Botany Lec 2 PDF - Plant Cell Walls

2. The Cell Wall  It is a protective layer, characteristic of all plant cells and located outside the cell membrane.  It is composed of cellulose, hemicellulose and pectic substances.  It varies in thickness depending on the function of cells and their position within the plant body.  It is...

2. The Cell Wall  It is a protective layer, characteristic of all plant cells and located outside the cell membrane.  It is composed of cellulose, hemicellulose and pectic substances.  It varies in thickness depending on the function of cells and their position within the plant body.  It is considered to be non living and is typically permeable.  It is strong, porous, rather rigid and elastic (i.e. has the ability to alter its shape and composition as the cell grows). 2.1. Functions of the cell wall:  2.2. General structure of the cell wall The cell wall mainly consists of three layers. Each layer has its unique structure and function.  1. The Middle Lamella (ML): It is a thin layer between adjacent cells. It forms the intercellular substance that cements together the walls of two adjacent cells. It is the first layer which develops during cell division. It is composed of calcium, magnesium and pectin granules that increase in size and number to form cell plates.  2. The Primary Wall (P-Wall): This is the first true visible layer of cell wall, secreted by the protoplast. It develops on both sides of the middle lamella when two cells are adjacent. It is elastic, permeable to water and small molecules, and capable of stretching and growth in surface area and thickening. It is composed of cellulose as the main component accompanied by other non- cellulosic substances such as hemicellulose and pectic compounds. Cells with only primary walls have living contents and can revert to the meristematic condition.  3. The Secondary Walls (S-Wall): These are thickened structures formed on the inner surface of the primary wall of specialized cells such as vessel elements, fibers…..etc. It is non elastic, so it is not formed until the cell is mature and has ceased to grow. It is composed of cellulose as the main component accompanied by various non- cellulosic substances such as lignin, subrin, cutin. It may be considered as a supplementary wall whose principal function is mechanical support. Impermeable to water and gases. Cells with secondary walls generally have no living contents when mature and cannot revert to the meristematic condition. The secondary cell walls are usually seen to be three-layered, differ from one another in the orientation of their cellulose microfibrils, the center layer is usually broader. Fine structure of the cell wall  The basic unit of the cell wall is based on cellulose. Cellulose molecule consists of long chains of about 3000 glucose residues condensed together to develop enormously strong chain. 60-70 cellulose molecules are bundled together and twisted along their long axes to form microfibrils which are laid down in a coordinated fashion. A cell wall typically has several layers of microfibrils, running in different directions to increase strength.  Other molecules act as a glue-like matrix around the fibers to increase strength. The spaces between these microfibrils may be filled with, hemicellulose in primary wall, or non-cellulosic matrix (e.g. lignin, subrin, cutin) in secondary walls. In primary wall, microfibrils run in all directions so give it the flexibility, while in secondary walls, microfibrils are arranged in the direction of cell elongation  Chemical composition of cell wall Hemicellulosic c.w. The primary and secondary walls differ also in their chemical composition. The primary wall consists mainly of cellulose, small amounts of hemicellulose and pectic substances (cellulosic cell wall). In secondary wall, sometimes hemicellulose deposited irregularly on the cell wall (e.g. seed endosperm of dates) as a main reserve carbohydrates, thus forming (hemicellulosic cell wall)  Sometimes, fatty compounds like a-: Cutin: It forms a distinct impermeable layer inside or outside epidermal cells. The deposition of cutin between microfibrils of secondary cell wall is called cutinization and the wall is called cutinized cell wall; while the deposition of cutin on the surface of epidermal cells is known as cuticularization and the wall is known as cuticularized cell wall. b. Suberin: It is a fatty acid associated with cellulose in the cell wall of cork cells of periderm. It makes the wall impermeable, and the wall is known as suberized cell wall and the process is known as suberization.  Some phenolic compounds like: Lignin: It is associated with cellulose in some tissues to give them strength (e.g. fibers, sclereids and xylem vessels) forming lignified cell wall which is impermeable layer. The process of lignin deposition is known as lignification. xylem vessels  Pits Pits are non thickened areas in the cell walls of the plant cells, therefore appear as depressions. Each pit has a complementary pit exactly opposite to it in the wall of the neighboring cell, thus called pit-pair.  Pit consists of: (1) Pit aperture (opening) (2) pit cavity (3) pit membrane (perforated-like sieve) Two principal types of pits are recognized: 1. Simple pits: These are developed in pairs on the primary cell walls of two adjacent cells. Through these pits there is a diffusion of soluble substances from one cell to the other. Usually found in parenchyma cells, fibers and sclereids. In places where the secondary wall is very thick, the pit cavity has the form of a canal; sometimes this canal is branched and then the pit is called a branched pit. 2. Bordered pits: It is more complicated than the simple pits in its structure. Formed by overarching of the secondary wall to form border that enclosed the pit chamber. Two openings are distinguishing: that facing the cell lumen is termed the inner aperture and that near the pit chamber is the outer aperture. In some plants the pit membrane is thickened in its center portion to form torus. Found in water conducting and mechanical cells such as xylem vessels, fibers and tracheids. Plasmodesmata: Thin cytoplasmic strands which pass through a pore in the cell wall, and usually connect the protoplast of two adjacent cells to permit the regular transport between the cells, and also conduction of stimuli