Cell Wall By:- A J

Contacts Introduction to cell wall What is cell wall -3 Synthesizing of cell wall -5 Structure of cell wall -8 Composition of cell wall -9 Properties of cell wall -13 Function of the cell wall -14

What is a cell wall? The cell wall surrounding plant cells is an extracellular matrix which can be viewed as a multifunctional sub-cellular compartment with many important functions in plant growth ( Ref:-The Cell wall by Hugo Zheng) The actual structure and content of a cell wall may vary with plant species. One of the most remarkable features of plants is that they have no bones or related skeletal structures, and yet exhibit remarkable strength. This strength is provided by the rigid cell walls that surround all plant cells. And cell wall also determine the shape of a cell Due to the shape of cell wall different shape of cells had been identified. Such as Parenchymatous, Collenchymatous cells

A major function of the cell wall is to prevent cell from over expansion. Which will happens when water enters the cell. Cell wall are also found in plant cells ,fungi cells ,algal cells and prokaryotic cells(Bacterial and Archaeal) The rigidity of the cell wall makes cell movements virtually impossible. At the same time, sturdy cell walls enable plant cells to withstand the considerable turgor pressure that is created by the uptake of water. Which is vital in cell expansion

Synthesizing of cell wall (Ref :- World of cell By Jeff Hardin ) The plant cell wall components are secreted from the cell stepwise, creating a series of layers in which the first layer to be synthesized ends up farthest away from the plasma membrane. The first structure to be synthesize is called the middle lamella. It is shared by neighbouring cell walls and holds adjacent cells together. The next structure to be formed is called the primary cell wall, which forms when the cells are still growing. Primary walls are about 100–200 nm thick. The primary cell wall consists of a loosely organized network of cellulose microfibrils associated with hemicelluloses, pectins, and glycoproteins. The cellulose microfibrils are generated by cellulose-synthesizing enzyme complexes called rosettes that are localized within the plasma membrane. In some plant cells, development of the cell wall does not proceed beyond this point. However, many cells that have stopped growing add a thicker, more rigid set of layers that are referred to collectively as the secondary cell wall.

The components of the multi-layered secondary wall are added to the inner surface of the primary wall after cell growth has stopped. Cellulose and lignin are the primary constituents of the secondary wall, making this structure significantly stronger, harder, and more rigid than the primary wall. Each layer of the secondary wall consists of densely packed bundles of cellulose microfibrils, arranged in parallel and oriented to lie at an angle to the microfibrils of adjacent layers. This organization imparts great mechanical strength and rigidity to the secondary cell wall. As the secondary walls form, microtubules located beneath the plasma membrane will orientate in the same direction as cellulose micro fibrils formed. microtubules will guide the movement of these rosette aggregates, which synthesize large parallel bundles of cellulose microfibrils as they move.

Cell walls consist of 3 types of layers (ref:- Middle lamella(inter cellular matrix):- This is the first layer formed during cell division. It makes up the outer wall of the cell and is shared by adjacent cells. It is composed of pectin compounds and protein. Primary wall: This is formed after the middle lamella and consists of a rigid skeleton of cellulose micro fibrils embedded in a gel-like matrix composed of pectin compounds, hemicellulose, and glycoproteins. It is thin and permeable Secondary wall: formed after cell enlargement is completed. The secondary wall is extremely rigid and provides compression strength. It is made of cellulose, hemicellulose and lignin. The secondary wall is often layered. And in some plants cells there occurs another cell wall beneath the secondary cell wall which called tertiary cell wall and it differ from other two. And it made of a substrate called Xylan.

