Plant Cell Wall PDF

Ms. Uma Sharma MSc,MPhil,NET COB DUVASU Mathura PLANT CELL WALL Introduction : Cell is described as fundamental unit and ―building bloc...

Ms. Uma Sharma MSc,MPhil,NET COB DUVASU Mathura PLANT CELL WALL Introduction : Cell is described as fundamental unit and ―building blocks‖ of an organism. It is functionally and structurally working unit of life. An English natural philosopher ‘Robert Hooke’ in the year 1665 identified and coined the term Cell. It was first seen in cork cells and named (simply as a "wall") by Hooke. Cells of plant, fungi, bacteria and many protists are surrounded by rigid structure called cell walls, an integral part of the cell. The eukaryotic cell wall (including fungi and plants) are primarily composed of polysaccharides. Chitin is the basic structural polysaccharide of fungal cell walls. The plant cell wall is mainly composed of cellulose, a single most abundant polymer on earth. Along with cellulose (long fibers of carbohydrates), plant cell wall also contains hemicelluloses, pectin, suberin or chitin and lignin. It provides strength, protection and structural support to the cell, and can also control the movement of molecules entering or leaving the cell to some extent, acts as a filtering mechanism. Structure of cell wall : A multilayer plant cell wall is primarily made up of three parts i.e., middle lamella (outer layer of cell wall), primary cell wall and secondary cell wall (inner layers of cell wall). Primary cell wall and middle lamella are found in all plant cells but secondary cell wall is not found in all. In some tissues a tertiary cell wall is formed on the inner surface of the secondary cell wall. This layer is very thin and is found in the xylem tracheids of gymnosperms, it is not  Ms. Uma Sharma MSc,MPhil,NET COB DUVASU Mathura found in all the cells. It is mainly composed of chemical substances xylan instead of cellulose. Typically, it does not contain any cellulose microfibrils.  Ms. Uma Sharma MSc,MPhil,NET COB DUVASU Mathura 1. Middle Lamella ⮚ Middle lamella is the first layer which is deposited at the time of cytokinesis. ⮚ It is the outermost layer of the cell, which forms the boundary between the neighboring cells of the plant and holds them together. It is a thin, amorphous and cementing layer between two adjacent cells, just like brick work of the common wall between two adjacent rooms. ⮚ Middle lamella contains polysaccharides called pectins (also known as pectic polysaccharides), is rich in galacturonic acid and facilitate in cell adhesion by serving the cell walls of neighboring cells to join to one another. ⮚ It is made up of calcium and magnesium pectates. The softening of ripe fruits is caused by partial solubilization of pectic compounds to produce jelly-like consistency. ⮚ In plant biology, pectin consists of a complex set of polysaccharides that are present in most primary cell walls and are particularly abundant in the non-woody parts of terrestrial plants. Pectin is a major component of the middle lamella, where it helps to bind cells together, but is also found in primary cell walls. ⮚ Pectin is deposited by exocytosis into the cell wall via vesicles produced in the Golgi. It is an important cell wall polysaccharide that allows primary cell wall extension and plant growth. 2. Primary Cell Wall ⮚ Primary cell wall layer is built up between plasma membrane and the middle lamella in a growing plant cells. It is typically a flexible, thin (~0.1 to 3.0 μm), and extensible layer formed while the cell is rising. ⮚ The primary cell wall provides the strength and flexibility needed to allow for cell growth. ⮚ It is found in all plant cells and is formed during the early stage of growth and development. I ⮚ t is principally composed of cellulose microfibrils enclosed inside a gel-like matrix of pectin polysaccharides and hemicellulose fibers. The chief components are hemicellulose (53%), cellulose (30%), pectin (5%), and lipid (7%).  Ms. Uma Sharma MSc,MPhil,NET COB DUVASU Mathura ⮚ The primary cell wall is elastic and undergoes extension with the growth of the cell. In many roots, fleshy stems, fruits and leaves the cells contain only the primary cell wall and middle lamella. ⮚ Cellulose molecules are nothing but long chains of about 3000 glucose units. ⮚ In the cell wall, these molecules are arranged parallel to one another to form microfibrils. Each microfibril contains about 2000 cellulose molecules and has a diameter of 100-250 angstrom (Å) and a length of several microns. ⮚ Cellulose microfibrils are the units of cell wall structural organization. A layer of pectin is also seen in between two layers of microfibrils. 