Plant Physiology - Musienko, M. M. 2001

Chemical and molecular composition, structure, and functions of the plant cell
Cell wall

In most plant Cells, the Plasmalemma is surrounded by a rigid Cell wall that limits the size of the protoplast, determines cell Morphology and, consequently, function, and also plays a direct role in regulating growth during the elongation phase. The Cell wall formed during Cell Division is called the primary wall. Later, As a result of thickening, the secondary wall is formed. In higher plants, the cell wall has three layers: the middle lamella, the primary wall, and the secondary wall (Fig. 5).

Middle Lamella

During cell division, the first layer laid down is the so-called middle lamella. It cements together the walls of adjacent daughter cells and consists of adhesive, gelatinous pectic substances. The middle lamella contains no Cellulose and, therefore, no microfibrils. Golgi vesicles supply low-molecular-weight polygalacturonic acid for its formation. Long chains of pectic acid apparently arise only after the formation process of the middle lamella is complete. Pectic acid later forms protopectin, the molecules of which intertwine with each other and with hemicellulose molecules. Together, they form a reticular Structure that serves as The basis of the middle lamella (Fig. 6). Pectic substances include calcium pectates, which effectively cement and glue daughter cells together. In the case of calcium deficiency, pectic substances become mucilaginous, causing cells to separate (tissue maceration). For example, during fruit ripening, the pectic substances of the middle lamellas that glued the cells together transition into a soluble form, making the fruit soft.

Primary Cell Wall

In young cells, the subsequent layer of the cellulosic envelope, laid down by the protoplasts of adjacent cells on both sides of the middle lamella, is called the primary cell wall. It consists of cellulosic micro- and macrofibrils embedded in a matrix. Each microfibril is composed of numerous cellulose chains connected to each other via Hydrogen Bonds.

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Fig. 5. Structural Features of the cell wall Organization:

a — a region of the wall clearly showing a pit (/), the middle lamella (2), the three-layered secondary wall (3), and the primary wall (4); b — the main component of the cell wall, cellulose, which makes up macrofibrils; c — each macrofibril, in turn, consists of microfibrils up to 100 A thick; d — a region of microfibrils with individual micelles (5); e — a micelle fragment showing how chains of cellulose molecules form a lattice

Microfibrils combine into macrofibrils and together form the framework of the cell wall, which is embedded in the matrix.

Fig. 6. Scheme of the network formation from pectic acid molecules in the middle lamella. The connection of individual molecules through The formation of double salts (Ca, Mg) and Esterification (arabinose, phosphate) is shown

The matrix consists of Polysaccharides — Pectins, hemicelluloses, and protein. Thanks to the presence of the matrix, microfibrils are held together. The primary wall protein extensin is a glycoprotein containing up to 90% of the cell's total hydroxyproline and plays an important role in cell wall expansion. Hydroxyproline residues are linked by O-glycosidic bonds to arabinose residues of hemicellulose fibers, giving the wall matrix a certain structure: elongated extensin molecules form a framework to which hemicellulose molecules attach. The cell wall also contains specific carbohydrate-binding Proteins called Lectins, which ensure Cell Recognition and interaction, protect against infections, and perform receptor Functions. Enzymes involved in cellulose synthesis have also been detected in the cell wall, along with protein granules up to 8 nm in diameter located on the outer surface of the plasmalemma and deeper within the primary wall matrix. It is well known that the primary material for Cell wall formation is synthesized by the protoplast and supplied via the ER and Golgi dictyosomes, though the participation of the detected enzymes in their synthesis is not excluded.

During cell growth, the primary cell wall undergoes intense pressure from the protoplast. This is how its expansion and the incorporation of new material are achieved. Expansion can be reversible (elastic) and partially irreversible (plastic). The primary wall not only stretches but also thickens. The enlargement of cell walls during their growth occurs either through the intercalation of new cellulose and protopectin molecules into the existing framework (intussusception) or by the deposition of new layers onto the previously formed surface (apposition).

Meristematic cells that divide continuously, as well as most specialized mature cells (e.g., leaf mesophyll), possess only primary cell walls. Such cells with a primary cell wall and living contents retain The ability to change shape, divide again, and differentiate into a new cell type. They participate in wound healing and Plant Regeneration. Primary cell walls vary in thickness and contain thin areas known as primary pit fields. Plasmodesmata, or strands of Cytoplasm connecting adjacent cells, pass through them. However, in most cells, additional layers of cellulose are deposited on the inner surface (outward from The Plasma Membrane), forming the secondary cell wall. This typically occurs when the cell stops growing and the area of the primary wall no longer increases.

Secondary Cell Wall

During its formation, many secondary layers are laid down over the primary wall, with the cellulose content reaching 60%. Its fibrils in each layer lie parallel, while in adjacent layers they are oriented at an angle to one another. Various inclusions such as lignins, suberin, cutin, and Waxes appear in the matrix. This gives the secondary wall considerable strength and rigidity, causing it to lose elasticity, while the cell loses its ability for further growth. The main component of this rigid wall is Lignin, the presence of which is typical for the secondary walls of wood cells. Lignin binds cellulose fibers together and holds them in place. Xylem elements and mechanical sclerenchyma cells undergo intense lignification. Suberin, in combination with waxes, causes the suberization of the cell wall, reducing its permeability. Cutin forms a cuticular layer on the outside of the wall, making it almost impermeable.

Functions of the Cell Wall

The cell wall is a secretory product of the protoplast, and in a mature cell, it is presumably a non-living part of it. However, it contains proteins, some of which possess enzymatic activity.

Cell walls provide mechanical strength and support to individual cells and the plant as a whole. They determine the size, shape, and Stability of the plant cell, protecting the protoplasmic membrane from destruction under METABOLISM/18.html">The Influence of turgor pressure generated inside the cell. The cell wall acts as an anti-infective barrier and participates in the uptake, transport, and excretion of substances. The system of interconnected cell walls is called the apoplast, which serves as the main pathway for the Movement of Water and dissolved substances throughout the plant. Cytoplasmic strands pass through pits in the cell walls, connecting the contents of individual cells and thus uniting all protoplasts into a single system—the symplast.

There is evidence that carbohydrate Components of the cell wall, by interacting with phytohormones, trigger a series of physiological changes within the cell.