• 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 division involves synthesis of new cell wall
• Cell enlargement involves changes in cell wall composition
• Cell differentiation involves changes in cell wall composition

Middle lamella: 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 pectic compounds and protein.

Primary wall: This is formed after the middle lamella and consists of a rigid skeleton of cellulose microfibrils embedded in a gel-like matrix composed of pectic compounds, hemicellulose, and glycoproteins.

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.

Composition of cell wall

Pectic acid
- polymer of around 100 galacturonic acid molecules
- very hydrophilic and soluble - become very hydrated
- forms salts and salt bridges with Ca++ and Mg++ that are insoluble gels
- major component of middle lamella but also found in primary walls

galacturonic acid

Pectic acid with salt bridges

Because the carboxyl groups on the galacturonic acid molecules are weak acids, they can exist in negatively charged and uncharged states depending on protonation (see fig below). The extent to which the molecules are protonated is pH dependant and related to the pKa (the pH at which the two forms are in equilibrium).

Pectin

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 micro fibers can interact to form fibers. Certain cells, like those in cotton ovules, can grow cellulose fibers of enormous lengths.

Cellulose fibers usually consist of over 500,000 cellulose molecules. If a fiber consists of 500,000 cellulose molecules with 5,000 glucose resides/cellulose molecule, the fiber would contain about 2.5 billion H-bonds. Even if an H-bond is about 1/10 the strength of a covalent bond, the cumulative bonding energy of 2.5 billion of them is awesome. It is the H-bonding that is the basis of the high tensile strength of cellulose.

Hemicellulose is a polysaccharide composed of a variety of sugars including xylose, arabinose, mannose. Hemicellulose that is primarily xylose or arabinose are 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. Hemicellulose is abundant in primary walls but is also found in secondary walls.

Structural 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. Strucural 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 positive charges 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, the 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.

Cell walls also contain functional proteins. Enzymatic activities in cell walls include:

• Oxidative enzymes - peroxidases
  
• Hydrolytic enzymes - pectinases, cellulases
  
• "Expansins" - enzymes that catalyze cell wall "creep" activity

Cell expansion involves loosening of existing wall materials and production of new material. Cell wall loosening can occur by at least 3 mechanisms:

1) Wall acidification - H+ATPase in plasma membrane 'pumps" H+ from cytoplasm into cell wall. The pH of the wall drops and carboxylic acids become protonated and 'salt bridges" are broken.

In addition, the enzyme "expansin" is activated and causes cellulose micro fibers to slip (mechanism of expansin action is unknown). This results in cell wall "creep".

Hydrolytic enzymes like cellulase and pectinase, "degrade" cell walls by breaking polymers into smaller subunits or by breaking crosslinks.

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