Chapter 2: Microbial Cell Structure and Function

Prokaryotic Cell Envelope

The prokaryotic cell envelope consists of multiple layers that surround the cytoplasm, providing protection and mediating interactions with the environment. The envelope is essential for maintaining cell integrity and facilitating various cellular functions.

• Components: The main components of the prokaryotic cell envelope include the cytoplasmic membrane, cell wall, outer membrane (in some bacteria), and S-layer (surface layer).

• Functions: These structures protect the cell, regulate transport, and contribute to cell shape and rigidity.

• Variations: The envelope structure varies among different prokaryotes, such as Gram-positive, Gram-negative, and wall-less bacteria (e.g., Mycoplasma).

Example: Electron micrographs show differences in envelope structure among various bacteria, such as thick cell walls in Gram-positive bacteria and the absence of a cell wall in Mycoplasma.

Cytoplasmic Membrane Structure

The cytoplasmic membrane is a selective barrier that separates the cytoplasm from the external environment. It is primarily composed of a phospholipid bilayer with embedded proteins.

• Phospholipid Bilayer: Consists of hydrophilic head groups facing outward and hydrophobic fatty acid tails facing inward.

• Proteins: Includes integral and peripheral membrane proteins that facilitate transport, signaling, and structural support.

• Function: Controls the movement of substances into and out of the cell, maintains homeostasis, and supports energy generation.

Example: The fluid mosaic model illustrates the dynamic nature of the membrane, with proteins and lipids moving laterally within the bilayer.

Archaeal vs. Bacterial Membranes

Archaeal membranes differ significantly from bacterial and eukaryotic membranes in their chemical composition and structure.

• Linkages: Archaeal phospholipids contain ether linkages, while bacterial and eukaryotic phospholipids have ester linkages.

• Lipid Composition: Archaeal lipids are based on isoprene units instead of fatty acids, forming glycerol diethers and tetraethers.

• Membrane Structure: Archaeal membranes can exist as monolayers, bilayers, or mixtures, providing stability under extreme conditions.

Example: Chemical diagrams show the difference between ester-linked fatty acids in bacteria/eukarya and ether-linked isoprenes in archaea.

Additional info: Archaeal membranes are more resistant to heat and chemical stress due to their unique lipid composition, which is an adaptation to extreme environments.