3. Secondary Cell Wall ⮚ The secondary cell wall is a thick layer that forms between the plasma membrane and the primary cell wall after the cell membrane is fully developed in some cells. ⮚ Secondary wall may be absent it is not found in all cell types, irregularly deposited or formed uniformly in the cells, which results in differentiation of cells: parenchyma, collenchyma, sclerenchyma, tracheids and vessels. ⮚ It consists of mainly cellulose with glycoprotein, lignin and other polysaccharides and provides additional protection to cells and rigidity and strength to the larger plant. ⮚ At one stage when the primary cell walls stop growing and dividing, they become thick to form a secondary cell wall, which is an inflexible layer that strengthens and supports the cell.  Ms. Uma Sharma MSc,MPhil,NET COB DUVASU Mathura ⮚ These walls are constructed of layered sheaths of cellulose microfibrils, wherein the fibers lie close, parallel within each layer and at an angle to the longitudinal axis of the cell. ⮚ It sometimes consists of three distinct layers - S1, S2 and S3 - where the direction of the cellulose microfibrils differs between the layers. A number of different materials may be deposited in the wall. ⮚ Along with hemicellulose and cellulose, several secondary cell walls also contain lignin. The incorporation of lignin reduces the flexibility of secondary cell walls, and makes it less permeable to water than the primary cell wall. Lignin provides strength to the cell wall and helps them to conduct water to the vascular tissue cells of the plant. In addition to giving more resistant against cell degradation, the hydrophobic nature of lignin is essential for keeping water within the vascular tissues transported throughout the plant. 4. Tertiary wall : ⮚ The innermost layer of secondary wall is sometimes distinct both chemically as well as in staining properties due to the presence of xylans (a group of hemicelluloses). It is then called tertiary wall, e.g., tension wood in gymnosperms. ⮚ The secondary cell wall has different ratios of constituents compared to the primary wall. For example, secondary wall in wood contains polysaccharides called xylan, whereas the primary wall contains the polysaccharide xyloglucan. The cellulose fraction in secondary walls is also higher. ⮚ Pectins may also be absent from the secondary wall, and unlike primary walls, no structural proteins or enzymes have been identified. Because of the low permeability through the secondary cell wall, cellular transport is carried out through openings in the wall called pits. PLASMODESMATA ⮚ Plasmodesmata (singular: plasmodesma) are intercellular organelles found only in plant and algal cells. The animal cell "equivalent" is called the gap junction. ⮚ Plasmodesmata are narrow channels that act as intercellular cytoplasmic bridges to facilitate communication and transport of materials between adjacent plant cells. ⮚ The plasmodesmata consist of minute pores or channels lying between individual plant cells, and connect the symplastic space in the plant.  Ms. Uma Sharma MSc,MPhil,NET COB DUVASU Mathura ⮚ They form a protoplasmic continuum called symplast. ⮚ Cell wall and intercellular spaces of cells form a non-living component of the plant body called apoplasm. ⮚ The actual air space separating the cells is called the desmotubule. ⮚ The desmotubule possesses a rigid membrane that runs the length of the plasmodesma. Cytoplasm lies between the cell membrane and the desmotubule. ⮚ Plasmodesmata are extremely specialized channels that allow for intercellular movement of water, various nutrients, and other molecules (including signalling molecules). ⮚ They are located in narrow areas of cell walls called primary pit fields, and they are so dense in these areas that they make up one percent of the entire area of the cell wall. ⮚ The plasmodesmata separate the outer cell membranes of the plant cells. The entire plasmodesma is covered with the smooth endoplasmic reticulum of the connected cells. ⮚ Plasmodesmata are formed during the periods of cell division and during plant development. They form when