The Cell Wall

Unlike the plasma membrane, which is a living, dynamic structure common to every cell, the cell wall is a non-living, rigid structure found only in plant cells (and, in a chemically different form, in bacteria and fungi), lying entirely outside the plasma membrane. In a plant cell, the wall is built mainly from the polysaccharide cellulose, together with hemicellulose, pectin, and structural protein, and it is this wall -- not the plasma membrane -- that gives a mature plant cell its comparatively fixed, often box-like shape.

The cell wall of a young, growing plant cell is laid down in two stages. As the cell first forms and continues to grow, it secretes a thin, flexible primary wall, capable of expanding as the cell enlarges. As the cell matures and growth slows or stops, additional layers of wall material -- the secondary wall -- are deposited on the inner face of the primary wall, next to the plasma membrane, progressively thickening and stiffening the wall. In many mature, non-growing cells, particularly those specialised for mechanical support or water conduction, this secondary wall becomes the dominant, and often the thickest, part of the wall.

Where two adjacent plant cells meet, their individual walls are cemented together by a thin, shared layer called the middle lamella, composed mainly of calcium pectate. At the corners where three or more cells meet, the middle lamella is often absent or reduced, leaving small gaps known as intercellular spaces, which allow for the diffusion of gases between cells.

Functionally, the cell wall serves several essential roles. It gives the plant cell its shape and provides crucial mechanical support and protection to the delicate protoplasm within, allowing plant tissues to remain upright and resist physical damage without the aid of an internal skeleton. It also protects the cell against many potential pathogens, forming a physical barrier that many microorganisms cannot easily breach, and it acts as a selective barrier limiting the entry of undesirably large macromolecules into the cell. Because of its porous, meshwork structure, the cell wall is freely permeable to water, and it plays a central role in the plant's overall water relations, contributing to the turgor pressure that keeps plant tissues firm and upright. …