Essay on the Structure, Development and Chemical Composition of the Plant Cells

Plant cells are characteristically enclosed by cell walls which serve the mechanical functions of the plant body. It provides shape, support and rigidity.

It is also involved in the maintenance of a balance between the osmotic pressure of the intra-cellular fluid and the extra-cellular environment. It is this equilibrium osmotic pressure that maintains the turgidity of the plant parts.

The growth and differentiation of plant cells such as formation of xylem vessels and phloem tissue is the result of special synthesis and assembly of cell wall.

Structure:

The cell wall consists of a middle lamella and primary, secondary and tertiary walls.

The primary wall is the first player to be laid down. It consists of a framework of microfibrils, made up of the polysaccharide, cellulose. This thin and elastic layer contains other substances like hemicellulose, pectic substances, lipids, proteins, some minerals and water.

Between adjacent primary walls, there is a middle lamella, which is composed mainly of pectic substances with salts of calcium and magnesium.

This acts as an inter-cellular cementing substance, binding cells together. It appears soon after cytokinesis and always, remains outside the primary wall.

While the cell is enlarging, the primary wall is thin, elastic and capable of extension. Some thickening of the wall may occur during elongation. Generally, this happens after the cell has reached to a maximum size.

When the cell has reached to its mature size, usually with the development of large vacuoles, the secondary wall appears. The secondary wall may be thick or thin. It consists of material that adds to the inner surface of the primary wall either as a homologous thickening or in localized thickenings the secondary wall varies in hardness and colour.

It is these part of the cell that diferentiates as wood and plant fibers (cotton, flax, hemp). The secondary wall mainly consists of cellulose and hemicellulose with little amount of pectin.

In xylem development, a further differentiation consists of the penetration of lignin from outside into the secondary wall. This hydrophobic polymer replaces water and finally encrusts all the microfibrils and the matrix. At this stage, the wall is lignified and the cells die.

Typical cell wall structure of a mature and lignified plant cell, (a) Cross sectij showing arrangement of various layers and the complex structure of the secondary wall, longitudinal section through a similar cell, (c) Electron micrograph of the cell walls of ih adjacent cells: the darker middle area is the middle lamella; the lighter portions, the primary.

In some cases, a tertiary wall is formed in the cell wall. It is thought to be the dry residue of the protoplasm. The tertiary wall is composed mainly of cellulose, hemicellulose xylan.

Development of the cell wall:

Development of the cell wall starts with the development of middle lamella formed by cell plate after cell division. Under an electron microscope, it is observed that the inter-zol microtubules at the equatorial plane have scattered patches of vesicles and dense materials attack to their surface.

The vesicles are derived from the Golgi complexes, which are found nil regions adjacent to the cell plate or phragmoplast.

These migrate to equatorial plane toll clustered around the microtubules. Although the phragmoplast is initially found in the mid-regl of the cell, subsequently, it grows centrifugal by addition of microtubules and vesicles it in real in size until two daughter cells separate by a fairly continuous plasma membrane. At this time, Phragmoplast changes into the cell plate.

The Golgi vesicles in the phragmoplast are already filled with a secretory material, consist] mainly of pectin. The fusion of vesicles results in the combining of pectin in the extra-cellu space between the daughter cells, thereby forming the main body of the primary cell wall.

Chemical composition:

As described earlier, cellulose is the major chemical component of the cell wall. The microfibrils of cellulose associate among themselves and non-cellulose polysaccharides and proteins to form the cell wall. Cellulose is synthesized by wide variety of cells that include bacteria, algae, fungi, cryptogams and seed plants.

The major polysaccharide fractions are (1) pectin substances containing galactose, arabinose and galacturonic acid (2) hemicelluloses, composed of glucose, mannose and glucuronic acid.

Lignin is found only in mature cell walls and is made of an aromatic polymer resulting from long chains of phenolic alcohols. Some cell walls may have cuticular substances called cutin, waxes and mineral deposits in the form of calcium and magnesium carbonates and silicates. In most fungi and yeast, the cell wall is composed of chitin. This is a polymer of glucosamine.