Plant Physiology - Lecture Notes - O. M. Tarnopilska 2019

2. Structural Components of the Plant Cell
2.4 Cell Wall, Its Structure and Functions

The Cell wall is formed during the telophase stage of mitotic Cell Division from The Cell plate, which develops at the boundary between adjacent Cells through the fusion of pectic vacuoles. Cellulose microfibrils—also produced via the secretory activity of the Golgi apparatus—are embedded into or deposited onto this pectic matrix. This is how the primary cell wall is formed. It largely consists of a pectic matrix interspersed with a specialized protein called extensin. Extensin is composed of Glycoproteins, with 20% of its composition attributed to L-hydroxyproline. The cellulose microfibrils are arranged loosely and randomly within it. As a result, the primary cell wall remains elastic, stretches easily, and grows concurrently with the cell, thus posing no barrier to cell expansion. Its pectic matrix readily absorbs Water, which is essential for cell growth during early developmental phases. Together with water, the matrix absorbs various ions and retains them via electrostatic attraction (the galacturonic acid residues that make up pectin carry a negative charge, while extensin Amino Acids carry both positive and negative charges). The strength of this attraction determines the rate at which ions enter the cell. The primary cell wall provides structural rigidity and helps protect the cell from mechanical damage. As the cell grows, cellulose microfibrils continue to be deposited in layers, significantly increasing packing density. Consequently, the proportional area occupied by the pectic matrix within the primary wall decreases substantially, leading to The formation of the secondary cell wall. Pectins and hemicellulose are transported to the wall by dictyosomes. Due to the limited volume of the pectic matrix, the wall loses its ability to expand. Cellulose microfibrils in different cells are arranged in various ways: transversely to the cell length, in rings—allowing these cells to elongate lengthwise (such as in stem vascular vessels)—longitudinally, or obliquely to form a spiral. All of this resembles reinforced concrete structures, where cellulose microfibrils act as iron rebar and pectic substances serve as cement.

Depending on cell specialization, the secondary cell wall undergoes various chemical modifications: cutinization (cutin), lignification (Lignin), and suberization (suberin). Walls impregnated with cutin can still permit water permeability because a network of pectic channels, or dendrites, is preserved within them. Upon lignification, the cell wall becomes poorly permeable to water, and upon suberization, it loses this capacity entirely, thereby forfeiting its physiological activity. At the same time, it becomes exceptionally rigid and predominantly performs a mechanical function: it limits Plasma Membrane stretching, prevents its overextension under increased hydrostatic pressure, acts as an external Cytoskeleton shielding the cell from damage and infection, provides structural support for plant Organs, and participates in the uptake, transport, and secretion of substances. The outer walls of epidermal cells are impregnated or covered with cutin and wax, protecting the cell against excessive Transpiration and pathogen invasion. The cell wall may also be impregnated with Mineral Substances, primarily calcium salts. The innermost, quite thin final layer of the secondary cell wall is conventionally referred to as the tertiary cell wall due to its distinct Structure and composition.

The cell wall contains unthickened regions (referred to as primary pit fields in the primary wall) that facilitate communication with neighboring cells. Fine cytoplasmic strands pass through these pit fields and pits. These are plasmodesmata—channels 100–200 nm in diameter that connect the Cytoplasm of adjacent cells. Their inner surface is lined with The Plasma Membrane. Passing through each plasmodesma is a tubular strand of The Endoplasmic reticulum known as a desmotubule. The number of plasmodesmata per cell ranges from 100 thousand to 500 million per 1 sq. mm of cytoplasmic membrane surface. Thus, Intercellular Communication can take place via the cytoplasm, plasma membrane, endoplasmic reticulum, and cell walls. The interconnected system of plant cell, tissue, and organ cytoplasm is called the symplast.