parts of the smooth endoplasmic reticulum from the parent cells become trapped in the newly formed plant cell wall. ⮚ Primary plasmodesmata are formed while the cell wall and endoplasmic reticulum are formed; secondary plasmodesmata are formed afterward. Secondary plasmodesmata are more complex and may have different functional properties in terms of the size and nature of the molecules able to pass through. Chemical composition : The polysaccharides of cell wall include cellulose, hemicelluloses, pectin compounds and  Ms. Uma Sharma MSc,MPhil,NET COB DUVASU Mathura lignins. (1) Cellulose : ⮚ Cellulose is a linear, unbranched polymer, consisting of straight polysaccharide chains made of glucose units linked by 1-4 beta glycosidic - bonds (called glycosidic bonds. ⮚ (Complete hydrolysis of cellulose yields D-glucose and its partial hydrolysis yields disaccharide units, cellobiose.) ⮚ These are the glucan chains which by intra-and intermolecular hydrogen bonding produce the structural units known as microfibrils, observable under electron microscopy and having toughness like the rubber. ⮚ Each microfibril is ribbon-like flat fibre being 10 nm wide and 3 nm thick (or 25 to 30 nm in diameter) and is composed of about 2000 glucan chains in it.  Ms. Uma Sharma MSc,MPhil,NET COB DUVASU Mathura ⮚ According to a classical estimate, each cellulose microfibril comprises three micelles or elementary fibrils : each elementary fibril contains about 100 cellulose molecules and each cellulose molecule is made up of 40 to 70 glucan chains (i.e., One microfibril = 3 ~ 100 ~ 70 = 21000 glucan chains). ⮚ Often numerous microfibrils get associated to form the macrofibrils having up to 0.5 micrometer diameter and observable under the light microscopy. ⮚ Cellulose is synthesized by a wide variety of cells that include bacteria (e.g., acetobacter,agrobacter and rhizobium), algae, fungi, cryptogams and seed plants. (2) Hemicellulose : Hemicelluloses are short but branched heteropolymers of various kinds of monosaccharides such as arabinose, xylose, mannose, galactose, glucose and uronic acid. Some of the common hemicelluloses go under the names xylans, arabinoxylans, glucomannans, galactomannans and xyloglucans.  Ms. Uma Sharma MSc,MPhil,NET COB DUVASU Mathura (3) Pectins are water soluble, heterogeneous branched polysaccharides that contain many negatively charged D-galacturonic acid residues along with D-glucuronic acid residues. Because of their negative charge, pectins are highly hydrated and intensely bind cations. When Ca2+ is added to a solution of pectin molecules, it cross-links them to produce a semirigid gel. Such Ca2+ cross-links are thought to help hold cell-wall component together. (4) Mannan is a homopolysaccharide of mannose and is found in the cell wall of yeast, fungi and bacteria. (5) Agar is a polysaccharide, found in the cell wall of sea weeds and containing D-and L-galactose residues. (6) Lignin is a biological plastic and non-fibrous material. It occurs only in mature cell walls and is made of an insoluble hydrophobic aromatic polymer of phenolic alcohols (e.g., hydroxyphenyl propane). (7) The chitin is a polymer of glucosamine. Glycoproteins (present up to 10 per cent in primary cell wall) are hydroxyproline- rich proteins (like the collagen). In them, many short oligosaccharide side chains are attached to hydroxyproline and serine side chains. Thus, more than half the weight of glycoprotein is carbohydrate. These glycoproteins are known to act like the glue to increase the strength of the wall. Cutin is also a biological plastic and is made of fatty acids (waxes). (8) Suberin is a water-resistant substance, comprisig of fatty acids and found in the cork and cell wall of many plants. (9) Sporopollenin is a lipoidal polymer forming tough wall (with species-specific patterns) of pollen grains. (10) Mineral deposits occur in cuticle in the form of calcium and magnesium carbonates and silicates. Deposits of calcium compounds are found in the cell wall of cruciferous and cucurbitaceous plants. Silicate deposits are common in the cell wall of Graminae family.  