A cell wall is a structural layer that surrounds the cells of many organisms, positioned immediately outside the cell membrane. It provides support, shape, and protection against external pressures. This outer covering also functions as a selective barrier, regulating substance passage and helping cells withstand osmotic pressure and mechanical stress. The specific materials that make up a cell wall vary significantly across different life forms, reflecting diverse evolutionary paths and environmental adaptations. While animal cells do not possess a cell wall, they are commonly found in plants, fungi, algae, and most prokaryotes.
Plant Cell Wall Components
Plant cell walls are complex structures primarily composed of polysaccharides like cellulose, hemicellulose, and pectin. Cellulose forms the main structural framework, consisting of long, unbranched chains of glucose units that bundle into strong microfibrils. These microfibrils provide tensile strength and rigidity to the cell wall.
Hemicellulose, a branched polysaccharide, interacts with cellulose microfibrils. It acts as a cross-linking agent, binding cellulose fibers and strengthening the cell wall network. Pectin, a gel-like polysaccharide, forms the matrix embedding the cellulose-hemicellulose network. Pectin also plays a role in cell-to-cell adhesion.
Some plant cell walls, particularly in woody tissues, incorporate lignin. Lignin fills spaces between other components, increasing rigidity and providing waterproofing. The plant cell wall can be differentiated into a primary cell wall, which is thin and flexible, and a secondary cell wall, which is thicker and provides additional mechanical support after cell growth stops.
Fungal Cell Wall Components
Fungal cell walls feature chitin as their primary structural polysaccharide. Chitin is a homopolymer made of repeating N-acetylglucosamine units, linked in a beta-(1,4) configuration. This structure allows chitin to form strong, fibrous microfibrils, providing durability and flexibility.
The presence of chitin in fungal cell walls is notable, as this same polysaccharide is a major component of the exoskeletons of insects and crustaceans. Beyond chitin, fungal cell walls also contain various glucans, which are glucose polymers. Beta-glucans, for instance, contribute to the structural integrity of the fungal cell. The proportion of chitin in the cell wall varies by fungal species and morphological phase, with filamentous fungi often having higher chitin content than yeasts.
Bacterial Cell Wall Components
Bacterial cell walls are characterized by a unique polymer called peptidoglycan, also known as murein. This mesh-like layer is composed of alternating sugar derivatives, N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM), cross-linked by short peptide chains. This network provides structural strength and protects the bacterial cell from osmotic lysis.
Bacteria are classified based on their cell wall structure using a technique called Gram staining. Gram-positive bacteria possess a thick, multi-layered peptidoglycan wall, which can be 20 to 80 nanometers thick and make up a substantial portion of the cell wall’s dry weight. In contrast, Gram-negative bacteria have a thinner peptidoglycan layer, 7 to 8 nanometers thick, which is covered by an outer membrane.
The outer membrane of Gram-negative bacteria contains lipopolysaccharide (LPS), a large molecule consisting of a lipid A component, a core oligosaccharide, and an O-antigen. LPS acts as a barrier against certain chemical attacks.
Diversity in Other Cell Walls
Cell wall composition diversifies in other organisms, such as algae and archaea, reflecting their varied habitats and evolutionary histories. Algal cell walls exhibit considerable variation, sometimes containing cellulose, similar to plants, but also incorporating other complex polysaccharides. For example, brown algae commonly possess alginates, gel-like substances that aid in water retention, while red algae have agar and carrageenan, known for their gelling properties. Diatoms, a type of algae, have intricate cell walls made of silica.
Archaea, a domain of single-celled microorganisms, have cell walls that differ from those of bacteria. Most archaeal cell walls lack peptidoglycan. Instead, some archaea, particularly certain methanogens, have cell walls made of pseudopeptidoglycan, also called pseudomurein. Pseudopeptidoglycan shares functional similarities with bacterial peptidoglycan but has a distinct chemical structure, substituting N-acetylmuramic acid with N-acetyltalosaminuronic acid and featuring different glycosidic linkages. Other archaea have cell walls composed primarily of surface-layer (S-layer) proteins, forming a regularly structured array around the cell.