Cell Wall – Definition, Function, Chemical Composition & Structure
Cell Wall Definition
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Do plant cells have a cell wall?
In plants almost all cells (except the reproductive cells like gametes, zoospores etc.) are provided with a rigid wall, called cell wall. The protoplast is enclosed within the cell wall; presence of cell wall is the characteristic feature of all plant cells.
The cell wall was discovered in the 17th century before the presence of protoplast was recognised, and since then the protoplast became the main object of study.
Nature
The cell wall, at first, is very thin and delicate, and is modified in various ways with the maturing up of cells. As the cells mature, some changes take place—these are increase in structure and in extent, in chemical composition, in the modification of gross physical structure owing to the absorption of end walls etc.
The cell wall is the secretory product of the protoplast and is a non-living boundary wall. For a long time, the cell wall was regarded as non-living excretion of the living cell matter (protoplasm)—hence, according to that concept, the cell wall does not form a living system.
However, the cell wall is by no means independent of the protoplasm (Cutter, 1978). Now-a-days much evidence has been found that in plant cells, specially in young stage, “organic unity exists between the protoplast and the wall” and that “the two together form a single biological unit” (Fahn, 1982).
The cell wall grows when comes in contact with the protoplast but outside of it. Cytoplasm is present in the wall of mature living cells in the form of plasmodesmata. It is still a question whether during growth of the cell the relation between cytoplasm and the wall is closer than at maturity.
According to some workers the cytoplasm penetrates the growing wall, but electron microscope studies of meristematic cells indicate the presence of ectoplast which delimits the cytoplasm from the cell wall.
Properties
Properties of plant cell wall are affected by the cell’s environment, nutrition and stage of differentiation. Cell walls exhibit different degrees of plasticity, elasticity and tinsel strength in relation to their chemical composition and their sub-microscopic and microscopic structure.
Plasticity is the property of becoming permanently deformed when subiected to changes in shape and size while elasticity is the property of recovery of the original size and shape after deformation.
Plasticity of walls is observed clearly by their permanent extension in some stages of growth of cells by volume (Heyne, 1940); similarly elasticity property of walls may be illustrated by the reversible changes in volume due to changes in turgor pressure (Frey-Wyssling, 1959). Tinsel strength is the remarkable characteristic of mechanical cells, specially of the extra xylary fibres of monocotyledons and dicotyledons.
Differences in optical and other physical properties of walls are interrelated with the orientation of microfibrils, e.g. walls having the microfibrils oriented parallel to the long axis of the cell do not show their anisotropy in transverse section and do not contract longitudinally ; while walls in which the microfibrils are oriented at right angles to the long axis of the cell are strongly birefringent in transverse sections and contract longitudinally on drying (Bailey, 1954).
Cell wall is mainly composed of Cellulose microfibrils : other substances e.g. lignin, suberin etc. add their properties or modify those imparted by cellulose. Tinsel strength is the remarkable characteristic of cellulose, while lignin increases the resistance of walls to pressure and protects the cellulose fibrils from becoming creased.
Cell Wall Function
(i) Cell wall protects the living protoplasm from external injury.
(ii) It also gives definite shape to the cell and texture to the tissue.
(iii) Cell walls have supportive function i.e. they provide mechanical strength to the cell.
(iv) Being permeable, the cell wall allows water and mineral salts to pass through it.
(v) Cell walls play an important role in some physiological activities such as absorption, transpiration, translocation, secretion etc. (Frey-Wyssling, 1959)
(vi) Cell walls also connect the living protoplasts of adjacent cells through plasmodesmata.
Plant Cell – Definition, Types, Structure With Diagram
Chemical Composition And Gross Structure Of The Cell Wall
A. CHEMICAL COMPOSITION OF THE WALL
Cell wall is made up of Cellulose. Cellulose is associated with various substances e. g. with other compound carbohydrates and with lignin, specially in the walls of woody tissues.
The common carbohydrate constituents of the walls other than cellulose are hemicellulose and pectic compounds. The fatty compounds, suberin, waxes etc. occur in varying proportions in the walls of many types of cells. Various other organic compounds and mineral substances may also be present.
Water is another most important and most variable component of cell walls—part of this water occurs in microcapillaries and is relatively free, the rest is associated with hydrophilic substances. A group of proteins containing hydroxyproline may be present in the primary walls of various tissues.
Cellulose — It is a relatively hydrophillic crystalline compound having the general empirical formula (C6H10O5)n. Cellulose is closely related to starch as a polysaccharide hexosan; the molecules of cellulose are chain- or ribbon- like structures with 1,000 or more of the glucose residues connected together by oxygen bridges with β- 1,4 glucosidic bonds. The length of each chain varies greatly and may be upto 4µ (Frey-Wyssling, 1959).
Hemicelluloses — These are like cellulose; they are built up not of glucose molecules, but of those of other sugars. Practically hemicelluloses are a group of polysaccharides of certain solubilities. Xylans, mannans, galactans, glucans etc. are examples of some of the individual members of hemicelluloses.
