Cell wall The cell wall is tough, flexible rigid structure that surrounds the cells. It is present outside the cell membrane and provides support and protection to the cell. Any cell that has lost its cell wall becomes amorphic i.e. it lacks a defined shape. The cell wall consists of 3 types of layers
i. Middle lamella is the first layer formed during cell division. Middle lamella makes up the outermost layer of the cell and is shared by adjacent cells. It is composed of pectic compounds and protein.
ii. Primary Wall is deposited by cells before and during active growth and consists of a rigid skeleton of cellulose microfibrils embedded in a gel-like matrix composed of pectic compounds, hemicelluloses and glycoprotein.
iii. Secondary wall is formed after growth stops or when the cells begin to differentiate. The secondary wall is extremely rigid and provides strength. It is made of cellulose, hemicelluloses and lignin. The secondary wall is often layered.
Composition of cell wall
• Pectic acid is a major component of middle lamella but is also found in primary walls. It is a polymer of around 100 galacturonic acid molecules and is hydrophilic. It forms salts and salt bridges with Ca++ and Mg++ that are insoluble gels. The carboxyl groups present on the galacturonic acid molecules are weak acids, so they can exist in negatively charged and uncharged states depending on protonation and related to the pKa
• Pectin is a polymer of around 200 galacturonic acid molecules. It is less hydrated than pectic acid but soluble in hot water. They are major component of middle lamella but also found in primary walls.
• Cellulose is a polymer of glucose and consists of 1,000 to 10,000 beta-D-glucose residues. It is a major component of primary and secondary wall layers that forms long fibers and gives the cell wall its rigidity. The cellulose polymers associate through H-bonds. The H-bonding of many cellulose molecules to each other results in the formation of micro fibers and the macro fibers that can interact to form fibers.
• Starch is also a polymer of glucose. However, instead of a -1,4 linkage between glucose molecules, starch uses an -1,4 linkage. The difference is due to the conformation of the ring structure. The -1,4 linkages cause the polymer to take on a twisted configuration instead of the linear shape of cellulose. Thus, starch forms globular structures. Starch molecules are often branched, which also prevents linear arrays from forming. In plants, starch is only found in plastids.
Hemicelluloses are a polysaccharide composed of a variety of sugars including xylose, arabinose and mannose. Hemicellulose that is primarily xylose or arabinose is referred to as xyloglucans or arabinoglucans, respectively. Hemicellulose molecules are often branched. Like the pectic compounds, hemicellulose molecules are very hydrophilic. They become highly hydrated and form gels. It is abundant in primary walls but is also found in secondary walls.
Lignin: Lignin is primarily a strengthening agent in the wall. It also resists fungal/pathogen attack.
Suberin, wax, cutin: A variety of lipids are associated with the wall for strength and waterproofing.
Water: The wall is largely hydrated and comprised of between 75-80% water. This is responsible for some of the wall properties. For example, hydrated walls have greater flexibility and extensibility than non-hydrated walls.
Proteins: In addition to carbohydrates, cell walls contain a variety of proteins. One type of cell wall proteins, called glycoproteins contains carbohydrate side chains on certain amino acids. One common group of cell wall proteins are characterized by having an abundance of the amino acid hydroxyproline.
Structural proteins are found in all layers of the plant cell wall but they are more abundant in the primary wall layer. Like the cell wall carbohydrates, glycoproteins are hydrophilic and can form H-bonds and salt bridges with cell wall polysaccharides. In addition to hydroxyproline, cell wall proteins are often high in the amino acids proline and lysine. The NH3+ on lysine provides positive charges along the peptide backbone. The positively charged residues can associate with negatively charged groups on pectic acids, etc. In addition to electrostatic interactions, H-bonds also form between amino acid side chains and cell wall carbohydrates.
Another type of structural cell wall protein, called extensin, can form covalent bonds with other extensin proteins through the amino acid tyrosine. In extensin, the tyrosines are evenly spaced and when they bond with tyrosine on another extensin molecule, they can wrap around other cell wall constituents knitting the wall together. The amount of extensin changes with development. Cells that have thick, hard walls are often rich in extensin (i.e., sclerids and fibers). The amount of extensin produced is dependent on mechanical wounding, infection and these responses are mediated by plant hormones.
Functions of cell wall
• It provides tensile strength and plasticity that are important for keeping cells from rupturing by turgor pressure.
• Thick walled cells provide mechanical support. It regulates the direction of growth.
• Cutinized walls prevent water loss.
• Act as physical barrier and provide protection from insects and pathogens.
• Physiological and biochemical activities
• Control plant morphogenesis
• Regulate diffusion of some substances, including proteins, to pass into the cell while keeping other substances out
• stores carbohydrates for use in plant growth
• Cell walls are important for products such as paper, wood, fiber, energy, shelter, and even roughage in our diet.