Structure and composition of the cell wall:

Transcription

1 Structure and composition of the cell wall: The cell wall is rigid and therefore limits the size of the protoplast, preventing rupture of the plasma membrane when the protoplast enlarges following the uptake of water. The cell wall largely determines the size and shape of the cell, the texture of the tissue, and the final form of the plant organ. Cell walls contain a variety of enzymes and play important roles in absorption, transport, and secretion of substances in plants. In addition the cell wall may play a role in defense against bacterial and fungal pathogens by receiving and processing information from the surface of the pathogen and transmitting this information to the plasma membrane of the host cell. Through gene-activated processes, the host cell may become resistant to attack through the production of phytoalexins antibiotics that are toxic to the pathogens or through the deposition of substances such as lignins, suberin, or callose, which may act as passive barriers to invasion. MACROMOLECULAR COMPONENTS OF THE CELL WALL Cellulose Is the Principal Component of Plant Cell Walls: The principal component of plant cell walls is cellulose, a polysaccharide with the empirical formula (C 6 H 10 O 5 )n. Its molecules are linear chains of (1 4)β- linked-d-glucan (repeating monomers of glucose attached end to end.these long, thin cellulose molecules tend to hydrogen bond together to form microfibrils. diameter of microfibrils range from 4 to 10 nanometers. The cellulose microfibrils wind together to form fine threads that coil around one another like strands in a cable. Each cable, or macrofibril, which is visible with a light microscope, measures about 0.5 micrometer. Cellulose has crystalline properties resulting from the orderly arrangement of cellulose molecules in the microfibrils. Such arrangement is restricted to parts of the microfibrils that are referred to as micelles. The crystalline structure of cellulose makes the cell wall anisotropic and, consequently, doubly refractive (birefringent) when viewed with polarized light. Hemicelluloses: The cellulose microfibrils of the wall are embedded in a cross-linked matrix of non-cellulosic molecules. These molecules are the polysaccharides known as hemicelluloses and pectins, as well as the structural proteins called glycoproteins. Xyloglucans: consist of linear chains of (1 4) β-d-glucan as in cellulose, with short side chains containing xylose, galactose, and often a terminal fucose. 1

2 Xylans: are the major non-cellulosic polysaccharides of the secondary walls of all angiosperms. Glucomannans: form the major hemicelluloses of the secondary cell walls of gymnosperms. Pectins: The pectins are probably the most chemically diverse of the non-cellulosic polysaccharides. They are characteristic of the primary walls of eudicots and to a lesser extent of monocots. Pectins may be lacking from secondary walls entirely. Proteins: In addition to the polysaccharides described above, the cell wall matrix may contain structural proteins (glycoproteins). Among the major classes of structural proteins are the 2

3 hydroxyproline- rich proteins (HRGPs), the proline-rich proteins (PRPs), and the glycinerich proteins (GRPs). Once believed to be associated with lignification in the xylem. Callose: a linear (1 3) β-d-glucan, is deposited between the plasma membrane and the existing cellulosic cell wall. It is probably best known in the sieve elements of angiosperm phloem, where it is associated with developing sieve pores and commonly found lining fully developed pores. Callose is deposited very rapidly in response to mechanical wounding and environmental- or pathogen-induced stress. Lignins: are phenolic polymers formed from the polymerization of three main monomeric units, the monolignols p-coumaryl, coniferyl, and sinapyl alcohols. The mechanical rigidity of lignin strengthens the xylem, enabling the tracheary elements to endure the negative pressure generated from transpiration without collapse of the tissue. Suberin: is a major component of the cell walls of the secondary protective tissue, cork, or phellem. CELL WALL LAYERS: 1- The Middle Lamella: The middle lamella is largely pectic in nature but often becomes lignified in cells with secondary walls. 2- The Primary Wall: The primary wall, composed of the first-formed wall layers, is deposited before and during the growth of the cell. Actively dividing cells commonly have only primary walls, as do most mature cells involved with such metabolic processes as photosynthesis, secretion, and storage. Current models of the architecture of the primary (growing) cell wall envision a network of cellulose microfibrils intertwined with hemicelluloses, such as xyloglucan, and embedded in a gel of pectins. In one model (Fig. A) the hemicelluloses coat the surface of the cellulose, to which they are non-covalently bonded, and form cross-links, or tethers, that bind the cellulose microfibrils together. It has been estimated that such a cellulose-xyloglucan network may contribute as much as 70% of the total strength to normal primary walls. 3

4 In an alternative model of the primary wall (Fig. B), there are no direct microfibril-microfibril links. Instead, hemicelluloses tightly bound to the microfibrils are sheathed in a layer of less tightly bound hemicelluloses, which in turn are embedded in the pectin matrix, filling the spaces between microfibrils. In this model, wall strength may depend in part on the presence of many non-covalent interactions between laterally aligned matrix molecules. 3- The secondary wall: Although the secondary wall commonly is thought of as being deposited after the increase in surface area of the primary wall has ceased. Secondary walls are particularly important in specialized cells that have strengthening function and in those involved in the conduction of water; in these cells the protoplast often dies after the secondary wall has been deposited. Cellulose is more abundant in secondary walls than in primary walls, and pectic substances are lacking; the secondary wall is therefore rigid and not readily stretched. Structural proteins and enzymes, which are 4

5 relatively abundant in primary cell walls, apparently are either absent or present in small amounts in secondary walls. *Lignin is common in the secondary walls of cells found in wood. In thick-walled wood cells, three distinct layers designated S1, S2, and S3, for outer, middle, and inner layer, respectively can frequently be distinguished in the secondary wall. The S2 layer is the thickest. The S3 layer may be very thin or lacking entirely. The separation of the secondary wall into the three S layers results mainly from different orientations of microfibrils in the three layers. Typically the microfibrils are helically oriented in the various layers. In S1, the fibrils run along crossed helices, which make a large angle with the long axis of the cell so that this layer is highly birefringent. In S2, the angle is small and the slope of the helix steep; hence the cellulose microfibrils in this layer do not show up with the polarizing microscope. In S3, the microfibrils are deposited as in S1, at a large angle to the long axis of the cell. Fig: represent secondary cell wall layer. 5