Prokaryotic Cell Structure: Membranes and Cell Walls

Cell (Plasma) Membrane

Structure and Function of the Plasma Membrane

The cell (plasma) membrane is a fundamental structure in all cells, serving as a selective barrier and site for essential cellular processes. In prokaryotes, it plays several critical roles:

• Site of ATP synthesis: The plasma membrane is the location of energy metabolism, where ATP is generated via processes such as oxidative phosphorylation.

• Transport: It regulates the movement of nutrients into the cell and the removal of waste products out of the cell through various transport proteins.

• DNA anchoring: The membrane helps anchor DNA during replication, ensuring proper segregation of genetic material.

• Cell wall synthesis: It provides a platform for the synthesis of the cell wall, which is essential for cell shape and protection.

Key components:

• Phospholipid bilayer: Forms the basic structure, providing fluidity and selective permeability.

• Proteins: Integral and peripheral proteins serve as channels, receptors, and enzymes.

• Carbohydrates: Attached to proteins and lipids, involved in cell recognition and signaling.

Example: The electron transport chain in bacteria is located in the plasma membrane, facilitating ATP production.

Prokaryotic Cell Walls

General Functions and Importance

The cell wall is a defining feature of most prokaryotes, providing structural integrity and protection:

• Shape and structure: Maintains cell shape and prevents lysis due to osmotic pressure.

• Attachment: Assists some cells in adhering to surfaces or other cells.

• Protection: Shields against environmental stress and some antimicrobial agents.

• Selective target: Absent in animal cells, making bacterial cell walls a prime target for antibiotics.

Example: Penicillin targets cell wall synthesis, making it effective against bacteria but not animal cells.

Prokaryotic Cell Wall Composition

Bacterial Cell Walls and Peptidoglycan

Most bacterial cell walls are composed of peptidoglycan, a complex polymer of sugars and amino acids:

• Peptidoglycan structure: Consists of alternating sugars N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM).

• Chemical structure:

$\text{NAG: C}_8\text{H}_{15}\text{NO}_6$ $\text{NAM: C}_9\text{H}_{17}\text{NO}_8$

• Crosslinking: Chains of NAG and NAM are connected by tetrapeptide crossbridges, which may be covalently bonded or linked by short amino acid chains (especially in Gram-positive bacteria).

Example: The rigidity of the bacterial cell wall is due to the extensive crosslinking of peptidoglycan.

Gram-Positive vs. Gram-Negative Cell Walls

Structural Differences and Staining Characteristics

Bacterial cell walls are classified into two main types based on their structure and response to Gram staining:

• Gram-positive:

  • One membrane

  • Thick layer of peptidoglycan

  • Contains unique polyalcohols called teichoic acids

  • Retains crystal violet dye in Gram staining, appearing purple

• Gram-negative:

  • Two membranes: inner and outer

  • Outer membrane contains porins (channels for molecule diffusion)

  • Thin layer of peptidoglycan

  • Outer membrane contains phospholipids, proteins, and lipopolysaccharide (LPS)

  • Does not retain crystal violet dye; stains pink after Gram staining

Example: Escherichia coli is Gram-negative, while Staphylococcus aureus is Gram-positive.

Summary Table: Gram-Positive vs. Gram-Negative Cell Walls

Feature	Gram-Positive	Gram-Negative
Number of Membranes	1	2
Peptidoglycan Layer	Thick	Thin
Teichoic Acid	Yes	No
Lipopolysaccharide (LPS)	No	Yes
Porins	No	Yes
Gram Stain Color	Purple	Pink

Additional info: The presence of LPS in Gram-negative bacteria contributes to their pathogenicity and resistance to certain antibiotics.