Here is a compilation of notes on Cell Wall. After reading these notes you will learn about:- 1. Origin and Structure of Cell Wall 2. Ultrastructure of Cell Wall 3. Plasmodesmata and Desmotubule 4. Chemical Constituents.
Note # 1. Origin and Structure of Cell Wall:
After nuclear division the phragmoplast or cell plate appears across the equator of the cell. At the end of mitosis granules originating from Golgi complex arrange themselves on the equator and finally fuse to form the cell plate. The cell plate grows in thickness by addition of new cell wall material.
Three layers can be distinguished in the cell wall. These are the middle lamella, the primary cell wall and the secondary cell wall (Fig. 2.28). Occasionally a tertiary cell wall may be present.
The middle lamella is formed between adjacent cells during cell division. It is of viscous substance and serves as a cementing material between adjacent cells. It is composed primarily of calcium pectate. The primary cell wall is formed during the early stages of growth and is 1-3 µm thick. It is composed chiefly of cellulose, hemicellulose and pectic compounds. It is elastic, extends with the growth of the cell.
The secondary cell wall is laid down on the primary cell wall on maturation of the growth of the latter. It is about 5-10 µm thick. The secondary cell wall has three layers, namely the outer layer, the middle layer and the inner layer. It contains lignin, suberin, pectin, cutin, etc. in addition to cellulose and hemicellulose.
A tertiary cell wall is formed in few cases on the inner surface of the secondary cell wall as in gymnosperm tracheids which is very thin and composed mainly of xylan, instead of cellulose.
Note # 2. Ultrastructure of Cell Wall:
The cell wall is composed of macro-fibrils, 0.5 µm in width, which again is a bundle of micro-fibrils, 250Å in diameter. The micro-fibril is visible only under the electron microscope and consists of a bundle of micelles or elementary fibrils, about 100Å in diameter. The micelle contains about 100 cellulose chains which is a polymer of glucose molecules (Fig. 2.29).
In primary cell wall the micro-fibrils are arranged at random and in the secondary cell wall they are closely packed and are arranged parallel to one another.
The micro-fibrils are embedded in matrix of polysaccharide material. The substances present in the matrix vary with the growth phase of the plant-pectic substances are predominant in earlier stages, and hemicellulose, xylan appear in later stages.
Complex polysaccharides present, linked to each other and to the cellulose micro-fibrils, are xyloglucans, arabinogalactans, rhamnogalacturonans. A glycoprotein called extensin, is also present.
The primary cell wall contains many small openings or pores situated in certain areas. The cytoplasm of adjacent cells communicates through these pores by means of cytoplasmic bridges termed as plasmodesmata. In some plants, the secondary cell wall has depressions or cavities called pits which may be simple or bordered.
Note # 3. Plasmodesmata and Desmotubule in Cell Wall:
Structure of Plasmodesmata and Desmotubule:
Plasmodesmata (PD) are the cell junctions in plants which form fine cytoplasmic channels (20-40 nm in diameter) between cells. Running through the centre of this channel is a narrow cylindrical structure called desmotubule, which is continuous with the endoplasmic reticula of adjoining cells.
Between the inner wall of plasmodesmata and the desmotubule, is a cytosolic annulus through which molecules can pass (Fig. 2.33).
Each plasmodesmata has:
(i) An outer sheath contiguous with the plasma membrane,
(ii) A central core of endoplasmic reticulum and
(iii) A collar or neck region.
Functions of Plasmodesmata and Desmotubule:
Plasmodesmata are known for long time playing a passive role in permitting free movements of small metabolites and growth hormones between plant cells. More recent work suggests that these are rather dynamic structure, rapidly altering their dimensions to allow transport of bigger molecules, like viral genomes and endogenous plant proteins.
Movement proteins (MP) of plant viruses also operate as endogenous PD transport system. Each MP has a transport signal which dictates transport, irrespective of the size of the molecule.
The cytoskeleton is also involved firstly, in tracking of macromolecules to PD and secondly, in gating of PD. Action filaments also traverse PD channels and may act as sphincter at the neck region of PD. Intercellular transport of RNA and protein molecules also takes place through PD in a regulated manner.
Such regulation of the movement of molecules by regulating the distribution and permeability of PD may help development and differentiation. Plasmodesmata are also implicated in the movement of numerous proteins (up to 70 KD) from companion cells to the enucleate sieve elements of the phloem.
Apoplast and Symplast:
The existence of plasmodesmata and intercellular spaces subdivides the plant body into two major compartments — symplast and apoplast. Symplast is the living part of the plant made up of the interconnected protoplast bound by single continuous plasma lemma.
