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Plant cell wall components

The document provides a detailed overview of plant cell wall components and their functions, highlighting structures such as the primary and secondary walls, plasmodesmata, and the roles of polysaccharides like cellulose, hemicellulose, and lignin. It explains the importance of these components in providing mechanical support, regulating growth, facilitating communication between cells, and protecting against environmental stresses. Additionally, it discusses the biochemical processes involved in the turnover and remodeling of cell wall components during plant growth and development.

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PLANT CELL COMPONENTS Plant systems: Extracellular matrix components of plants-cell wall, cellulose and hemicelluloses, extensins, WAKs, secondary wall structure, pits-primary and secondary pits and their development, plasmodesmota-structure and functions, pectins, cutins, lignins, turnover of cell wall Dr. M. THIPPESWAMY
• Can not make their own food so they have to eat food • Animal cells are more round shaped • Animal cells do not have a cell wall • Animal cells do not have a large vacuole • They do not have chloroplast • Animal cells can not make sugar • Plant cells go through photosynthesis • Plant cells are in the shape of a rectangle • Plant cells have a cell wall • Plant cells have a large vacuole • Plant cells have chloroplast • Plant cells use chloroplast to store energy Animal cell Plant cell
• The cell wall is a selective filter whose permeability is controlled largely by pectins in the wall matrix, including proteins larger than 20 kDa, is limited. • This limitation may account for why many plant hormones are small, water-soluble molecules, which can diffuse across the cell wall and interact with receptors in the plasma membrane of plant cells. • The primary cell wall, generally a thin, flexible and extensible layer formed while the cell is growing. • The secondary cell wall, a thick layer formed inside the primary cell wall after the cell is fully grown. It is not found in all cell types. Some cells, such as the conducting cells in xylem, possess a secondary wall containing lignin, which strengthens and waterproofs the wall. • The middle lamella, a layer rich in pectins. This outermost layer forms the interface between adjacent plant cells and glues them together. Plant cell wall
Primary wall composition and architecture • Primary walls composed of polysaccharides with lesser amounts of structural glycoproteins (hydroxyproline-rich extensins), phenolic esters (ferulic and coumaric acids), ionically and covalently bound minerals (e.g. calcium and boron), and enzymes. • The primary wall consists of 50% hemicellulose, 25% of cellulose and in lesser amount of fats, proteins etc….. • Extensin, a glycoprotein that contain hydroxyproline and serine. Most hydroxyproline residues are linked to short chains of arabinose and the serine residues are linked to galactose. • Lignin, a macromolecule composed of highly cross-linked phenolic molecules, is a major component of secondary walls. • Cellulose - a large, linear polysaccharide composed of 1,4-linked β-D-glucose polymerizes the enzyme called cellulose synthase, assembles spontaneously into microfibrils stabilized by hydrogen bonds.
Hemicellulose - branched polysaccharides that are structurally homolgous to cellulose because they have a backbone composed of 1,4-linked β-D- hexosyl residues. The predominant hemicellulose in many primary walls is xyloglucan. Other hemicelluloses found in primary and secondary walls include glucuronoxylan, arabinoxylan, glucomannan, and galactomannan. Pectin - a family of complex polysaccharides that all contain 1,4-linked α-D-galacturonic acid. To date three classes of pectic polysaccharides have been characterized: Homogalacturonans, rhamnogalacturonans, and substituted galacturonans. Some of the functions of the primary wall: •Structural and mechanical support. •maintain and determine cell shape. •resist internal turgor pressure of cell. •control rate and direction of growth. •ultimately responsible for plant architecture and form. • carbohydrate storage - walls of seeds may be metabolized. •protect against pathogens, dehydration, and other environmental factors. •source of biologically active signalling molecules. •cell-cell interactions.
• The thicker and stronger secondary wall, which accounts for most of the carbohydrate in biomass, is deposited once the cell has ceased to grow. • The secondary walls of xylem fibers, tracheids, and sclereids are further strengthened by the incorporation of lignin. • Chemically secondary walls are made up of cellulose, hemicellulose with lignin, chitin or suberin. • It sometimes consists of three distinct layers - S1, S2 and S3 - where the direction of the cellulose microfibrils differs between the layers. The secondary wall Unlike cellulose, pectin and hemicellulose are synthesized in the Golgi apparatus and transported to the cell surface where they form an interlinked network that helps bind the walls of adjacent cells to one another and cushions them.
