The plant cell wall serves a variety of functions. Along with protecting the intracellular contents, the structure bestows rigidity to the plant, provides a porous medium for the circulation and distribution of water, minerals, and other nutrients, and houses specialized molecules that regulate growth and protect the plant from disease.
Many plant cells have both a primary cell wall, which accommodates the cell as it grows, and a secondary cell wall they develop inside the primary wall after the cell has stopped growing. The primary cell wall is thinner and more pliant than the secondary cell wall, and is sometimes retained in an unchanged or slightly modified state without the addition of the secondary wall, even after the growth process has ended.
The plant cell wall is a product of protoplasmic activity and in the higher plants its development begins with the formation of the cell plate, immediately after nuclear division.
This thin cell plate quickly acquires the form of a primary cell wall, which is defined ?as the structure which encloses the protoplasts during the period of cell enlargement'.
Once the period of cell enlargement is over the cell wall becomes thickened to become the secondary wall.
The secondary wall is regarded as the structural component of the plant (that is the plant skeleton). Such walls are also the major components of the conducting vessels.
The constituents of the cell wall are cellulose, hemi-cellulose, pectic substances, lignin and proteins. Waxes, cutin, suberin and sporopollenin are also found.
The microfibrils may be arranged randomly or in a regular fashion proposed the ?unit cell' of cellulose.
The cellulose chains lie antiparallel in such a way that alternate chains point in opposite directions. Within the microfibrils themselves are smaller units, the micelles, which are small aggregations of cellulose molecules that lie parallel to one another and thus confer a crystalline structure upon the microfibrils.
The microfibrils are necessary to bear the stress in the wall due to turgor pressure.
T he pectic substances :Hemi celluloses like
HEMI LEGNIN: It is a structural material . this gives strength to wood. Its deposition of lignin between the existing cellulose framework there is always a swelling of the cell wall during lignification.
: proteins containing hydroxyproline are in the primary walls of various tissues. The amount present increases during growth and it is thought that the protein may serve enzymatic as well as structural functions.
. In most cases the secondary walls have a higher percentage of cellulose and lignin while the pectic substances are present only in trace quantities as compared to the primary walls.
The endoplasmic reticulum, Golgi bodies and microtubules have all been implicated in the synthesis and organised deposition of the material of the plant cell wall on the basis of their association in electron micrographs with developing cell walls .