In this article we will discuss about:- 1. Meaning of Cell Wall 2. Functions of Cell Wall 3. Chemical Composition 4. Structure 5. Cell Coat.
Meaning of Cell Wall:
It is the outer rigid protective supportive and semi transparent covering of plant cells, fungi and some protists. Cell wall was first seen in cork cells by Hooke in 1665. Its thickness varies in different types of cells from 0.1 µm to 10 µm. Cell wall is a non-living extracellular secretion or matrix of the cell which is closely appressed to it. It is, however, metabolically active and is capable of growth.
Functions of Cell Wall:
(i) Protects the protoplasm against mechanical injury,
(ii) Protects the cell from attack of pathogens,
(iii) Provides rigidity and shape to the cell,
(iv) Counteracts osmotic pressure.
(v) Gives strength to the land plants to withstand gravitational forces,
(vi) By its growth the wall helps in cell expansion,
(vii) Pits present in the wall help produce a protoplasmic continuum or simplest amongst cells,
(viii) Walls prevent bursting of plant cells by inhibiting excessive endosmosis.
(ix) Wall has some enzymatic activity connected with metabolism,
(x) In many cases, wall takes part in offence and defense,
(xi) Cutin and suberin of the cell wall reduce the loss of water through transpiration,
(xii) Walls of sieve tubes, tracheids and vessels are specialised for long distance transport,
(xiii) Some seeds store food in the form of hemicellulose in cell wall.
Chemical Composition of Cell Wall:
Water— 60%. Hemicellulose— 5- 15% Pectic Substances-2-8%. Lipids-0.5-3.0%. Proteins— 1-2%
2. Micro fibrils:
Cellulose/fungus cellulose— 10-15%.
3. Other Ingredients:
Lignin, cutin, suberin, silica (silicon dioxide), minerals (e.g., iron, calcium, carbonate), waxes, tannins, resins, gum— variable.
Structure of Cell Wall:
A cell wall can have upto three parts— middle lamella, primary wall and secondary wall.
It is a thin, amorphous and cementing layer between two adjacent cells. Middle lamella is the first layer which is deposited at the time of cytokinesis (Fig. 8.13). It is just like brick work of the common wall between two adjacent rooms.
Middle lamella is absent on the outer side of surface cells. It is made up of calcium and magnesium pectates. The softening of ripe fruits is caused by partial solubilisation of pectic compounds to produce jelly-like consistency.
Primary Wall (Fig. 8.14):
It is the first formed wall of the cell which is produced inner to the middle lamella. The primary wall is commonly thin (0.1-3.0 µm) and capable of extension. It grows by intussusceptions or addition of materials within the existing wall. Some cells possess only primary wall, e.g., leaf cells, fruit cells, of cortex and pith.
Primary wall consists of a number of micro fibrils embedded in the amorphous gel like matrix or ground substance. In the majority of plants, the micro fibrils are formed of cellulose. They are synthesized at plasma membrane by particle rosettes (terminal complexes) having cellulose synthetize enzyme.
The wall is made of a polymer of P, 1-4 acetyl glucosamine or fungus cellulose in many fungi. Fungus cellulose is similar to chitin present in the exoskeleton of insects. Micro fibrils are oriented variously according to the shape and thickening of the wall. Usually they are arranged in a loose network due to incomplete cross-linking.
The matrix of the wall consists of water, pectin, hemicelluloses and glycoproteins. Pectin is the filler substance of the matrix. Proteins are structural and enzymatic. Protein expansin (Me Queen-Mason et al, 1992) is involved in loosening and expansion of cell wall through incorporation of more cellulose. Hemicellulose binds micro fibrils with matrix.
It is produced in some mature cells when the latter have stopped growth, e.g., tracheids, vessel elements, fibres, collenchyma’s. Secondary wall is laid inner to the primary wall by accretion or deposition of materials over the surface of existing structure. It is thick (3—10 µm) and made up of at least three layers, sometimes more (e.g., latex tube of Euphorbia milli). They are named as S1, S2, S3, Sx, etc. (Fig. 8.15).
The innermost layer of the secondary wall is sometimes distinct both chemically as well as in staining properties due to the presence of xylans. It is then called tertiary wall, e.g., tension wood in gymnosperms. Secondary wall may be absent, irregularly deposited or formed uniformly in the cells. This results in differentiation of cells— parenchyma, collenchyma, sclerenchyma, tracheids and vessels.
The composition of secondary wall is basically similar to the primary wall in having cellulose micro-fibrils embedded in a matrix of pectin and hemicelluloses. Cellulose micro-fibrils of the secondary wall lie close, parallel and at an angle to the longitudinal axis of the cell. Their orientation is different in the different layers of the secondary wall. A number of different materials may be deposited in the wall.
The important ones are:
It reduces the water content of the wall matrix and increases its hardness. However, water permeability is not affected. The characteristic of lignification’s (and cutinisation) has evolved with the evolution of land plants,
The wall of cork and endodermal cells contains a special fatty substance called suberin. Suberin makes the walls impermeable,
The epidermal cells possess another fatty substance called cutin. Cutin is also laid as a distinct layer on the outside of the epidermal cell walls. It is known as cuticle. Cutin reduces the rate of epidermal or surface transpiration. Other substances which can be deposited in the cell wall are silica (e.g., grasses), minerals, waxes, tannins, resins, gums, etc.
Plasmodesmata (Fig. 8.16; singular- plasmodesma; Tangl, 1879; Strasburger, 1901) are cytoplasmic bridges between adjacent plant cells which develop in the minute pores of their walls. They form a protoplasmic continuum called symplast. Cell wall and intercellular spaces form a non-living component of the plant body called apoplasm. A plasmoderma is 40—50 nm in diameter. It may be simple or branched (Fig. 8.16).
Plasmodesma is lined by plasma membrane. It encloses tubular extention of endoplasmic reticulum called desmotubule (Fig. 8.17). The space between desmotubule and plasma membrane contains 8-10 microchannels. Plasmodesmata form channels for controlled passage of small sized particles between adjacent cells as well as transfer of some specific signals.
Pits are un-thickened areas in the secondary walls of plant cells. They, therefore, appear as depressions. Pits generally occur in pairs on the wall of two adjacent cells. A pit has a cavity or pit chamber and a pit membrane.
The pit membrane consists of primary wall and middle lamella. Pits are of two types, simple and bordered (Fig. 8.18). Simple pit has uniform width of the pit chamber. In bordered pit, the pit chamber is flask-shaped because the secondary wall overarches its mouth.
Pit membrane is permeable. It may have minute sub-microscopic pores. Therefore, pits help in rapid translocation between two adjacent cells.
Cell Coat (Fig. 8.19):
A distinct layer of glycocalyx is observed in the outer surface of cells in many animals and protistans. It is fibrous and is made of oligosaccharides. The latter are actually part of the plasma membrane. In some cases cell coat is thickened and strengthened by the deposition of silicon, calcium and other salts,
(i) Like cell wall, cell coat is protective in nature,
(ii) It provides shape to the cells,
(iii) Glycocalyx type of cell coat is useful in recognition between microbe and body cell by white blood corpuscles,
(iv) Cell coat helps in cell aggregation and tissue formation,
(v) It is involved in histocompatibility,
(vi) Blood grouping is based on specific antigens present in the cell coat of erythrocytes.