• The wall-associated kinases, or WAKs, are receptor-like kinases that are linked to the pectin fraction of the cell wall, and have a cytoplasmic protein kinase domain. • WAKs are required for cell expansion, cell elongation, morphogenesis, pathogen response, and their expression is activated by numerous environmental stimuli. • WAKs receptors for both pectin in the cell wall, and for pectin fragments, oligogalacturonic acids (OGs), generated during some pathogen attacks. Wall-Associated Kinases • These receptor-like proteins contain a cytoplasmic serine threonine kinase, a transmembrane domain, and a less conserved region that is bound to the cell wall and contains a series of epidermal growth factor repeats. • Undoubtedly, wall-associated proteins serve complex and biological roles with regard to wall structure.
In fungi, the cell wall is the outer-most layer, external to the plasma membrane. The fungal cell wall is a matrix of three main components: • Chitin: polymers consisting mainly of unbranched chains of β-(1,4)- linked-N-Acetylglucosamine in the Ascomycota and Basidiomycota, or poly-β-(1,4)-linked-N-Acetylglucosamine (chitosan) in the Zygomycota. Both chitin and chitosan are synthesized and extruded at the plasma membrane. • Glucans: glucose polymers that function to cross-link chitin or chitosan polymers. β-glucans are glucose molecules linked via β- (1,3)- or β-(1,6)- bonds and provide rigidity to the cell wall while α- glucans are defined by α-(1,3)- and/or α-(1,4) bonds and function as part of the matrix. • Proteins: enzymes necessary for cell wall synthesis and lysis in addition to structural proteins are all present in the cell wall. Most of the structural proteins found in the cell wall are glycosylated and contain mannose, thus these proteins are called mannoproteins or mannans. Fungal cell wall Chemical configurations of the different monosaccharides (glucose and N-acetylglucosamine) and polysaccharides (chitin and cellulose) A close-up of the wing of a leafhopper; the wing is composed of chitin.
Functions of cell wall • Cell walls primary function is mechanical support. It acts like a skeletal framework of the plants. • Cell wall is tough and has high tensile strength. Still plant cell is fully permeable to water and solutes. • Plant cell wall has minute water filled channels through which water, hormones and gases passes to and fro. • Cell wall shows plasticity and elasticity during cell growth. • It helps to maintain the balance of intracellular osmotic pressure with that of its surroundings • Lignification of secondary walls greatly enhances compressive strength permitting woody structures to reach the sky. • Cell wall upon lignification becomes dead as it becomes impermeable and thus protoplasm has no access to take up solutes that is why lignified tissue is always dead. • Lignin provided extra mechanical strength and also provides a water resistant channel for transport of solutes.
• The cytosols of adjacent animal or plant cells often are connected by functionally similar but structurally different “bridges” called gap junctions in animals and plasmodesmata in plants. • These structures allow cells to exchange small molecules including nutrients and signals, facilitating coordinated functioning of the cells in a tissue. • The cell wall is perforated by narrow pores or pits at many places. • Plasmodesmata are narrow channels through the wall bound by plasmalemmma containing cytoplasm and often a desmotubule. • The desmotubule is the cenrtal core and is composed of protein subunits consists of modified membranous structure continuous with the endoplasmic reticulum of the adjoining cells. Plasmodesmata
• These membraneous structures probably originate from the ER. • Desmotubule acts as valve to control the direction of flow of materials. • Plasmodesmata have been shown to transport proteins (including transcription factors), messenger RNA, viroids, and viral genomes from cell to cell. • One example of a viral movement proteins is the tobacco mosaic virus thought to bind to the virus's own genome and shuttle it from infected cells to uninfected cells through plasmodesmata. • It is concerned with the transport of materials from one cell to another and also conduction of stimuli. • In the case of certain parasites like Viscum, Loranthus, Cuscuta etc plasmodesmata connections exist between the haustoria and the cells of their host. • Through these channels food and virus are transported. The structure of a primary plasmodesma. CW=Cell wall CA=Callose PM=Plasma membrane ER=Endoplasmic reticulum DM=Desmotubule Red circles=Actin Purple circles and spokes=Other unidentified proteins.
