Most prokaryotic cells, which are single-celled organisms lacking a true nucleus and other membrane-bound internal structures, possess a cell wall. This outer layer surrounds the cell membrane, providing structural support and protection. While not present in all prokaryotes, such as some mycoplasmas, the cell wall is a common and important feature for bacteria and archaea. It functions as a semi-rigid barrier, separating the cell’s internal components from the external environment.
The Defining Layer: Peptidoglycan
The cell walls of most bacteria are characterized by a unique substance called peptidoglycan. This complex polymer forms a mesh-like layer that encases the bacterial cytoplasmic membrane. Peptidoglycan is composed of repeating disaccharide units, N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM), linked together to form long glycan strands. These strands are then cross-linked by short peptide chains.
This intricate, lattice-like structure gives the bacterial cell wall its strength and helps maintain the bacterium’s specific shape. The cross-linking of these sugar and amino acid components creates a robust sacculus around the cell. Peptidoglycan is a distinct feature of bacteria and is not found in the cell walls of other life forms.
Variations and Distinctions
While peptidoglycan is a hallmark of bacteria, the composition of prokaryotic cell walls can vary. Bacteria are broadly categorized into two groups based on their cell wall structure: Gram-positive and Gram-negative. Gram-positive bacteria possess a thick, multilayered peptidoglycan wall. These walls also contain teichoic acids, which are polymers embedded within the peptidoglycan layers.
In contrast, Gram-negative bacteria have a more complex cell wall structure. They feature a much thinner peptidoglycan layer, surrounded by an additional outer membrane. This outer membrane contains lipopolysaccharides and lipoproteins. The presence or absence of this outer membrane and the thickness of the peptidoglycan layer account for the different staining properties observed in the Gram stain procedure.
Archaea, another domain of prokaryotes, possess cell walls that are chemically distinct from those of bacteria, lacking peptidoglycan entirely. Instead, archaeal cell walls exhibit diverse compositions. Some archaea have cell walls made of pseudopeptidoglycan, which is structurally similar to bacterial peptidoglycan but differs in its sugar components and linkages. Other archaea may have cell walls composed of glycoproteins, polysaccharides, or a proteinaceous surface layer (S-layer).
Beyond Structure: The Cell Wall’s Crucial Roles
The prokaryotic cell wall performs several important functions. A primary role is protection against osmotic lysis, the bursting of the cell due to excessive water intake. Prokaryotic cells often live in environments where their internal solute concentration is higher than their surroundings, causing water to move into the cell. The rigid cell wall counteracts this internal pressure, preventing the cell membrane from expanding to the point of rupture.
The cell wall also helps maintain the specific shape of the prokaryotic cell, which is important for processes like nutrient acquisition and movement. It regulates the passage of certain molecules into and out of the cell. The bacterial cell wall, particularly its peptidoglycan component, is a target for certain antibiotics, such as penicillin. Penicillin interferes with the synthesis of peptidoglycan, preventing the proper formation of cross-links in the cell wall, which weakens the structure and can lead to cell lysis. This mechanism is effective because human cells do not possess cell walls.
Prokaryotic cell walls are distinct from eukaryotic cell walls found in plants and fungi. Plant cell walls are primarily composed of cellulose, a polysaccharide, which provides rigidity and support to plant cells. Fungal cell walls are typically made of chitin.