With a few notable exceptions, plant cells are encased in a complex polymeric wall that is synthesized and assembled by the cell during its growth and differentiation. Cell walls function as the major mechanical restraint that determines plant cell size and morphology. They enable cells to generate high turgor pressure and thus are important for the water relations of plants. Cell walls also act as a physical and chemical barrier to slow the invasion of bacteria, fungi, and other plant pests, and they also take part in a sophisticated signaling and defense system that helps plants sense pathogen invasion by detecting breakdown products from wall polysaccharides. Finally, cell walls glue plant cells together and provide the mechanical support necessary for large structures (the largest trees may reach 100 meters in height, generating tremendous compression forces due to their own weight).

Cell walls vary greatly in appearance, composition...