While many prokaryotic organisms feature a cell wall, this characteristic is not universal. The presence or absence of a cell wall, along with its specific composition, plays a significant role in the biology and classification of these diverse microorganisms.
Understanding Prokaryotes
Prokaryotes are single-celled organisms distinguished from eukaryotes by their simpler cellular organization, lacking a membrane-bound nucleus and other internal organelles. Their genetic material, typically a single circular chromosome, is located in a region of the cytoplasm called the nucleoid. Prokaryotes encompass two primary domains: Bacteria and Archaea, found in a vast array of environments, including extreme conditions.
Prokaryotic cells are generally much smaller than eukaryotic cells, ranging from 0.1 to 5.0 micrometers in diameter. This small size allows for efficient diffusion of molecules throughout the cell. Despite their structural simplicity, prokaryotes exhibit metabolic diversity, enabling them to thrive in nearly every habitat on Earth.
The Prokaryotic Cell Wall
For most prokaryotes, a cell wall is a structural component located outside the cell membrane. This rigid layer provides crucial structural support, helping to maintain the cell’s shape and protecting it from mechanical damage. A primary function of the cell wall is to prevent osmotic lysis, which is the bursting of the cell when it absorbs too much water.
The composition of the cell wall varies significantly between the two prokaryotic domains. Bacterial cell walls are primarily composed of peptidoglycan, a unique polymer of sugars and amino acids that forms a mesh-like sacculus. This peptidoglycan layer provides the cell with its structural strength. In contrast, archaeal cell walls do not contain peptidoglycan; instead, they are made of diverse materials such as pseudopeptidoglycan, polysaccharides, glycoproteins, or protein-based S-layers. These distinct compositions contribute to the ability of archaea to survive in extreme environments.
Prokaryotes Lacking a Cell Wall
While a cell wall is characteristic of most prokaryotes, notable exceptions exist. The most well-known group of bacteria lacking a cell wall is the genus Mycoplasma, which belongs to the class Mollicutes. These organisms are among the smallest and simplest self-replicating bacteria, notable for this fundamental absence. The lack of a rigid cell wall means Mycoplasma species are highly pleomorphic, adopting various forms from spherical to filamentous.
The absence of a cell wall has several implications for Mycoplasma. Their outer boundary is a triple-layered unit membrane that incorporates sterols, such as cholesterol, typically found in eukaryotic cell membranes. These sterols provide increased rigidity and stability to their cell membrane, compensating for the lack of a cell wall and making them more resistant to rupture. This adaptation is important as many Mycoplasma species live as parasites within host cells, where the surrounding environment is osmotically controlled. Consequently, Mycoplasma are resistant to many antibiotics, such as penicillin and other beta-lactam antibiotics, which specifically target cell wall synthesis.