Prokaryotes are diverse, ancient microscopic life forms found in nearly every environment. These single-celled organisms have a simple cellular structure, lacking a membrane-bound nucleus and other specialized compartments. Understanding them often involves examining their outer protective layers, particularly the presence or absence of a cell wall.
The Nature of Prokaryotes and Their Cell Walls
Most prokaryotes, broadly categorized into Bacteria and Archaea, possess a cell wall. This defining feature influences their survival and interaction with the environment. While a cell wall is common, its specific chemical makeup varies significantly between bacteria and archaea. This difference in composition distinguishes the two prokaryotic domains.
The prokaryotic cell wall provides a rigid outer layer surrounding the cell membrane. This structural component helps maintain cell shape and offers protection. Its presence enables them to thrive in diverse and often challenging habitats.
Building Blocks of Prokaryotic Cell Walls
Bacterial cell walls primarily consist of peptidoglycan, a unique polymer also known as murein. Peptidoglycan is a mesh-like layer formed from alternating sugar derivatives, N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM), linked by short peptide chains. These peptide cross-bridges create a strong, interconnected network around the bacterial cell.
Bacteria are classified as either Gram-positive or Gram-negative based on differences in their cell wall structure, which affects how they react to a staining procedure. Gram-positive bacteria have a thick peptidoglycan layer, sometimes up to 40 layers, positioned outside their single cell membrane. This robust wall also contains teichoic acids, polymers contributing to its stability. In contrast, Gram-negative bacteria possess a much thinner peptidoglycan layer, typically a few layers thick, located between two membranes. An outer membrane, containing lipopolysaccharides (LPS), envelops this thin peptidoglycan layer, adding another protective barrier.
Archaeal cell walls, unlike those of bacteria, do not contain peptidoglycan. Instead, archaea utilize various other materials for their cell walls. Some archaea have cell walls made of pseudopeptidoglycan, or pseudomurein, which shares structural similarities with bacterial peptidoglycan but has different chemical linkages and sugar components. Other archaea may have cell walls composed entirely of surface-layer proteins (S-layers), which form a crystalline array, or they might incorporate various polysaccharides or glycoproteins. This chemical diversity provides protection and structural integrity.
Essential Roles of the Cell Wall
The prokaryotic cell wall performs several functions. Its primary role is providing structural support, maintaining cell shape (rod-shaped, spherical, or spiral). It also acts as a protective barrier against physical damage and harmful substances.
Beyond structural integrity, the cell wall plays a role in osmotic regulation. It counteracts internal pressure from water influx, preventing the cell from bursting (osmotic lysis). It also contributes to cell division and attachment to surfaces, facilitating biofilm formation.
Prokaryotes Lacking a Cell Wall
While cell walls are widely present, some exceptions exist. One example is Mycoplasma, a genus of bacteria that naturally lacks a cell wall. Without this rigid outer layer, Mycoplasma species are pleomorphic, meaning they can adopt various shapes.
Mycoplasma compensates for the lack of a cell wall by incorporating sterols into their cell membrane, which provides increased strength and stability. They often thrive in osmotically protected environments, such as within host organisms, or as obligate intracellular parasites, where osmotic stress is minimized. The absence of a cell wall also renders Mycoplasma naturally resistant to many common antibiotics, such as penicillin, which target cell wall synthesis.
Another group, L-forms, consists of bacteria that can lose their cell wall under specific conditions, like environmental stress or antibiotic exposure. L-forms are sensitive to osmotic changes, though some can temporarily survive without their cell walls.