In this article we will discuss about the structure of cell wall of a mature plant cell with the help of suitable diagrams.
Plant cells are surrounded by a non-living and rigid coat termed cell wall. The main functions of cell wall are to provide plant cells a definite shape, and mechanical support and strength to tissues and organs. Cells in meristematic tissue have a relatively thin and elastic cell wall.
The differentiation of newly formed cells into various tissues is accompanied with a thickening of cell wall due to various types of depositions. The pattern of these depositions on the inner surfaces of cell walls shows a considerable variation among different various tissues.
Even walls of individual cells are not uniformly thick over their entire surface; variable frequencies of pits of different types are distributed over the centre cell wall. In plants, differentiation is primarily manifested in the structure of cell wall which visible a marked and consistent variation among various tissue. In fact, plant tissues are identified based on the structure of their cell walls.
The cell wall of a differentiated cell has three distinct parts:
(i) Middle lamella,
(ii) Primary cell wall, and
(iii) Secondary cell wall (Fig. 2.2).
(i) Middle Lamella:
In plants, the walls of contiguous (immediate neighbour) cells are joined by middle lamella, which is composed chiefly of pectin. The pectin of middle lamella is most likely in the form of calcium (Ca++) and magnesium (Mg++) salts. Adhesion of the walls of contiguous cells is primarily dependent on the presence of Ca++ and Mg++ ions in the middle lamella; a removal of these ions results in the separation of cells from each other.
Pectin is readily hydrolysed by the enzyme pectinase as well as by strong acids. This characteristic is exploited by cytologists to achieve cell separation in smears from various tissues. Cellulose is characteristically lack from middle lamella. Middle lamella is the first to originate following cell division. Three kinds of membranous structures accumulate at the equatorial plate of a cell during telophase.
These structures are:
(1) Small cisterns of ER origin,
(2) Spherical vesicles of 20 mµ diameter, and
(3) Phragmosomes of about 250 mµ diameter.
These membranous structures are arranged in a layer (Fig. 2.3). The cisterns and vesicles fuse with each other to produce the cell plate; the membrances of fusing cisterns and vesicles gives rise to the plasma lemma of daughter cells in the dividing area of the cytoplasm.
The 20 mµ vesicles are filled with pectin which is deposited between the two newly produced membrances; this pectin gives rise to the middle lamella. Phragmosomes possess a relatively more dense substance which probably is a polysaccharide; it is most likely used in the formation of plasma lemma. Phragmosomes disappear once the cell plate is formed. The 20 mµ vesicles are perhaps produced by golgi bodies.
(ii) Primary Cell Wall:
Primary cell wall is deposited after the formation of middle lamella, and lies between middle lamella and plasma lemma. Its major constituents are- hemicellulose (53%) and cellulose (30%); in addition, it contains pectin (5%), protein (5%) and lipid (7%).
Cellulose molecules are like fibres; each molecule is made up of upto 3000 glucose residues. Many cellulose molecules associate to form a cellulose microfibril; in a single microfibril upto 2000 cellulose molecules can be arranged parallel to each other.
Cellulose microfibrils are the units of cell wall structural organization. In primary cell walls, they are present in a dispersed and loose arrangement (Fig. 2.4 a). A layer of pectin is present between two layers of microfibrils, while hemicellulose is distributed between microfibrils of each layer.
Cells in meristematic tissues possess only primary cell wall. Primary cell wall is not deposited in the vicinity of plasmodesmata so that pores are produced in the cell walls wherever plasmodesmata are present (Fig. 2.3).
(iii) Secondary Cell Wall:
Secondary cell wall is the last to be deposited and lies between primary cell wall and plasma lemma; in a cell it is the innermost layer of wall. It is made up mainly of cellulose, but in several tissues it contains lignin, suberin and, in some cases, cutin. In secondary walls, cellulose microfibrils are relatively more closely packed and they are arranged more or less parallel to each other (Fig. 2.4 b). Many microfibrils associate to form a macrofibril, which is the structural unit of secondary cell walls.
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