Read this essay to learn about the structure of a cell wall.
Most of the plant cells have tough and semi rigid cell wall which is non-living in nature. The presence of this wall in plant cells distinguishes them from animal cells. The cell wall was actually noticed by the early workers in the seventeenth century long before the protoplasm was recognized and since then it received considerable attention by many researchers.
Various techniques of investigations such as chemical, physical, and morphological have been used. These investigations have been aided by organic chemistry, X-ray analysis, light, polarized and electron microscopy. The commercial importance of cellulose had given new impetus to the investigators to study the cell-wall.
The cell wall materials are synthesized and deposited by the protoplast. The cell walls are responsible for the shape of plant. They protect the protoplasts from adverse external influences and give mechanical stability to the cells. The walls of plant cells are analogous to the skeletons of animals. They also form a protective barrier against viral, bacterial and fungal attack. So, they function as the skeleton as well as the skin of plants.
The cell walls control the growth rate of the plant cell and thus of the whole plant. So the cell walls are fundamentally involved in several aspects of plant biology, including the morphology, growth and development of plant cells. The cell wall of each cell merges with the walls of adjacent cells, giving the tissue physical coherence and strength.
The walls are not simply rigid and static structures. The cell walls expand as the cells grow, and new components must be deposited onto the existing wall structure. With differentiation the walls change in size, shape and chemical composition.
The cell wall is formed during the cell division and it is comparatively thin in young cells than in fully mature cells. In mature cells the cell walls have the three general layers — the middle lamella, a thin primary wall and a thicker secondary wall. The primary walls are laid down by the still-growing undifferentiated cells. The primary wall is transformed into a secondary wall when the cells stop growing.
The cell wall is composed of cellulose, hemicellulose, pectic polysaccharide, structural protein and lignin. Structurally, the primary cell wall is composed of cellulose fibers embedded in an amorphous mixture of polysaccharides and glycoproteins. Lignin is a characteristic component of secondary walls.
The Middle Lamella:
The middle lamella is the cementing substance that holds the individual cells together to form the tissues and, accordingly, it is found between the primary cell walls of adjacent cells. It is an amorphous and colloidal substance and remains inactive in plane polarised light. In mechanical tissues it may also fill the intercellular spaces. In such tissues the middle lamella becomes quite obscure due to secondary depositions.
At such times, it becomes difficult to separate the middle lamella from the primary walls on the either side. Esau (1953) proposed the term compound middle lamella for such three-ply structure.
The middle lamella consists mainly of pectic substances such as calcium and magnesium pectate which can dissolve in strong acids. The enzyme pectinase and chemical reagents which digest pectins disintegrate tissues into individual loose cells — a procedure called maceration.
The Primary Wall:
The primary wall is the first product of cell wall synthesised by the protoplast. During cell enlargement the primary wall remains relatively thin and elastic. Thickening and rigidity comes only after the completion of cell enlargement. In many cells, primary wall is the only cell wall as the middle lamella is not the wall proper. This wall is optically active. It gets more and more stretched during the growth of the cell.
Later on, secondary wall materials are deposited on the primary wall. The primary walls of two adjacent cells are present on either side of the middle lamella.
It is composed of cellulose, hemicellulose, polysaccharides and many other pectic substances. Electron microscopic study reveals that during the development of middle lamella and primary wall, small openings are left at many places in walls between the adjacent cells.
These openings are known as primary pit fields. Through these pits cytoplasmic continuity between the neighbouring cells is maintained. These cytoplasmic connections are called plasmodesmata.
Secondary Cell Wall (S- Wall):
It develops on the inner surface of the primary wall, mainly when the cell growth has ceased. It is usually made up of a matrix of cellulose, hemicelluloses and polysaccharides. During maturation, lignin, suberin, waxes, tannins and calcium carbonate are also deposited on the secondary wall giving rise to different irreversible modifications.
At the onset of secondary wall formation the cell wall becomes much less flexible and, finally, almost inelastic. That is why cell elongation ceases with the onset of secondary wall formation. However, the uniform layering of the secondary wall materials is discontinuous and may be localised to certain areas to form some special patterns.
The secondary cell wall is optically active. It gives very high mechanical strength to the plants. In the majority of tracheids and fibres the secondary walls have three layers — the outer layer (S1), the central layer (S2), and the inner layer (S3) (Fig. 5.18). The central layer usually is the thickest layer.
