Cell walls

Cell walls (plant)

The cell wall is the layer of material secreted by the plant cell outside its plasma membrane. All plants have cell walls that are generally very similar in chemical composition, organization, and development. The walls of the Chlorophyta (green algae) show characteristics virtually identical to those of flowering plants, an indication that flowering plants are derived evolutionarily from this division of algae. The wall serves as the first point of entry of materials into cells, functions in the movement of water throughout the plant, and is one of the major mechanical strengthening factors. In addition, the wall must be sufficiently flexible and plastic to withstand mechanical stresses while still permitting the growth of the cell. See Cell membranes

The plant primary wall is initiated during the process of cell division. After chromosomes line up along the metaphase plate and begin to be pulled apart toward the poles of the cells by the spindle fibers (the anaphase portion of mitosis), a cell plate or phragmoplast can be observed at the equator of the dividing cell. Vesicles line up on both sides of the equator to form the proteinaceous cell plate. Elements of the endoplasmic reticulum fuse with the cell plate, marking the location of plasmodesmatal pores and pits which will eventually provide the intercellular connections between adjacent cells. The cell plate forms the matrix within which the middle lamella and primary walls are formed. The middle lamella is composed of pectic substances which are polymers of pectins plus smaller amounts of other sugars. The middle lamella provides some of the observed plasticity and extensibility of cell walls during cell growth, and it has also been suggested that pectins are capable of hydrogen-bonding to the cellulose that forms the plant cell primary wall. During the early stages in cell wall formation, the cellulose wall is isotropic without any ordered orientation, but as cell walls continue to develop in area and in thickness and the cell grows to mature size, the walls become anisotropic, or highly ordered. See Cytokinesis

Cellulose, like starch, is basically a polymer of glucose, a six-carbon monosaccharide. Each chain of cellulose may be as long as 8000 to 12,000 glucose monomers, or up to 4 micrometers long. These are arranged linearly, with no side branching. Cellulose chains are aggregated into bundles of approximately 40 chains each, the cellulose micelles, which are held together by hydrogen bonds. The micelle is a very regular, quasicrystalline structure.

The micelles are embedded in a matrix of other polysaccharides, the hemicelluloses. Hemicellulose serves to bind the micelle into a fairly rigid unit which retains a good deal of flexibility. Micelles, in bundles of variable number, are bound together into the cellulose microfibril, a unit sufficiently large to be seen under the electron microscope; these, in turn, are bound together into macrofibrils which are observable under the light microscope.

During the formation of the primary wall, at locations predetermined by attachments of endoplasmic reticulum to the middle lamella, cellulose microfibrillar deposition is minimal, leaving a thin place in the primary wall which forms the plasmodesmatal connections. Running through these pores are fine strands of protoplasm, the plasmodesmata proper, which contain a tube of endoplasmic reticulum–like material. The plasmodesmata provide a cytoplasmic connection between adjacent cells. Such connections are found among all the living cells of a plant, a fact which has led to the concept that all plant cells are so interconnected that the entire plant is a cytosymplast or single unit.

Although there are differences in nomenclature and terminology, secondary walls of plant cells are defined as those laid down after the primary wall has stopped increasing in surface area, essentially at that time when the plant cell has reached mature size. This is particularly true of those cells that, at maturity, have irreversibly differentiated into specialized cells, some of which are destined to lose their cytoplasm and become functional only as dead cells, including xylem vessels and tracheids, and sclereids. The secondary wall of most plants seems to have the same chemical structure and physical orientation of fibrils and hemicelluloses as do primary walls. While there may be little orientation of fibrils in young primary walls, the secondary walls are composed of fibrils that are highly ordered. In most secondary walls, and particularly those of the xylem, the fibrillar structure of the primary as well as the secondary walls may become impregnated with more substances, the most prominent of these being lignin. The chemical nature and biological role of lignin is of considerable interest because of the use of wood in the lumber and pulpwood-paper industries. The primary roles of the lignins include their ability to render walls mechanically strong, rigid, and—at least to some extent—water-impermeable. It has been suggested that lignins may also serve to make wood less subject to microbially caused decay. See Plant cell, Plant growth, Wood anatomy