Rakuseinad

Rakuseinad, or cell walls, are rigid or semi-rigid extracellular layers that enclose the plasma membrane of many organisms. They provide mechanical support, determine cell shape, protect against osmotic lysis, and influence growth and development. Animal cells generally lack walls, while plants, fungi, bacteria, algae, and some archaea possess them.

Composition varies across lineages. Plant and algal walls are typically rich in cellulose and hemicelluloses, with pectins forming the glue in the middle lamella. Fungal walls are dominated by chitin and β-glucans. Bacterial walls are primarily composed of peptidoglycan, with differences in thickness and outer membranes between Gram-positive and Gram-negative types. Some archaea use pseudopeptidoglycan or other polymers such as glycoproteins. In many algae, walls may include silica, calcium carbonate, or alginates depending on species.

Structure often includes a primary wall that allows growth, and, in some cells, a thicker secondary wall for added strength. In plant cells, the middle lamella rich in pectins glues adjacent cells, while plasmodesmata traverse primary walls to enable intercellular transport.

Functionally, walls regulate the movement of molecules into and out of the cell, contribute to tissue rigidity, and participate in defense against pathogens. Biosynthesis and remodeling of walls are dynamic during development and environmental responses, and they are common targets in agriculture, industry, and medicine (for example, antibacterial and antifungal strategies).