A non-cellulosic tertiary wall may also develop on the inner side of the secondary wall particularly in the tracheary elements. Frey- Wyssling (1976) suggests that in addition to the inner layer of the secondary wall an innermost layer may also be formed in some cases. According to him, this layer should be termed tertiary lamella consisting of a membranogenic stratum and a warty stratum.
Secondary walls are usually present in the non-living cells like sclereids, fibres, tracheary elements etc. On the other hand, the xylem parenchyma and ray cells, though living, possess secondary walls.
Fine Structure of the Cell Wall:
Cellulose is the principal scaffolding component of all plant cell walls. It is the most abundant plant polysaccharide and exists in the form of microfibrils, which are paracrystalline assemblies of several dozen (1→4) β-D-glucan chains hydrogen-bonded to one another along their length.
In higher plants each microfibril contains 36 individual cellulose chains, but that of algae can form either large, round cables or flattened ribbons of several hundred chains. Angiospermic microfibrils are 5 to 12 nm wide in the electron microscope.
Each (1→4) β-D-glucan chain may be about 2 to 3 µm long, but individual chains begin and end at different places within the microfibril which is analogous to a spool of thread that consists of thousands of individual cotton fibers, each about 2 to 3 cm long.
By electron diffraction, it has been observed the (1→4) β-D-glucan chains of cellulose are arranged parallel to one another with all of the reducing ends of the chains remain in the same direction. Recently, bacterial cellulose was shown to be synthesised by the adding of glucose units to the non-reducing ends of glucan chains.
Callose is another type of wall material. It is made by a few cell types at specific stages of wall development, such as in growing pollen tubes and in the cell plates of dividing cells. It differs from cellulose in consisting of (1→3) β-D-glucan chains, which can form helical duplexes and triplexes. It is also made in response to wounding or to attempted penetration by invading fungal hyphae.
Each cellulose molecule is 0.8 nm in diameter. The long chain molecules are again arranged in bundles to form elementary fibril. Each elementary fibril has a diameter of about 3 nm and contains 40-100 cellulose molecules. Both the cellulose molecules and the fibrils are ribbon-like structures.
Elementary fibrils contain some smaller units called micelles or crystallites which are small aggregations of cellulose molecules that lie parallel to one another. The elementary fibrils are again arranged in bundles called microfibrils (Fig. 5.18). Each microfibril is 25 nm in diameter and contains about 2,000 cellulose molecules. Microfibrils are aggregated into macrofibrils. Each macrofibril has a diameter of 0.4µ and contains about 500,000 cellulose molecules.
Cross-linking glycans, which are often called “hemicelluloses”, interlock the cellulosic scaffold. The term hemicellulose is used for all materials extracted from the cell wall with molar solutions of alkali, regardless of the structure. They are a class of polysaccharide that links the microfibrills together to form a network.
There are two major cross-linking glycans found in primary cell walls of flowering plants — xyloglucans (XyGs) and glcuronoarabinoxylans (GAXs).
The XyGs consist of Hnear chains of (1→4) β-D-glucan with numerous α-D-xylose (Xyl) units linked at regular sites to the 0-6 position of the glucose (Glc) units. Some of the xylosyl units are substituted further with α-L-Arabinose or β-D-Galactose, depending on the species, and sometimes the Galactose is substituted further with a-L-Fuctose.
The XyGs are constructed in block-like unit structures containing 6 to 11 sugars, the proportions of which vary among tissues and species. The XyGs can form three major structural variations. All of the non-commelinoid monocots and most of the dicots are fucogalacto-XyGs. The fundamental structure is composed of nearly equal amounts of XXXG and XXFG (Fig. 5.19).
But variations may occur in which α-L- Arabinose is added at some places along the glucan chain. In Solanaceae and peppermint arabino-XyGs are formed in which only two of every four glucosyl units contains a xylose unit, and the xylosyl units are substituted with either one or two α-L-Arabinose units to produce a mixture of AXGG, XAGG, and AAGG subunits.
The commelinoid monocots also contain small amounts of XyG, but these include random additions of xylosyl units. All angiosperms also contain at least small amounts of GAXs, but their structure may vary considerably with respect to the degree of substitution and position of attachment of α-L-Ara residues.