Apoplast is the non-living part of the plant cell external to plasma lemma and composed of cell walls, intercellular spaces and the dead cell lumens such as xylem vessels. Both the compartments are utilized for the transport of materials through the plant (Fig. 2.34).
In apoplastic transport, the movement of substances between neighbouring ceils occurs through the matrix of cell wall and across the plasma membrane, whereas in symplastic transport, the movement occurs via plasmodesmata.
The size of the transporting substances is important in apoplastic movement because the micro-fibrils and matrix polymer of cell wall which form the sieve like structures, inhibit the entry of larger molecules.
Note # 4. Chemical Constituents of Cell Wall:
(i) Cellulose (β-1-4 glucans):
It is the main chemical component of the cell wall (20-90%). Cellulose is a polymer of 8000-15000 glucopyranose residues covalently linked by β-1-4 glycosidic bonds to form a ribbon like structure.
Intermolecular hydrogen bonds between adjacent cellulose molecules cause them to adhere strongly to one another in an overlapping manner which leads to the formation of bundles of 60-70 cellulose chains with same polarity. These bundles are described as cellulose micro-fibrils which are connected with each other through long hemicellulose molecules that make hydrogen bonds to the surface of the micro-fibrils.
(ii) β -1 -4 Mannans:
The un-branched polysaccharide composed of D-mannopyranose residues are linked together by β-1-4 glycosidic linkage.
(iii) ) β -1 -3 Xylans:
The un-branched chains of D-xylopyranose residues are linked together by β-1-3 glycosidic linkage.
The hemicellulose molecules are heterogeneous group of branched polysaccharides, bound to one another and to micro-fibril making a ‘hemicellulose complex network’ (Fig. 2.31). There are many classes of hemicelluloses of long, linear backbone composed of one type of sugar, from which short side chains of other sugars protrude. It consists of arabinose, xylose, galactose, mannose and uronic acid.
According to the predominant monosaccharide, these molecules are divided into three groups — xylans, mannans and galactans. Xylans are linear chain of D-xylopyranose residues linked by β-1-4 glycosidic linkages; mannans consist of β-D-mannopyranose and β-D-glu- copyranose residues (3:1) linked by β-1-4 glycosidic linkage (glucomannan); galactans of β- galactopyranose units linked by β-1-3 glycosidic linkages.
Other hemicelluloses are xyloglucan, glucan, galactomannan, glucuromannan, arabinogalactan, etc.
There is another network of pectin which are heterogeneous group of branched polysaccharides containing many negatively charged α-D-galacturonic acid (GalUA) and glucuronic acid residues along with rhamnose, arabinose and galactose (Fig. 2.32). Due to their negative charge, pectins are hydra- ted and remain associated with cations which make cross-links with them.
Pectin polysaccharides exist in the cell wall in two forms:
(a) A linear copolymer of α-(1-4)-linked galacturonic acid (GalUA) forming smooth regions or
(b) They have attached α-(1-2)-linked rhamnosyl residues forming hairy regions.
Lignins are phenolic-derived aromatic polymers which interact with other wall polymers to provide structural integrity. Lignin is formed by polymerization of a mixture of three monolignols — coumaryl, sinapyl and coniferyl alcohol.
III. Non-polysaccharide Components:
The cell wall in commelinoid group contains non-cellulosic components — ferulic acid and glucuronoarabinoxylans.
Proteins are also present in cell wall, some of them having high levels of hydroxyproline and serine which strengthen the cell wall. Proteins constitute 5-10% and are divided into two classes — enzymes and structural proteins.
Numerous hydrolases including invertase, glucanase, pectin methyl esterase, ATPase, DNase, RNAse, ascorbic acid oxidases, laccase and phosphatase are present in cell wall.
(ii) Structural proteins:
In primary cell wall, glycoproteins are found, concerned with cell-wall extension, called ‘extensin’.
V. Incrusting Substances:
(i) Cutin and Wax:
In cell walls of epidermal cells, cutin and waxes may produce impermeable surface coating. Cutin consists of a complex mixture of hydroxyfatty acids linked together by ester bonds to give a three dimensional network.
Suberin is a polymeric substance built by repeating units like a co-saturated or monounsaturated dicarboxylic acids ranging from C16 to C22. It replaces cutin in cells of underground plant parts.
(iii) Inorganic compounds:
Inorganic compounds containing calcium carbonate and silicate deposit in the cell wall of some plants.
Water, an important structural component of cell wall, causes reversible changes in the texture of cell wall by sol-gel transformation of pectin. It increases wall permeability to ions and also involves in hydrolysis of glycosidic bonds during cell growth.