A cell wall is a protective layer located outside the cell membrane in certain cells. It provides structural support and shape, forming a boundary between the cell’s interior and its environment. This layer can be tough, flexible, or rigid depending on the organism.
Structure and Occurrence
The composition of cell walls varies across different groups of organisms. In land plants, the cell wall primarily consists of polysaccharides like cellulose, hemicelluloses, and pectin. Cellulose forms strong microfibrils that provide tensile strength, while hemicelluloses link these microfibrils, and pectin forms a hydrated matrix embedding this network. Plant cell walls also incorporate lignin, a complex polymer that adds rigidity and waterproofing. They are organized into multiple layers, including a primary wall, a secondary wall, and a middle lamella that glues adjacent cells together.
Fungi possess cell walls composed mainly of chitin, a polymer of N-acetylglucosamine, along with glucans and glycoproteins. Chitin provides structural rigidity. The specific arrangement and types of glucans and glycoproteins can vary among different fungal species.
Bacterial cell walls feature a unique component called peptidoglycan. This complex polymer is made of sugar derivatives cross-linked by short peptide chains, forming a mesh-like structure that surrounds the cell membrane. Bacteria are categorized as Gram-positive or Gram-negative based on their cell wall structure; Gram-positive bacteria have a thick, multi-layered peptidoglycan wall, while Gram-negative bacteria have a thinner peptidoglycan layer surrounded by an additional outer membrane. Animal cells do not possess a cell wall, being enclosed only by a flexible plasma membrane that allows for greater cellular movement and adaptability.
Primary Roles
Cell walls provide structural support, contributing to the cell’s shape and integrity. This outer layer helps maintain the cell’s form, preventing collapse or deformation under pressure. In multicellular organisms, particularly plants, the collective rigidity of cell walls allows for upright growth and the formation of stable tissues and organs.
The cell wall offers protection to the cell. It acts as a physical barrier against mechanical stress, shielding internal cellular components from damage. This protective function is important for organisms exposed to environmental elements. The cell wall also prevents osmotic lysis, where a cell bursts from excessive water intake. Water moves into cells via osmosis, creating internal turgor pressure; the rigid cell wall resists this pressure, allowing cells to swell without rupturing and maintaining cellular turgor.
The cell wall also functions as a selective filter. While permeable to water, ions, and small molecules, it limits the entry of larger or potentially harmful substances. This selective permeability helps maintain a stable internal cellular environment.