The presence of a cell wall, above all other characteristics, distinguishes plant cells from animal cells. Its presence is the basis of many of the characteristics of plants as organisms. The cell wall is rigid and therefore limits the size of the protoplast, preventing rupture of the plasma membrane when the protoplast enlarges following the uptake of water. The cell wall largely determines the size and shape of the cell, the texture of the tissue, and the final form of the plant organ. Cell types are often identified by the structure of their walls, reflecting the close relationship between cell wall structure and cell function.
Once regarded as merely an outer, inactive product of the protoplast, the cell wall is now recognized as a metabolically dynamic compartment having specific and essential functions Accordingly the primary cell wall the wall layers formed chiefly while the cell is increasing in size—has been variously characterized as a “vital” or “indispensable organelle” a “special subcellular compartment outside the plasma membrane” and a “vital extension of the cytoplasm”. Cell walls contain a variety of enzymes and play important roles in absorption, transport, and secretion of substances in plants. Experimental evidence indicates that molecules released from cell walls are involved in cell-to-cell signaling, influencing cellular differentiation. In addition the cell wall may play a role in defense against bacterial and fungal pathogens by receiving and processing information from the surface of the pathogen and transmitting this information to the plasma membrane of the host cell. Through gene-activated processes, the host cell may become resistant to attack
through the production of phytoalexins—antibiotics that are toxic to the pathogens or through the deposition of substances such as lignins, suberin, or callose, which may act as passive barriers to invasion