasm and mediate interactions with the environment.

• Cytoplasmic membrane: Present in all cells; primary permeability barrier.

• Cell wall: Found in most bacteria; provides rigidity and shape.

• Outer membrane: Present only in Gram-negative bacteria.

• S-layers: Protein or glycoprotein layers found in some bacteria and archaea.

Gram-Positive and Gram-Negative Bacteria

Cell Envelope Differences

Bacteria are classified based on their cell wall structure, which is revealed by the Gram stain.

• Gram-positive: Thick peptidoglycan layer, no outer membrane.

• Gram-negative: Thin peptidoglycan layer, outer membrane present, periplasmic space between membranes.

The Cytoplasmic (Cell) Membrane

Structure and Function

The cytoplasmic membrane is a phospholipid bilayer with embedded proteins, serving as a selective barrier and site for energy metabolism.

• Main function: Selective permeability—controls transport of nutrients and waste.

• Membrane proteins: Facilitate transport and energy-related processes.

Phospholipid Structure: Hydrophilic head (glycerol + phosphate + functional group) and hydrophobic tails (fatty acids).

Membrane Proteins

• Integral proteins: Embedded within the membrane.

• Transmembrane proteins: Span the entire membrane.

• Peripheral proteins: Loosely attached to the membrane surface.

Variations Across Domains

• Bacteria & Eukarya: Ester linkages in phospholipids; fatty acid tails.

• Archaea: Ether linkages; isoprene chains instead of fatty acids; can form lipid monolayers for increased stability.

Domain	Lipid Type	Linkage	Side Chain
Bacteria/Eukarya	Phospholipid	Ester	Fatty acid
Archaea	Phosphoglycerol diether/tetraether	Ether	Isoprene

Additional info: Archaeal membranes are more chemically stable due to ether linkages and branched isoprene chains.

The Cell Wall

Function and Composition

The cell wall provides structural support, maintains shape, and prevents lysis due to osmotic pressure.

• Peptidoglycan: Rigid polysaccharide layer unique to Bacteria; not found in Archaea or Eukarya.

• Structure: Alternating N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM) with β-1,4 linkages; short peptide chains attached to NAM.

• Cross-linking: Peptide bonds between glycan strands provide strength.

Gram-Positive vs. Gram-Negative Cell Walls

• Gram-positive: Thick peptidoglycan (20–35 nm), multiple layers, teichoic acids present.

• Gram-negative: Thin peptidoglycan, outer membrane with lipopolysaccharide (LPS), periplasmic space.

Destruction of Peptidoglycan

• Lysozyme: Enzyme that cleaves glycosidic bonds, found in human secretions.

• Penicillin: Antibiotic that blocks peptide cross-link formation.

Archaeal Cell Walls

Unique Features

• Lack peptidoglycan; typically lack outer membrane.

• Most have S-layer (protein shell) instead of polysaccharide wall.

• Methanogens possess pseudomurein: alternating N-acetylglucosamine and N-acetyltalosaminuronic acid, β-1,3 linkages, all L-amino acids, resistant to lysozyme and penicillin.

Feature	Bacteria	Archaea
Main wall polymer	Peptidoglycan	Pseudomurein or S-layer
Linkage type	β-1,4	β-1,3
Lysozyme/penicillin sensitivity	Yes	No

Summary Table: Key Cell Envelope Differences

Feature	Gram-Positive	Gram-Negative	Archaea
Peptidoglycan thickness	Thick	Thin	Absent
Outer membrane	Absent	Present	Absent
Teichoic acids	Present	Absent	Absent
LPS	Absent	Present	Absent
S-layer	Sometimes	Sometimes	Common