Ms. Uma Sharma MSc,MPhil,NET COB DUVASU Mathura CELL WALL FORMATION ⮚ During cytokinesis, middle lamella is constituted first formed by the cell plate, and later the primary cell wall is deposited within the middle lamella. ⮚ The cell wall is made during cell division when the cell plate is formed between daughter cell nuclei. This plate undergoes chemical and physical changes and then finally converted into the intercellular substance, the middle lamella. ⮚ A series of vesicles formed by the Golgi apparatus makes cell plate. The vesicles travel along the cytoskeleton and move to the cell equator, where it mobilizes and deposits their contents. These vesicle membranes now turn into the new cell membrane. ⮚ The Golgi apparatus synthesizes the non-cellulosic polysaccharides, which is composed of about 25–35% of plant cell walls. Golgi vesicles typically have pectic polysaccharides that are used to make the middle lamella. ⮚ The protoplast goes on secreting cell wall materials on the middle lamella, and ultimately a soft, delicate and plastic wall is formed, called primary cell wall. This is really the first formed cell wall, which may persist in many cells as the only wall. ⮚ It consists mainly of cellulose and pectic compounds and may also contain other polysaccharides. In plants, cellulose is synthesized at the plasma membrane by rosette terminal complexes (RTCs) that contain the cellulose synthase enzymes which synthesize the individual cellulose chains. ⮚ Each RTC floats in the cell's plasma membrane and "spins" a microfibril into the cell wall. Initially, cellulose synthase prepared by the ribosomes (of rough ER) is formed and travel through vesicles to Golgi and finally to cell membrane. ⮚ Notably, the wall is made from the outside in and when the wall gets thicker, the lumen gets smaller. The actual structure of the cell wall is not clearly defined, and several models exist such as the covalently linked cross model, the tether model, the diffuse layer model and the stratified layer model. However, the primary cell wall can be defined as composed of cellulose microfibrils aligned at all angles and are joined together by H-bonds to give high flexible strength. ⮚ The cells are retained and share a gelatinous membrane called the middle lamella, which contains salts of pectic acid, calcium and magnesium pectates. Cell to cell interaction are  Ms. Uma Sharma MSc,MPhil,NET COB DUVASU Mathura made via plasmodesmata, the inter-connecting channels of cytoplasm which connect to the protoplasts of neighboring cells across the cell wall. CELL WALL FUNCTIONS: ⮚ Protection: It protects the cell against physical damage, the protoplasm against mechanical injury and protects the cell from attack of invading pathogens. Cell wall also helps to stop water loss and acts as a physical obstacle to guard against viruses and other plant pathogens. ⮚ Support: It provides structural support and maintains the shape of the cell. The cell wall gives mechanical strength and rigidity, as well as controls the orientation of cell growth. It gives strength to the land plants to withstand gravitational forces. ⮚ Withstand turgor pressure: Turgor pressure is the force exerted on the cell wall, and has a major role in plant growth by promoting cell volume expansion. Its magnitude is determined by osmotic potential and water potential, as the turgor pressure increases, the cytoplasm pushes the cell membrane against the cell wall. It helps a plant to remain rigid and straight, but it can also cause a cell to burst. The cell wall prevents rupture of the cell membrane by preventing excessive endosmosis in a hypotonic medium.  Ms. Uma Sharma MSc,MPhil,NET COB DUVASU Mathura ⮚ Regulate growth: It controls the direction of cell growth and regulate cell volume. The cell wall has some enzymatic activity along with metabolism. It also sends a signal to the cell, which enters the cell cycle to divide and grow. ⮚ Regulate diffusion: The porous cell wall allows certain substances, including proteins, to pass into the cell, keeping the other substances out. It, therefore, acts as a selective permeable membrane that allows the small molecules to enter and also counteracts the osmotic pressure. Cutin and suberin of the cell wall reduce the loss of water through transpiration. ⮚ Communication: Cells communicate via plasmodesmata; these are pathways or pores found in plant cell walls, which allow communication signals and molecules to pass from one cell to another. ⮚ Storage: It also functions as a storage component as it stores carbohydrates like starch and glycogen, which are mainly used at growing period of a plant, especially in germination of seeds.