Pectic substances — These substances occur in cell walls in three forms such as protopectic, pectin and pectic acid. Pectic substances belong to the polyuronids i.e. polymers composed mainly of uronic acid. They are related to hemicellulose but have different solubilities.
Pectic compounds are amorphous, colloidal, plastic and highly hydrophilic. Pectic compounds not only constitute the intercellular substance in the middle lamella, but also occur associated with cellulose in the primary wall.
Gums and mucilages are also regarded as compound carbohydrates of the cell walls. These substances are related to pectic compounds owing to same property of swelling to water. The mucilages occur in some mucilaginous and gelatinous types of cell walls—such walls are found in the outer cell layers of many aquatic plant bodies and in seed coats.
Lignin is another most important composition of the plant cell wall. The structure of lignin is not fully understood. It is not a carbohydrate, but a polymer made up of units of phenylpropane derivatives.
Physically lignin is rigid. It is an end product of metabolism and after formation it functions mainly as a structural component of the cell wall. Lignin may be present in the middle lamella, the primary wall and the secondary wall.
Mineral substances like silica, calcium carbonate etc. and various organic compounds like resins, tannins, fatty substances, volatile oils, acids, crystalline pigments etc. may impregnate walls.
Cutin, suberin and waxes are most important fatty substances present in the walls. Suberin and cutin are highly polymerised compounds composed of fatty acids, they are closely related, not meltable and insoluble in fat solvents.
Waxes are meltable and soluble in fat-solvents. Cutin forms a continuous layer termed cuticle on the surface of epidermis of aerial plant parts, it also occurs with cellulose in the outer walls of epidermis.
Suberin occurs with cellulose in cork cells of the periderm. Waxes may be associated with suberin and cutin and are present in the surface of cuticle in various forms.
B. GROSS STRUCTURE OF THE CELL WALL
On the basis of development and structure. three parts are generally recognised in the cell wall, such as:
(i) Middle lamella
The middle lamella i.e. intercellular substance occurs between the primary walls of two contiguous cells. The middle lamella is amorphous and optically inactive i.e. isotropic. It is mainly composed of a pectic compound possibly combined with calcium.
The middle lamella in woody tissues is commonly lignified (Esau, 1965). It can be dissolved by various substances including the enzyme pectinase. Kerr and Bailey (1934) used the term compound middle lamella when dealing with the wood tissue. This term is used to refer to the complexes of the more or less homogeneous lignified layers.
The compound middle lamella may be either three-layered or five-layered. When three-layered, it refers to the middle lamella proper and the adjoining primary walls. When five layered, it (i.e. compound middle lamella) refers to the middle lamella proper, the primary walls and the outer layer of the secondary walls of the adjoining cells.
(ii) Primary cell wall
The primary wall is the first true cell wall to be formed by the developing cell. In many cells it is the only cell wall, as the middle lamella is regarded as intercellular substance and not a proper wall. This wall is formed on either side of the middle lamella by the contiguous cells.
It contains cellulose, hemicellulose and some pectin, it may be lignified. Owing to the presence of cellulose, the primary wall is optically active (anisotropic), thin, very elastic and capable of great extension.
All meristematic cells and also many mature cells having living contents have primary walls. Primary wall grows both in surface as well as in thickness. Changes of thickness of the wall during growth are considered to be reversible. The primary walls of the endosperm of Phoenix dactylifera, Diospyros virginiana, Strychnos nuxvomica etc. are very thick and serve as a source of reserve carbohydrate.
(iii) Secondary cell wall
Secondary walls are generally laid down after the primary wall ceases to increase in surface area, hence secondary walls do not extend to any considerable degree.
Secondary wall is formed on the inner surface of the primary wall, next to the cell lumen. It consists mainly of cellulose or of varying mixtures of cellulose and hemicelluloses; lignin and various other sustances may be deposited in the wall. Because of its high content of cellulose, the secondary wall is very strongly anisotropic i.e. optically active.
In many cases e.g. secondary walls of tracheids and fibres are three layered such as (a) the outer layer, (b) the middle layer and (c) the inner layer. Hence a cell wall may consists altogether of five layers viz. the middle lamella, the primary wall and a three-layered secondary wall. Of these layers the middle layer is the thickest.
In some cells of the stem of Linum usitatissimum more than three layers may be present (Fahn, 1982). The inner layer of the secondary wall is also termed as tertiary wall or tertiary layer by some authors (Bailey, 1957 ; Meier, 1957).
Frey-Wyssling (1976) suggests that an innermost layer with properties differing from those of the secondary wall may be present in addition to the inner layer of the secondary wall. According to him this layer should be termed tertiary lamella which may be differentiated into two strata viz, a membrano genoic stratum and a warty stratum.
Secondary walls are usually present in cells which are non-living at maturity e,g, sclereids, fibres, tracheary elements etc. But cells with active living protoplasts e.g. xylem parenchyma and xylem ray cells, may have secondary walls.
