Plant Cell Wall - PDF

THE PLANT CELL STRUCTURE Two different organization levels: Prokaryotes Bacteria Archaea Eukaryotes Protists Plants Fungi Animals The three domains of life THE EUKARYOTIC CELL  2 MAIN DISTINCTIVE CHARACTERISTICS COMPARED TO PROKARYOTIC CELLS...

THE PLANT CELL STRUCTURE Two different organization levels: Prokaryotes Bacteria Archaea Eukaryotes Protists Plants Fungi Animals The three domains of life THE EUKARYOTIC CELL  2 MAIN DISTINCTIVE CHARACTERISTICS COMPARED TO PROKARYOTIC CELLS bigger size compartmentation (cell organelles) THE CELL WALL  Functions of the cell wall Mechanical support for the cell (cellular «exoskeleton») Osmotic role Controls the rate and the direction of the cell growth Defenses plants against pathogens Energy source for cells (in few cases, seeds) Site of signal molecules (elicitors, oligosaccharides) Plant cell walls determine the mechanical strength of plant structures, allowing those structures to grow to great heights. The expansive growth of plant cells is limited principally by the ability of the cell wall to expand. The wall acts as a diffusion barrier that limits the size of macromolecules that can reach the plasma membrane from outside, and it is a major structural barrier to pathogen invasion.  CELL WALL AND MEMBRANE: don’t confuse!!! CELL WALL MEMBRANE Thichness (m) Thickness (nm) Chemical composition: mainly Chemical composition: polysaccharides phospholipids and proteins Mechanical resistance Chemical barrier The cell wall is not uniform, but greatly varies in appearance and chemical composition in different cell types, and depending on the growth stage of the cell Cell walls are commonly classified into two major types: PRIMARY WALLS and SECONDARY WALLS PRIMARY WALLS : formed by growing cells and rather similar in its architecture in all cell types (variations in primary cell wall were also observed in different cell types) SECONDARY WALLS: formed after the cell growth (enlargement) has ceased. They are not present in all cell types and more specialized!! CELL WALL IS PENETRATED BY PLASMODESMATA  Chemical composition of primary cell wall Cellulose microfibrils embedded in a highly hydrated matrix This combination provides both strenght and flexibility Matrix: two major groups of polysaccharides: 1. Hemicelluloses 2. Pectins 3. Small amounts of structural proteins Cellulose microfibrils The individual glucans that make up the microfibril are closely aligned and bonded to each other to make a highly ordered (crystalline) ribbon that excludes water and is relatively inaccessible to the enzymatic attack. Cellulose is very strong and very stable and resists degradation. Single glucose chains are composed of 2000 to more than 25000 glucose residues.  HEMICELLULOSES Flexible polysaccharides that bind to the surface of cellulose. They may form tethers that bind cellulose microfibrils together into a cohesive network or they may act as a slippery coating to prevent direct microfibril-microfibril contact. Polymers are named based on the main sugar they contain  PECTINS They form a highly hydrated gel in which the cellulose- hemicellulose network is embedded They act as a hydrophilic filler and provide the flexibility to cell wall  Polygalacturonic acid (or pectic acid) Non-esterified carboxylic group with a negative charge Esterified carboxylic group without negative charge (neutral pectins) Formation of a pectin network involves ionic bridging of the non-esterified carboxyl groups (COO-) by calcium ions  Structural proteins Their role is still not very clear, but they probably add mechanical strength to the wall and assist in the proper assembly of other wall components.  WATER Very important component of the primary cell wall, mostly located in the matrix The hydration degree of the cell wall is very important for a normal growth  Where are cell wall components synthesized? Matrix polysaccharides: Golgi apparatus (dictyosomes) Cellulose: plasma membrane (cellulose synthase) The orientation/arrangement of cellulose microfibrils in the primary wall controls the direction of growth  SECONDARY WALL After the wall expansion ceases, some cells continue to synthesize a wall, called SECONDARY WALL Thicker than the primary cell wall, often multilayered, and with a different chemical composition  SECONDARY WALL Higher cellulose content Different hemicellulose composition Different arrangement of cellulose microfibrils (more neatly aligned and paraller to each other)  MODIFICATIONS OF THE CELL WALL In specialized cells the wall is very intricately sculpted and impregnated with particular substances: CUTIN, SUBERIN, WAXES, LIGNIN, SILICA Secondary walls can be impregnated with LIGNIN