Essay on the Structural Components of Cell Wall

Cell wall is a fairly rigid, protective and supportive layer surrounding the cell external to plasma membrane of plants, bacteria, archaea, fungi and algae.

It is absent in animals and most protists. It was first observed by Robert Hooke in 1665. It is not an entirely rigid structure as the cell wall expands due to the growth of the cell.

It varies in thickness from o.1mm to I0mm.in most of the cells, cell wall is present from the very beginning to the last, but in slime molds (myxomycetes) cell wall is present for a very short time.

The cell wall is made of different materials depending on cell type. Only plant cell walls have cellulose which is an unbranched polysaccharide of D-Glucose linked together by p – 1-4 glycoside bonds.

Bacterial cell walls are composed of peptidoglycan. The cell walls in Archea are composed of various substances including glycoprotein s-layers, pseudo peptidoglycan or polysaccharides. Fungi have cell walls made up of chitin and algae have glycoprotein and polysaccharides. However, certain algae have silicic acid and often other accessory molecules found anchored to the cell wall.

In general, the plant cell wall consists of middle lamella, primary wall and secondary wall and is derived from the living protoplast.

8.1.1 Middle Lamella It is the cementing layer of a thin amorphous substance between two adjacent cells. This layer is the first formed layer and is laid down during cytokine sis. The primary wall is deposited inner to middle lamella.

The surface cells on their outer or exposed sides lack middle lamella. It is constituted by Calcium and Magnesium pectate. The actual structure is not clearly defined and several models are there to explain the structure of the middle lamella.

They are, covalently linked crossed model, tether model, diffuse layer model and stratified layer models. The pectic substances of this layer are enzymatic ally converted to partially soluble

Primary wall:

This layer is laid down inner to the middle lamella. During the cell enlargement the primary wall remains relatively thin and elastic. Thickening and rigidity come only after the completion of cell enlargement. Major carbohydrates making up the primary wall are cellulose, hemicelluloses and pectin.

The cellulose micro fibrils Plasmodesmata present between adjacent cells, (b) Layers of cell wall are linked via hemi cellulose tethers to form the cellulose-hemi cellulose net work which is embedded in pectin matrix.

Out of the three subgroups of hemicelluloses like the xylems, the manna’s and the galactans, xylans or xyloglycans is the most common in primary wall. Pectin, the polymers built up mainly of a-D galacturonic acid fills the spaces of the matrix. In brown alga the polyatomic acid constituting pectin is structurally different and is called alginic acid.

The cellulose micro fibrils are aligned at all angles and are held together by hydrogen bonds to provide high tensile strength. Plant cell walls also incorporate a number of enzymatic and structural proteins. Numerous hydrolyses have been found in cell walls including invertase, glucanases, pectin methyl esterases and various phosphates.

Several oxidases are also present including ascorbic acid oxidize and laccase involved in lignin formation. The most abundant structural proteins are hydroxyl-proline rich glycoprotein (HRGP) also called extensin; the arabinogalactan protein (AGP), the glycine rich proteins (GRPS) and Proline rich protein (PRP). Except GRPS the rest are glycoproteins and contain hydroxyl- proline. Extensin connects pectin and hemicelluloses.

Secondary wall

In some plants and cell types, after the maximum growth is reached a secondary wall is laid down inner to the primary wall. Unlike the Primary wall the cellulose micro fibrils are aligned mostly in same direction and with each additional layer of the secondary wall the direction of alignment slightly changes.

The secondary wall is multilayered consisting of at least three layers. The secondary wall is strengthened by the deposit of lignin. In general lignin is found in mechanical and conducting (Vascular) tissues. Lignin also occurs in the cell walls of pith, roots, fruits, buds, bark and cork. Lignins are polymers of phenyl propane residues.

The lignin of monocots, dicots and gymnosperms are structurally different. In the walls of cork and endodermal cells a special fatty substance called suberin is deposited making the walls impermeable to water. Another fatty substance called cutin is laid down as a distinct layer on the outside of epidermal cell walls.

This layer is known as cuticle that checks transpiration from the surface cells. The secondary wall is laid down by a process called accretion or deposition of materials over the surface of existing structure. Secondary walls are laid down in trachieds, vessels, fibers and collenchymas etc. In certain tissues of gymnosperms the innermost layer of the multilayered secondary wall is chemically different from other layers due to the presence of xylans.

This layer is then called tertiary wall. Inorganic compounds like calcium carbonates and calcium silicate are found in the cell walls of some plants. Siliceous cell walls occur amongst the diatoms (Algae), Equisetaceae (pteridophyte), cyperaceae, and poaceae (Angiosperms).

The growth of the cell wall takes place in two ways: (1) Intussusceptions – growth from within. Area of the cell wall increases in this way. The primary wall is stretched and material of secondary wall is deposited.

(ii) Apposition – growth from outside. The thickness of the cell-wall increases in this way. Materials of secondary walls are deposited in thin layers.

The cells having secondary wall are rigid. They communicate with adjacent calls through pits in the secondary wall that allow plasmodesmata to connect through. Pits develop as depressions on secondary wall forming a pit chamber.

The primary wall and middle lamellum constitute the pit membrane. The pits may be simple or bordered. In simple pits the pit chamber has uniform width. The bordered pits have flask-shaped pit chambers.

The plasmodesmata are cytoplasm bridges between adjacent cells that develop through minute pores of cell wall. They form a protoplasmic continuum called symplast.

It is lined by plasma membrane and has tubular extension of endoplasmic reticulum called dcsmotubule. Tangle in 1879 first observed plasmodesmata and these were extensible studied by Strasburg in 1901.Plasmodesmata can be simple or branched.

Function

1. Protects the cell and gives a definite shape to it

2. It provides a filtering mechanism for the cell

3. It prevents the cell from over expansion and bursting due to endosmosis.

4. It forms a protective barrier against pathogens.

5. It protects the protoplasm from mechanical injury.

6. Cell wall having cutin and suberin checks water loss from the cell. Intercellular spaces: In mature cells three kinds of cavities or space are found, which are of following types:

(i) Schizogenesis cavities (Schizoid-to split) The cell walls of mature cells separate and form schizogenesis cavities, e.g. resin canals in Pinus.

(ii) Litigious cavities (Lyses-breakdown) these cavities are formed by the breakdown of cell walls, e.g. Citrus oil cavities.

(iii) Schizo-lysigenous cavities these cavities are formed both by the separation and breakdown of cell walls e.g. protoxylem of maize.