Cell Wall Composition of Prokaryotic Cells
Introduction
Prokaryotic cells, which include bacteria and archaea, are characterized by the presence of a rigid cell wall. The cell wall provides structural support, determines cell shape, and protects the cell from osmotic pressure. The composition of the prokaryotic cell wall differs significantly between bacterial species, especially between Gram-positive and Gram-negative bacteria.
General Composition
The main component of the bacterial cell wall is peptidoglycan, also known as murein. Peptidoglycan is a complex polymer consisting of sugar chains cross-linked by short peptides. It provides mechanical strength and is unique to bacteria, making it an ideal target for antibiotics like penicillin.
1. Peptidoglycan Structure
- Composed of alternating units of N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM)
- Short peptide chains attach to NAM residues and link adjacent strands
- Forms a mesh-like structure around the cell membrane
Types of Bacterial Cell Walls
1. Gram-Positive Bacteria
Gram-positive bacteria have a thick peptidoglycan layer and stain violet in the Gram stain procedure.
- Thick, multilayered peptidoglycan (up to 90% of the wall)
- Contains teichoic acids and lipoteichoic acids that provide rigidity and help in ion transport
- No outer membrane
2. Gram-Negative Bacteria
These have a thinner peptidoglycan layer but a more complex cell wall structure.
- Thin peptidoglycan layer (about 10% of the wall)
- Surrounded by an outer membrane composed of lipopolysaccharides (LPS), phospholipids, and proteins
- Periplasmic space between the inner membrane and outer membrane containing enzymes and transport proteins
3. Acid-Fast Bacteria
Like Mycobacterium, these have cell walls rich in mycolic acids, giving them a waxy texture and resistance to many antibiotics and stains.
4. Archaea
Unlike bacteria, archaea do not have peptidoglycan. Instead, their cell walls contain pseudopeptidoglycan, proteins, or polysaccharides.
Functions of the Prokaryotic Cell Wall
- Maintains cell shape (e.g., cocci, bacilli, spirilla)
- Protects against osmotic lysis
- Acts as a barrier to harmful substances
- Anchors flagella in motile bacteria
- Plays a role in pathogenicity (especially LPS in Gram-negative bacteria)
Antibiotic Target
Peptidoglycan synthesis is a target for antibiotics. Drugs like penicillin and cephalosporins inhibit the enzymes that cross-link the peptidoglycan layers, leading to cell lysis in actively growing bacteria.
Conclusion
The cell wall is a defining feature of prokaryotic cells, particularly bacteria. Its composition varies among different groups but always plays a critical role in survival and interaction with the environment. Understanding the cell wall structure is crucial for microbiology, medicine, and biotechnology.