In the elongating elements of the protoxylem the secondary wall is not continuous but is formed in annular and helical bonds. The secondary cell wall is considered a supplementary wall whose main function is mechanical.
Ultrastructure and components of the cell wall
The principal component of plant cell wall is cellulose, the ultrastructure of cell walls is therefore based on cellulose. Work with electron microscope shows that cellulose in cell walls occurs in the form of long-chain molecules (Roelofsen, 1959 ; Albersheim, 1975 ; Frey-Wyssling, 1969-’76 ; Frey-Wyssling and Muhlethaler, 1965).
These chain-like molecules may be arranged randomly or in a more or less regular fashion. Each such cellulose molecule has 8Å maximum width.
Again cellulose molecules are regularly arranged in bundles—each such bundle forms an elementary fibril. Each bundle of elementary fibril contains 40-100 cellulose molecules in a transection, and is about 3.5 nm wide and 3 nm thick—some authors suggest that it has a widest diameter of 100Å.
Both the cellulose molecules and the elementary fibrils are ribbon-like structures. Within the elementary fibrils themselves are smaller units termed micelles or crystallites (Wardrop, 1962), which are small aggregations of cellulose molecules that lie parallel to one another and thus confer a crystalline structure upon the elementary fibril; only very small part of elementary fibril, that are presumably arranged at random, may be paracrystalline.
It was observed that the number of glucose residues in cellulose molecules of fibre cell varies from 500 to 10,000 and the length of such molecules varies from 0.25—5µm.
The elementary fibrils are again arranged in bundles, each such bundle is called microfibril which is 250Å wide and contains 2,000 cellulose molecules in transection. Microfibrils are combined into macrofibrils, each of which is 0.4µ wide and contains 500,000 cellulose molecules in transection.
In case of the secondary wall of ramie (Boehmeria) fibre, about 2000,000,000 cellulose molecules were found in a transection. This is the structure that has been studied in case of the secondary wall. The primary wall is similar in structure to the secondary wall in that it consists of crystalline (anisotropic) cellulose microfibrils and a non cellulosic matrix.
Microfibrils consists of bundles of cellulose molecules partly arranged into orderly three dimensional lattices i.e. the micelles. The crystalline properties of micelles are due to regular spacing of glucose residues which are connected by β-1-4 glucosidic bonds.
The spaces between the randomly arranged molecules in the microfibril are filled up with water, pectic substances, hemicellulose and in secondary walls lignin, cutin, suberin etc. The swelling of the cell wall during lignification is mainly due to this depositions of lignin between the existing cellulose framework of the wall.
The occurrence of a group of proteins containing hydroxyproline in the primary walls of various tissues has also been demonstrated by N. J. King and S. T. Bavley (1965), these proteins may serve enzymatic as well as structural functions. The protein may be involved in the orientation of the fibrils (Muhlethaler, 1967).
Many workers think that the wall-protein plays an important role in cell extension, and accordingly that protein has given the name “extensin”. Experiments with radioactive isotopes suggests that the protein is deposited throughout the wall matrix (Roberts and Northcote, 1972).
In most dicotyledons the cellulose microfibrils are coated with a single layer of hemicellulose of the nature xyloglucan. The hemicellulose is cross linked by pectic polymers, so that elementary fibrils are interconnected. In monocotyledons similar structures are found except that arabinoxylan takes the place of xyloglucan (Albersheim, 1974).
The microfibrils are arranged variously in cell walls. In the secondary walls they are arranged more regularly. In the primary walls microfibrils are often arranged in a direction more or less transverse to the longitudinal axis.
The microfibrils become arranged more longitudinally during growth of the cell. Microfbrils become oriented more and more longitudinally when even subsequent wall layers are formed. This transition is gradual.
Studies of cell walls of different cell types stained with permanganate have shown that in parenchyma cell walls the fibrils are laid down in lamellae—here the direction of fibrils alternate in successive lamellae; walls of sieve elements and collenchyma cells are polylamellate (Deshpande, 1976).
Different orientation of the wall layers in the secondary wall is also observed; here the layer S1 is the outermost and adjacent to the primary wall; within this layer S2 and S3 are laid down—in S1 and S3 the fibrils form a lax helix while in S2 fibrils form a steep helical structure.
Differences between cell wall and cell membrane :
| Cell wall | Cell membrane |
| 1. Cell wall is present only in plant cells. 2. Present outside the cell membrane of a plant cell. 3. Non-living material made up of cellulose and pectin. 4. The wall is thick and rigid. 5. Permeable membrane. 6. Shows different patterns of ornamentation. 7. Made up of middle lamella, primary wall and secondary wall. | 1. Present both in plant and animal cells. 2. Present outside the protoplasm of all living cells. 3. Living material made up of lipid and protein. 4. The membrane is thin and elastic. 5. Semi-permeable membrane. 6. Shows no pattern of ornamentation. 7. Made up of three consecutive layers like protein, lipid and protein. |