Structure of cell wall (Ref:- World of cell By Jeff Hardin) Like the extracellular matrix of animal cells, plant cell walls consist predominantly of long fibers in a network of branched molecules. Instead of collagen and proteoglycans, however, plant cell walls contain cellulose microfibrils in a complex network of. branched polysaccharides and glycoproteins called extensins. The two main types of polysaccharides are hemicelluloses and pectins. On a dry- weight basis, cellulose typically makes up about 40% of the cell wall, hemicellulose account for another 20%, pectin represent about 30%, and glycoproteins make up about 10%. Cellulose, hemicelluloses, and glycoproteins are linked together to form a rigid interconnected network in a pectin matrix. Pectins are also branched polysaccharides but with backbones called rhamnogalacturonans that consist mainly of negatively charged galacturonic acid and rhamnose.

Composition of cell wall (Ref:- Plant cell biology By William V Composition of cell wall (Ref:- Plant cell biology By William V.Dashek) Cellulose :- polymer of glucose (typically consisting of 1,000 to 10,000 beta-D- glucose. major component of primary and secondary wall layers. Cellulose polymers associate through H-bonds. Which is important for the formation of micelles(Fibrils) and micro fibrils that forms A bundle size Macro fibrils and that forms the main frame work of Cell wall. Pectin:- polymer of galacturonic acid molecules. Major component of middle lamella but also found in primary cell walls.

Hemicellulose :- A polysaccharide composed of a variety of sugars including xylose, arabinose, mannose. Hemicellulose is largely numbered in primary walls but is also found in secondary walls. Pectic acid:- polymer of galacturonic acid molecules. Highly hydrated than pectin. Major component of middle lamella but also found in primary walls

Structural proteins:- In addition to carbohydrates, cell walls contain a variety of proteins as well. Major protein in the cell wall is Glycoprotein contains carbohydrate side chains on certain aminoacids. Structural proteins are found in all layers of the plant cell wall but they are more found in the primary wall layer. Cell wall also consist of minerals such as Ca, Mg in a form of Carbonates, Silicates.And we can find cutin of fatty acids and lignin forms from coniferyl Alcohols and chitin of polymer glucosamine. In bacteria we can found cell wall consist of peptidoglycan. Archean cell wall consist of glycoprotein S-layers, pseudo peptidoglycan, or polysaccharides In Algae glycoproteins and polysaccharides are the main parts Fungi have cell walls made of the glucosamine polymer chitin

Cell walls also contain functional proteins. Oxidative enzymes - peroxidases Hydrolytic enzymes - pectinases, cellulases "Expansins" - enzymes that catalyse cell wall "creep" activity(Ref:-Cell Wall Loosening by Expansins Daniel J. Cosgrove )

Properties of cell wall :- Flexibility and Rigidity (The plant cell wall by :- Jocelyn K.C.Rose) The cell wall gives cells rigidity and strength, offering protection against mechanical stress. In multicellular organisms, such as plants it permits the organism to build and hold its shape. The Extensin cross linking provide additional rigidity to the wall. This ability helps to create a physical barrier for pathogens It further permits the creation of a stable osmotic environment by preventing osmotic lysis and helping to retain water. Cytolysis, or osmotic lysis occurs when a cell bursts due to an osmotic imbalance that has caused excess water to move into the cell. The primary cell wall is flexible. In plants, a secondary cell wall is a thicker additional layer of cellulose which increases wall rigidity. Additional layers may be formed containing lignin in xylem cell walls, or containing suberin in cork cell walls

Permeability The primary cell wall is a free permeable surface. So small molecules like water and carbon di oxide can easily pass through. This distribution towards cell wall is called as apoplast pathway Function of cell walls Provide tensile strength and limited plasticity which are important for: keeping cells from rupturing from turgor pressure turgor pressure provides support for non-woody tissues Thick walled cells provide mechanical support Tubes for long-distance transport Cutinized walls prevent water loss Provide mechanical protection from insects & pathogens Physiological & biochemical activities in the wall contribute to cell-cell communication

Cell wall’s participation in Plasmodesmata (Ref :- World of cell by Jeff Hardin) Plant cells are capable of intercellular communication such as that afforded by the gap junctions of animal cells. Plasmodesmata are cytoplasmic channels through relatively large openings in the cell wall, allowing continuity of the plasma membranes from two adjacent cells.