Fig (1):Layers of seconday cell wall

Transcription

1 Dr. Alaa J. Taha, Dr. Rana Alroomi and Dr. Hadeel Al-Newani :Secondary cell wall Although the secondary wall commonly is thought of as being deposited after the increase in surface area of the primary wall has cease but evidence has been reported for both conifer tracheids and for the fibers in bamboo culms that secondary wall deposition begins before cell expansion ceases. Secondary walls are particularly important in specialized cells that have strengthening function and in those protoplast often dies after the secondary wall has been deposited. In thick-walled wood cells, three distinct layers has been designated S1, S2, and S3, for outer, middle, and inner layer, respectively can frequently be distinguished in the secondary wall as shown in fig (1). The S2 layer is the thickest. The S3 layer may be very thin or lacking entirely. Some wood anatomists consider the S3 layer to be sufficiently distinct from the S1 and S2 layers to be called tertiary wall. The separation of the secondary wall into the three S layers results mainly from different orientations of microfbrils in the three layers. Typically the microfibrils are helically oriented in the various layers. Fig (1):Layers of seconday cell wall

2 Chemical components of secondary cell wall 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 relatively abundant in primary cell walls, apparently are either absent or present in small amounts in secondary walls. As mentioned previously, an extensin-like protein has been localized in the secondary walls of mature loblolly pine wood. Lignin is common in the secondary walls of cells found in wood. Pits: A pit is a depression occurs in secondary cell wall. A pit in a cell wall usually occurs opposite a pit in the wall of an adjoining cell, and the two opposing pits constitute a pit-pair. Pits arise during ontogeny of the cell and result from differential deposition of secondary wall material; none is deposited over the pit membrane so that the pits are actual discontinuities in the secondary wall. Pits are serving as areas through which sap may pass from one cell to anthor, primary pits, which are thin areas, not interruptions, in the primary wall. Pit structure: 1- Pit membrane (the space middle lamella and the two primary walls between the two pit cavities. Fig 2: Pit structure 2- Pit cavity 3- Pit pore Types of pits:

3 Typical, there are four types of pits can be recognized with secondary walls. 1- Bordered Pits: the secondary wall arches over the pit cavity and narrows down its opening to the lumen of the cell. The overarching secondary wall constitutes the border. They usually found on the walls of hard wood vessels, tracheids, and fibers. In bordered pits, the part of the cavity enclosed by the border is called the pit chamber, and the opening in the border is the aperture. Fig (3): Bordered pit 2- Simple pits: In simple pits, no such overarching occurs. Simple pits are found in certain parenchyma cells, in extraxylary fibers, and in sclereids. In a simple pit, the cavity may be uniform in width, or it may be slightly wider or slightly narrower toward the lumen of the cell. If it narrows down toward the lumen, the simple pit intergrades with the bordered pit in its structure. 3- Branched or Ramiform Pits: Fig (4): Simple pit

4 Pits may coalesce as the wall increases in thickness and form. 4- Vestured pits: In some dictoleydonous woods, particularly members of Leguminosae, small outgrowth are present in pit cavity and on adjacent cell wall of vessel members, sometimes these outgrowth occlude the pit. Pit combination: A combination of simple pits is termed a simple pit pair, and of two opposing bordered pits a bordered pit-pair. Combinations of simple pits and bordered pits, called half-bordered pit-pairs, are found in the xylem. A pit may have no complementary structure, for example, as when it occurs opposite an intercellular space. Such pits are called blind pits. In addition two or more pits may oppose a single pit in an adjoining cell, a combination that has been named unilaterally compound pitting. Plasmodesmata: They are narrow strands of cytoplasm are interconnected the protoplasts of adjacent plant cells, which provide potential pathways for the passage of substances from cell to cell. They were first described by Tangl in Consequently the plant body essentially is partitioned into two compartments, the symplast (or symplasm) and the apoplast (or apoplasm). The symplast is composed of the plasma membrane-bound protoplasts and their interconnections, the plasmodesmata; the apoplast consists of the cell wall continuum and intercellular spaces. Accordingly the movement of substances from cell to cell by means of plasmodesmata is called symplastic transport (symplasmic transport), and the movement of substances substances in the cell wall continuum is called apoplastic transport (apoplasmic transport).

5 Fig (5): Plasmodesmata Types of plasmodesmata: 1- Primary plasmodesmata: Many plasmodesmata are formed during cytokinesis as strands of tubular endoplasmic reticulum become entrapped within the developing cell plate. 2- Secondary plasmodesmata: These post cytokinetically formed plasmodesmata are referred to as secondary plasmodesmata, and their formation is essential to establish communication between ontogenetically unrelated cells. According to one proposed mechanism, secondary plasmodesmata development involves the activity of localized wall degrading enzymes: pectinases, hemicellulases, and possibly cellulases that allow cytoplasmic strands to penetrate the otherwise intact wall. The control of these enzymes presumably is mediated by the plasma membrane. Plastids:

6 Plastids are characteristic components of plant cells. Each plastid is surrounded by an envelope consisting of two membranes. Internally the plastid is differentiated into a more or less homogeneous matrix, the stroma, and a system of membranes called thylakoids. The principal permeability barrier between cytosol and plastid stroma is the inner membrane of the plastid envelope. The outer membrane, although a barrier to cytosolic proteins, has generally been assumed to be permeable to low molecular weight solutes (<600 Da), an assumption that may be in need of re-evaluation. Plastids are semiautonomous organelles widely accepted to have evolved from free-living cyanobacteria through the process of endosymbiosis. Mature plastids are commonly classified on the basis of the kinds of pigments they contain. 1- Chloroplasts: It is the sites of photosynthesis, contain chlorophyll and carotenoid pigments. The chlorophyll pigments are responsible for the green color of these plastids, which occur in green plant parts and are particularly numerous and well differentiated in leaves. In seed plants, chloroplasts are usually disk-shaped and measure between 4 and 6 micrometers in diameter. The number of chloroplasts found in a single mesophyll (middle of the leaf) cell varies widely, depending on the species and the size of the cell. Chloroplasts often contain starch, phytoferritin (an iron compound) and lipid in the form of globules called plastoglobuli (singular: plastoglobule). The starch grains are temporary storage products and accumulate only when the plant is actively photosynthesizing. 2- Chromoplasts:

7 They are also pigmented plastids of variable shape, they lack chlorophyll but synthesize and retain carotenoid pigments, which are often responsible for the yellow, orange, or red colors of many flowers, old leaves, some fruits, and some roots. Chromoplasts are the most heterogeneous category of plastids and are classified entirely on the structure of the carotenoid-bearing components. 3- Leucoplasts ( Nonpigmented Plastids): Structurally the least differentiated of mature plastids, leucoplasts generally have a uniform granular stroma, several nucleoids, and, despite reports to the contrary, typical 70S ribosomes. They lack an elaborate system of inner membranes. Some store starch (amyloplasts), others store proteins (proteinoplasts), fats (elaioplasts), or combinations of these products. Amyloplasts are classified as simple or compound. Simple amyloplasts, such as those of the potato tuber, contain a single starch grain, whereas compound amyloplasts contain several often tightly packed starch grains as in the endosperm of oats and rice. Fig (6): Amyloplasts