The pits are formed in pairs lying against each other on the opposite sides of the wall, and morphologically more correct they are called ‘pit pairs’. • Simple pits: The simple pit is circular, oval, polygonal, elongated and occur in parenchyma cells, in medullary rays, in phloem fibres, companion cells, and in tracheids of several flowering plants. • Bordered pits: They overarching secondary wall found in the vessels of many angiosperms and in the tracheids of many conifers. They are more complex and variable in their structure than simple pits. The overarching secondary wall which encloses a part of the pit cavity is called, the pit border, which opens outside by a small rounded mouth known as pit aperture. • Half bordered pit pair: In some cases, bordered pit has a complementary simple pit. Such a pit pair is called half bordered pit pair. Pit Pairs: Structure and Types
• Blind pits: Some pits do not hate ally complementary pit. Such pits are called blind pits. • Compound pits: Sometimes, there is one pit on side. But there are two or more complementary pits on opposite side. Such pits are called compound pits. • Torus: The torus may remain in central position or it may shift to the lateral position. As the torus is shifted to the lateral position the pit aperture closes, and the passage of the protoplasm may take place only by diffusion through torus. Patterns of bordered pits: The bordered pits in vessel walls of angiosperms show three main types of arrangement: (i)Scalariform pitting (ladder like arrangement) (ii)Opposite pitting and (horizontal pairs) (iii)Alternate pitting. (rectangular outlines)
• Cutin is one of two waxy polymers that are the main components of the plant cuticle, which covers all aerial surfaces of plants. • The other major cuticle polymer is cutan, which is much more readily preserved in the fossil record. • Cutin consists of omega hydroxy fatty acids and their derivatives, which are interlinked via ester bonds, forming a polyester polymer of indeterminate size. • There are two major monomer families of cutin, the C16 and C18 families. The C16 family consists mainly of 16- hydroxy palmitic acid and 9,16- or 10,16-dihydroxypalmitic acid. • The C18 family consists mainly of 18-hydroxy oleic acid, 9,10-epoxy-18-hydroxy stearic acid, and 9,10,18- trihydroxystearate. • The primary function of the plant cuticle is as a water permeability barrier that prevents evaporation of water from the epidermal surface, and also prevents external water and solutes from entering the tissues. • "The waxy sheet of cuticle also functions in defense, forming a physical barrier that resists penetration by virus particles, bacterial cells, and the spores or growing filaments of fungi”. Cutin
• Pectin is present in the middle lamella, primary cell and secondary walls and is deposited in the early stages of growth during cell expansion. • It pectin plays an important role in the formation of higher plant cell walls, which lend strength and support to a plant and yet are very dynamic structures. • In general, the polymeric composition of primary cell walls in dicotyledonous plants consists of approximately 35% pectin, 30% cellulose, 30% hemicellulose, and 5% protein. • Grasses contain 2–10% pectin and wood tissue ca 5%. In cell walls of some fruits and vegetables, the pectin content can be substantially higher and the protein content lower. • Second, pectin influences various cell wall properties such as porosity, surface charge, pH, and ion balance and therefore is of importance to the ion transport in the cell wall. • Pectin oligosaccharides induce lignification and accumulation of protease inhibitors in plant tissues. Pectin
• Lignin is a class of complex organic polymers that form important structural materials in the support tissues of vascular plants and some algae. • Lignins are particularly important in the formation of cell walls, especially in wood and bark, because they lend rigidity and do not rot easily. Chemically, lignins are cross-linked phenolic polymers • The composition of lignin varies from species to species. In gereral 63.4% carbon, 5.9% hydrogen, 0.7% ash (mineral components), and 30% oxygen (by difference), corresponding approximately to the formula (C31H34O11)n. • Lignin fills the spaces in the cell wall between cellulose, hemicellulose, and pectin components, especially in vascular and support tissues: xylem tracheids, vessel elements and sclereid cells. Lignin
• The plant cell-wall compartment can have high metabolic activity and may be engaged in important events related to plant cell function and development, including control of growth and morphogenesis, cell-cell recognition, disease resistance, and signaling. • Changes in the chemical structure of the cell wall polysaccharides underlie the process of cell wall loosening leading to cell elongation. • Cell wall of the plant is composed of many complex carbohydrates and proteins, which undergo rapid turn over during the process of cell elongation. • Cellulose microfibrils are linked by two major groups of polysaccharide network, i.e. pectic substances and xyloglucans. • Cell wall components turnover occur through a variety of chemical modification by cleavage (hydrolysis) of cell wall polysaccharides. • Cell wall polymer breakdown has been associated with cell division, growth, xylem differentiation, the abscission of various plant organs and pollen germination and pollen tube growth. Turnover of cell wall components
• The proteins play a important role in cell wall turnover by hydrolyzing the polysaccharides of cellulose and hemicellulose with cellulases and glucanases. Glycosidases are enzymes which hydrolyze oligosaccharides to monomers. • Plant cell growth is the result of turgor-dependent extension of the cell wall that the primary growth promoting actions of auxin weakens the wall which allowed it to stretch. • The observation that increased RNA and protein synthesis accompanied hormone-promoted tissue elongation led to the proposal that enhancement of de novo synthesis of hypothetical wall-loosening enzymes was the primary action of auxin. • The pulse-chase study of cell wall turnover in epicotyl segments were studied with labelled 14C-glucose. The radioactive epicotyl segments were incubated for 3 hrs in unlabeled glucose. • IAA was then provided and changes in labeled wall sugars were detected an auxin-dependent loss of 14C- xylose and 14C-glucose from the epicotyl wall. • The synthetic and degradative reactions are balanced to provide coordination of wall turnover with all of the other processes that are essential to plant cell development.
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