Why do plants have a cell wall

Algae are the most easily organized plants. Many of them are unicellular, some without a cell wall, others with cell walls that differ significantly in their composition and structure from those of higher plants. With them we can trace the evolution of the cell wall back. Primitive cell walls do not meet the requirements that apply to higher plants.

A structure like the cell wall has undoubtedly emerged several times in the course of the evolution of organisms. All archaebacteria, eubacteria, and with them the blue-green algae (Cyanophyta or Cyanobacteria) have complex walls, the biosynthesis of which is very expensive in terms of energy. Neither in terms of composition nor mode of biosynthesis, they have anything in common with plant cell walls.

Although the origin of the plants from early eukaryotic cells is unclear in detail, there is consensus that the primitive algae are flagellates and are close to the other, non-green flagellates. Many, but not all, species at this level of organization, including the Euglenophyta as typical green representatives, have no cell walls. However, they are not only separated from the environment by a simple membrane, but by a pellicle, which is already quite complex. It essentially consists of glycoproteins that are organized in regular patterns like a two-dimensional crystal. At regular intervals, the cell surface is wound with helically arranged ribs and intervening furrows.

Real cell walls can be found in the mostly unicellular algae, including the Volvocales. The most extensively studied species is Chlamydomonas reinhardii. Long-chain fibrillar carbohydrates are absent from their wall. Glycoproteins make up the main mass, with a hydroxyproline-rich protein similar to extensin already occurring here. The detected sugar residues include arabinosyl, galactosyl and mannosyl residues. From an electron microscope, the wall appears to be made up of seven layers. The middle one contains an extensive grid-like grid of polygonal plates, which mainly consist of the glycoprotein mentioned, while the approaches above and below show fibrous structures. The outer layer varies in thickness because it is made up of components that the cell takes from its surroundings.

One of the essential tasks of the cell wall of simple, unicellular species becomes clear: It is the mediator between the cell and the environment, so it not only serves to protect it but also to communicate with similar and different cells. It must be permeable for metabolites and regulator molecules and / or carry receptor molecules with the help of which one cell can establish contact with another. The variety of these tasks (and specificities) results in the evolution of a variety of differently structured cell walls.

In plant multicellular cells, communication along the entire cell surface is largely restricted. Neighborhood relationships between cells develop as tissues develop. Strength is a crucial criterion. Substance exchange between cells takes place via specific openings in the wall (pits, plasmodesmata). The functions originally performed by one unit are thus divided into two functional structures.

The essential structural elements of all plant cells are polysaccharides, the different chemical composition of which basically results in different physical properties. There is no plant cell wall that consists of only one class of molecules (the interactions between the various molecules lead to characteristics that distinguish cell walls of certain classes from one another.

Cellulose is the main structural element of the wall in many algae classes. The fibril structure, on the other hand, still shows considerable variations. There is definite X-ray structural analysis evidence that it is also crystalline in algal cell walls - at least over large areas. Differences in the type of diffraction reflexes (a measure of periodic intervals in the molecular range), which vary considerably from species to species (particularly large in the Rhodophyta [red algae]), indicate that cellulose is in many more or less uniform crystalline forms can be stored together.

With some classes of algae there are only scattered textures, with others (especially with many species of the Chlorophyta) there is a higher degree of organization (layers of parallel microfibrils). However, such layers usually alternate with layers of amorphous material. With most algae there is no clear distinction between primary and secondary walls, and where they exist, their existence is based on a different mode of formation than with the higher green plants.

In a number of marine green algae (Codium, Dasycladus, Acetabularia i.a.), and in the wall of some red algae (Porphyra, Bangia), mannans form the main structural elements. They too are linear, and the mannosyl residues are beta 1> 4 linked together glycosidically. Hydrogen bonds can be formed; as with cellulose, they are also the cause of the partially crystalline organization in microfibrils. At Codium are they firmly associated with the protein.

Polymers in which beta-D-xylosyl residues are linked via 1> 3 and 1> 4 glycosidic bonds. They have been identified as structural elements in some red and green algae. In contrast to the polymers discussed so far, we are dealing here - in part at least - with branched structures; however, species whose walls contain xylans have a layered structure and an orientation of microfilaments. In them, linear polymers are predominant.

Alginic acid and its salts, the alginates, are important components of the walls of Phaeophyta (brown algae). You are unique in many ways. They consist exclusively of uronic acids: mannuronic acid and beta-L-glucuronic acid in varying proportions, and in smaller amounts also from beta-D-glucuronic acid.

Homopolymers occur alongside heteropolymeses, some species-specific differences can be found, which in turn indicates that the individual species are equipped with a different range of enzymes.

The alginates of brown algae occur both inside and outside the cell wall (in the intercellular substance). Their proportion of the cell wall can make up to 40 percent of the dry matter. They have a high affinity for divalent cations (calcium, strontium, barium, magnesium) and the property to gel. Most of the magnesium ions that can be isolated from brown algae come from the alginic acid fraction.

Polysaccharides, the monomers of which are esterified with sulfonic acid residues and also partially methylated, have been found in almost all marine algae. They occur partly in the cell wall itself, partly in the intercellular substance. Sulphonated galactans are typical for many red algae, according to their origin they are called agarose, carrageenan, porphyran, furcelleran and funoran.

Both L- and D-galactose linked by beta 1> 3 or alpha 1> 4 glycosidic bonds, form the basic pattern of agarose and porphyran (there: alternating L- and D-galactosyl residues), while carrageenan and furcelleran only contain the D-form. As with alginates, gelatinization is one of the most important physical properties of this family of molecules. Agar, the basic unit of which is agarose, is primarily derived from the genera of red algae Gelidium and Gracillaria won.

The unusual types of binding found in agarose and carrageenan lead to specific tertiary structures.

A number of algae contain mineral cell wall components. Silicon is known as an essential component of the diatom shells, but it also occurs in the cell walls of other groups of algae, e.g. the chrysophyte Synura surrounded by silicon-containing scales, in some brown algae and in green algae Hydrodictyon it is part of the wall. Diatoms absorb silicon as a silicate. The process is dependent on oxygen and temperature, consumes energy and relies on the presence of divalent sulfur.

Sporopollenin is an isoprene derivative. It is part of pollen cell walls, but it was also found in the walls of some green algae (Chlorella, Scenedesmus i.a.) proven.

Calcium deposits in cell walls have been described variously. They seem to be particularly common in species of tropical marine waters. Some of the species are involved in reef formation. Calcium is consistently deposited as calcium carbonate, of which (at least) two different crystalline forms occur: Calcite (formed in the walls of some groups of red algae and Charophyceae) and argonite [formed by some green algae (Acetabularia i.a.), brown and red algae]. Mixtures of both forms in one species do not occur.