The cell wall is a rigid layer that surrounds the cell membrane in a variety of organisms, including plants, fungi, and bacteria. This structural casing is tough, yet sometimes flexible, and is positioned immediately outside the cell membrane. The cell wall provides a defined shape and acts as an external skeleton. Unlike animal cells, which lack this layer, cells with a wall rely on this exterior structure for survival and interaction with their environment.
Fundamental Purpose
The cell wall’s most basic function is to provide structural support, allowing organisms like plants to stand upright against the force of gravity. This mechanical strength maintains the cell’s fixed shape, preventing it from collapsing or deforming under external pressure. The wall offers rigidity and mechanical stability to the entire organism.
Beyond structural integrity, the cell wall serves as a physical barrier against external threats. It protects the interior of the cell from environmental toxins and mechanical stress, acting as a selective filter. The wall defends against biological invaders, physically blocking the entry of viruses or harmful pathogens.
Diverse Building Blocks
The chemical composition of the cell wall varies across different life forms. Plant cell walls are primarily constructed from cellulose, a long-chain polysaccharide, along with other carbohydrates like hemicellulose and pectin. Pectin helps to “glue” adjacent plant cells together, while cellulose provides the high tensile strength required for structural support.
The cell walls of fungi are composed mainly of chitin, a polymer also found in the exoskeletons of insects. Chitin provides a tough, flexible, and resilient framework distinct from the cellulose structure of plants.
In bacteria, the wall is made from peptidoglycan, a unique mesh-like layer of sugars cross-linked by short peptides. The thickness of this peptidoglycan layer is a defining feature used to classify bacteria as either Gram-positive or Gram-negative.
Turgor Pressure Regulation
The cell wall plays a primary role in regulating turgor pressure, a force generated by water moving into the cell through osmosis. Water flows inward because the concentration of dissolved substances inside the cell is higher than the external environment. This influx of water pushes the plasma membrane outward against the surrounding cell wall.
Turgor pressure is the resulting hydrostatic force that pushes back, and the rigid wall resists this expansion. This resistance prevents the cell from taking in too much water and bursting (lysis). Maintaining this internal pressure makes living plant tissue firm and rigid, enabling plants to remain upright and supporting processes like the opening and closing of stomata.
Medical and Agricultural Importance
The unique construction of the cell wall has implications for human medicine, particularly in the development of antibiotics. Because bacterial cell walls are constructed from peptidoglycan, a material not found in human cells, this structure becomes a target for drugs. Antibiotics like penicillin work by interfering with the enzymes that cross-link the peptidoglycan chains during cell wall synthesis.
By disrupting the formation of a complete, strong cell wall, the antibiotic causes the bacteria to lose their ability to withstand turgor pressure, leading to cell rupture and death. This targeted action allows the medication to destroy infectious bacteria without harming the surrounding human cells. The ongoing fight against antibiotic-resistant bacteria often involves developing new drugs that target different steps in the peptidoglycan synthesis pathway.
In agriculture, the cell wall is fundamental to crop structure. The cellulose and lignin in plant cell walls provide the raw materials for timber, paper, and textiles. Furthermore, the indigestible plant cell walls form the dietary fiber essential for human and animal digestive health. Understanding and manipulating the cell wall’s composition is a major focus in efforts to improve crop yield, wood quality, and biofuel production.