In the commelinoid monocots, where GAXs are the major cross linking polymers, the Arabtnose units are invariably on the 0-3 position. The α-L-Ara units are more commonly found at the 0-2 position where XyG is the major cross-linking glycan.
A third major cross-linking glycan, called “mixed-linkage” (1→3), (1→4) β-D-glucans, is found in the cereals. It distinguishes the members of Poales from the commelinoid species.
These unbranched polymers consist of 90% cellotriose and cellotetraose units in a ratio of about 2 : 1 and connected by (1→3) β-D- linkages. These units form corkscrew-shaped about 50 residues long polymers which are intervened by oligomers of four or more contiguous (1→ 4) Glc units.
Other, much less abundant non-cellulosic polysaccharides, such as glucomannans, galactoglucomannans, and galactomannans, potentially interlock the microfibrils in some primary walls. These mannans are found in all angiosperms.
Pectins are a mixture of heterogeneous, branched, and highly hydrated polysaccharides rich in D-galacturonic acid. They are extracted from the cell walls by Ca2+-chelators such as ammonium oxalate, EDTA, EGTA, or cyclohexane diamine tetra-acetate.
These wall materials perform many functions such as:
(a) They bring about wall porosity
(b) Provide charged surfaces that modulate wall pH and ion balance
(c) Regulate cell-to-cell adhesion at the middle lamella
(d) Serves as recognition molecules to alert plant cells from the presence of pathogens, insects and symbiotic organisms
(e) Cell wall enzymes bind to the charged pectin network to function at local regions
Two fundamental constituents of pectins are homogalacturonan (HGA) and rhamnogalacturonan I (RG I).
HGAs are homopolymers containing as many as 200 GalA (α-D-galacturonic acid) units and are about 100 nm long. RG I is a rod-like heteropolymer of repeating (1→2) α-L-Rha- (1→4) α-D-GalA disaccharide units.
Although the scaffold structure of the cell wall is largely carbohydrate, structural proteins may also form networks in the walls.
There are four major classes of structural proteins such as:
(1) Hdroxyproline-rich glycoproteins (HRGPs),
(2) Poline-rich proteins (PRPs),
(3) Gycine-rich proteins (GRPs) and
(4) Aabinogalactan proteins (AGPs). Their relative amounts vary among tissues and species.
These proteins are cotranslationally inserted into the endoplasmic reticulum (ER). Thus the mRNAs of these proteins encode signal peptides that target the proteins in secretory pathway. The conformational structure of PRPs is unknown. GRPs form a β-pleated sheet structure rather than a rod shape.
Extensin is one of the well-studied HRGPs. It is a rod-like molecule encoded by a multigene family. It consists of repeating Ser-(Hyp)4 and Try-Lys-Try sequences that are important for secondary and tertiary structure.
AGPs are named proteoglycans as they may contain more than 95% carbohydrate. These proteins form a broad class of molecules located in Golgi derived vesicles, the plasma membrane, and the cell wall. The exact function of AGPs is still unknown.
Aromatic substances are present in the non- lignified walls of commelinoid species. These moncots along with certain members of Chinopodiaceae (such as sugar-beet and spinach) contain significant amounts of aromatic substances in their non-lignified cell walls. The major components of these aromatics are hydroxycinnamic acids such as ferulic acids and p-coumaric acids.
The walls of most dicots and non-commelinoid monocots contain about equal amounts of XyGs and cellulose. These kinds of wall are called Type I walls. The commelinoid monocots have Type II walls which contain GAXs (glucuronoarabinoxylans) as principal interlocking polymers instead of XyGs. These walls are pectin poor, but contain α-L-GIcA units on GAX.
In Type I walls, the cellulose-XyG framework is embedded in a protein matrix that controls wall porosity. Type II walls have very little structural protein compared with dicots and other monocots. There are extensive interconnecting networks of phenylpropanoids.
The polymers remain soluble until they can be cross-linked at the cell surface. Different types of cross-linking possibilities exist such as, hydrogen-bonding, ionic-bonding with Ca2+ ions, covalent ester linkages, ether linkages, and van der Waals’ interactions etc.
Assembly of the wall materials occur in an aqueous environment. Water is one of the major components of the cell wall and maintains the polymers in their proper conformations. It also serves as the medium for enzyme action as well as allows the passage of ions and signalling molecules through the apoplast.