Microbial Cell Structure and Function: The Cell Envelope

The Cell Envelope

Overview of the Cell Envelope

The cell envelope is a series of layered structures that surround the cytoplasm of microbial cells, governing interactions with the environment. It is essential for maintaining cell integrity, shape, and mediating transport and communication with the external environment.

• Cytoplasmic membrane

• Cell wall

• Outer membrane

• S-layers

The Cytoplasmic Membrane

Structure and Function

The cytoplasmic membrane surrounds the cytoplasm, separating it from the environment. Its main function is selective permeability, allowing nutrients to enter and waste products to exit. Membrane proteins facilitate these processes and play a role in energy metabolism.

• Phospholipid bilayer with embedded proteins

• Contains both hydrophobic (fatty acid tails) and hydrophilic (glycerol + phosphate + functional group) components

• Fatty acids form the hydrophobic interior; hydrophilic head groups face the cytoplasm or external environment

Membrane Proteins

• Integral membrane proteins: embedded within the membrane

• Transmembrane proteins: span the entire membrane

• Peripheral membrane proteins: loosely attached to the membrane surface

Archaeal Cytoplasmic Membranes

• Contain ether linkages in phospholipids (unlike ester linkages in Bacteria and Eukarya)

• Archaeal lipids have isoprenes instead of fatty acids

Major Functions of the Cytoplasmic Membrane

• Permeability barrier: restricts passage of polar and charged molecules; transport proteins accumulate solutes against concentration gradients

• Protein anchor: holds proteins involved in transport, bioenergetics, and chemotaxis

• Energy conservation: site of generation and dissipation of the proton motive force

Transporting Nutrients into the Cell

Active Transport Mechanisms

Cells use active transport to accumulate solutes against their concentration gradients. This process is energy-driven and involves specialized transport proteins.

• Simple transport: uses transmembrane transport proteins, driven by the proton motive force

• Group translocation: substance is chemically modified during transport; driven by energy-rich organic compounds

• ABC system: consists of a binding protein, transmembrane transporter, and ATP-hydrolyzing protein

Types of Simple Transport

• Symport: solute and H+ cotransported in the same direction (e.g., E. coli lac permease)

• Antiport: solute and H+ transported in opposite directions

The Cell Wall

Function and Classification

The cell wall provides strength to withstand osmotic/turgor pressure, preventing cell lysis and maintaining cell shape and rigidity. Most bacteria are classified as Gram-positive or Gram-negative based on cell wall structure and Gram stain reaction.

Gram-Positive vs. Gram-Negative Cell Envelopes

• Gram-positive: cytoplasmic membrane + thick cell wall

• Gram-negative: cytoplasmic membrane, thin cell wall, outer membrane, and periplasmic space

Bacterial Cell Walls

• Peptidoglycan: rigid polysaccharide layer providing strength; found in all bacteria with a cell wall, absent in Archaea and Eukarya

• Gram-positive cell envelope: thick peptidoglycan layer (20–35 nm), up to 90% peptidoglycan, stabilized by peptide cross-links (e.g., glycine interbridges in Staphylococcus aureus)

• Teichoic acids: acidic molecules embedded in the cell wall, sometimes covalently linked to membrane lipids (lipoteichoic acids)

• Lysozyme: enzyme that cleaves glycosidic bonds in peptidoglycan, found in human secretions as a defense against bacterial infection

• Penicillin: antibiotic that blocks formation of peptide cross-links in peptidoglycan

Archaeal Cell Walls

• Structure differs from Bacteria; lack peptidoglycan and typically lack outer membrane

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

• Pseudomurein cell wall in methanogens: similar to peptidoglycan but resistant to lysozyme and penicillin

LPS: The Outer Membrane

Structure and Function

The outer membrane is a second lipid bilayer external to the cell wall in Gram-negative bacteria. It contains the lipopolysaccharide layer (LPS), which is important for surface recognition, virulence, and structural strength.

• LPS: composed of core polysaccharide, O-polysaccharide, and Lipid A (toxic endotoxin component)

• LPS replaces most phospholipids in the outer half of the outer membrane

• Porins: transmembrane proteins that allow entrance and exit of solutes

• Periplasm: space (~15 nm wide) between cytoplasmic and outer membranes, housing many extracellular proteins

Diversity of Cell Envelope Structure

S-Layers

• S-layer: paracrystalline structure of protein or glycoprotein, always the outermost layer if present

• Functions: strength, protection from lysis, shape determination, creation of periplasmic-like space, cell surface interactions, adhesion, and defense against host immune responses

Alternative Configurations

• Outer S-layer can surround Gram-positive or Gram-negative bacteria

• Many Archaea have only an S-layer outside the cytoplasmic membrane

• Pseudomurein cell walls in Archaea may exist with or without S-layer

• Some Archaea possess an outer membrane

• Some Bacteria (e.g., Mycoplasmas) and Archaea (e.g., Thermoplasma) lack cell walls but have tough cytoplasmic membranes containing sterols

Table: Comparison of Cell Envelope Structures

Type	Main Components	Unique Features
Gram-positive Bacteria	Cytoplasmic membrane, thick peptidoglycan wall	Teichoic acids, lipoteichoic acids
Gram-negative Bacteria	Cytoplasmic membrane, thin peptidoglycan wall, outer membrane (LPS)	Periplasm, porins, endotoxin (Lipid A)
Archaea	Cytoplasmic membrane, S-layer or pseudomurein wall	Ether-linked lipids, isoprenes, resistance to lysozyme/penicillin
Wall-less Bacteria/Archaea	Tough cytoplasmic membrane (sterols)	Examples: Mycoplasmas, Thermoplasma

Additional info: The proton motive force ($ext{PMF}$) is generated by the movement of protons across the membrane, creating an electrochemical gradient used for ATP synthesis and transport processes.