Cell Wall

 

The plant cells possess a cell wall, and this feature distinguishes them from the animal cells. The cell wall constitutes a kind of exoskeleton that provides protection and mechanical support for the plant cell. The cell wall is non-living structure which is formed by the living protoplast. Cell wall gives shape, rigidity, mechanical support and protection to the cell. It is permeable to all substances. It was first discovered by Hanstein.

Cell wall formation takes place during cell division at the time of cytokinesis. A barrel shaped body called the phragmoplast appears in the region of equatorial plane of the dividing mother cell. The microtubules of mitotic spindle pass through the phragmoplast. The small vesicles coming from the Endoplasmic reticulum migrate to the equatorial plane and fuse with one another to form a discontinuous membrane called cell plate.

The cell plate enlarges and soon reaches the side walls of the dividing mother cell. The cell plate gradually undergoes changes to form the middle lamella.

Growth of cell wall occurs by two methods such as intussusception and apposition. In intussusceptions, new cell wall materials are introduced into the spaces present within the existing cell wall. As a result, area of the cell wall increases. In apposition, new cell wall materials are deposited on the surface of the existing cell wall in the form of thin layers. It results in increase in the thickness of the cell wall.

Structure

The cell wall shows differentiated into three layers –

Middle lamellae:

The jelly like intercellular substance which acts like a cementing material between the two adjoining cell is called the middle lamellae. It is composed of calcium and magnesium pectates. Pectic substances are the mixture of polygalacturon (D-galacturonic acid) and polysaccharides. The cell plate formed during cell division develops into middle lamellae. Golgi vesicles and Golgi vacuoles play an important role in the formation of middle lamellae.

The characteristic softening of fruits during ripening is mainly due to the increase in the solubility of middle lamellae by the action of pectolytic enzymes.

Primary Cell Wall:

It is formed during the early stages of growth and development. It is found in all plant cells, is 1 to 3 µm thick. It is formed as a deposition of cellulose, hemicellulose, pectin and other polysaccharides on either side of the middle lamellae.

The primary wall of a epidermal cells of leaf and stem also possess cutin. The primary wall of fungi is composed of chitin.

The primary cell wall is thin and elastic and allows the increase of cell volume. As cell matures it becomes inelastic and rigid.

 Additional layers are deposited on the inner surface of the primary wall when the cell stops growth and begin to differentiate. This thickened later formed wall layer is called the secondary cell wall. It is present in between the primary wall and plasma membrane. It is about 5 to 10 µm thick.

The secondary cell wall is made up of  cellulose, hemicelluloses, lignin, suberin and pectin. Secondary cell wall is hard, tough and inelastic due to the presence of wall materials like lignin. It gives rigidity .

The secondary cell wall has 3 layers outer (SW1) , middle (SW2) and inner (SW3). Of these, the middle layer is usually the thickest.

Tertiary Wall:

In some tissues a tertiary cell wall is formed on the inner surface of the secondary cell wall. This layer is very thin and is found in the xylem tracheids of Gymnosperms. It is mainly composed of xylan instead of cellulose.

Ultrastructure of Cell Wall:

The cell wall under the electron microscope shows two parts i) a gelatinous amorphous matrix and the ii) cellulosic paste consisting of polysaccharides.

The cellulose in the cell wall occurs in the form of fine strands called the macrofibrils. The macrofibrils are 0.5µm in width and about 1 µm in length and are visible under the light microscope.

Each macrofibril consists of bundle of about 250 microfibrils, each of which are about 25nm or 250 A^o thick. The microfibrils are visible under the electron microscope.

The microfibril in turn consists of a bundle of micelles or elementary fibrils. Each micelle is about 100 A^o in diameter. Each micelle is made up of 100 parallely  arranged  cellulose chains. Each cellulose chain is a polymer of D-Glucose units linked by β-1,4 glycosidic bonds.

In primary cell wall the macrofibrils are arranged at radom. In secondary cell wall they are closely packed and are arranged parallel to one another.

Pits

The unthickened areas in the cell  wall are called pits.

Pits are formed in the cell wall during the deposition of the wall material. While primary wall material is depositing over the middle lamellae, some areas are left unthickened. These are called as primary pit fields.

Very thin protoplasmic connections occur between the adjoining cells through the primary pit fields. These protoplasmic connections are known as plasmodesmata. Plasmodesmata are relatively more visible in the endosperm of seeds (Phoenix, Diospyros) and in cotyledons of some plants.

The secondary cell wall has unthickened areas which are very much distinct, these are called as pits. Cytoplasmic continuity between adjacent cells is maintained through pits. To facilitate this, each pit has a complimentary pit exactly opposite to it in the wall of the neighbouring cells. These two pits of adjacent cell walls are called pit pair.

Adjacent pits are separated by the middle lamellae and the primary cell wall, which together from the pit membrane.

The opening of the pit through which pit opens into the cell lumen is called pit aperture. The cavity of pit is called pit cavity.

There are two types of pits – simple pits and bordered pits.

In simple pits the pit cavity is uniform and pit membrane is not swollen. In bordered pits, the secondary wall develops over the pit cavity to form an over arching roof with a narrow pore in its centre.

If both the pits of a pair are simple, it is known as simple pit pair; and if they are bordered, it is called bordered pit pair. But sometimes, one of the pit of a pair may be simple and the other bordered. Then it is termed as half bordered pit pair. A pit without any complementary pit on the opposite side is known as blind pit.

Bordered pits are abundantly found in the tracheary elements of pteridophytes and gymnosperms and also in the xylem elements of angiosperms.

In gymnosperms, the pit membrane of a bordered pit pair sometimes get thickened in its central position. This thickened part is disc shaped and is known as torus. Torus is formed by circular deposition of microfibrils. The part of the membrane surrounding the torus is called margo.

Functions of the cell wall:

1. it provides a definite shape to the cell.

2. Cell wall acts as a exoskeleton for the plant.

3. Orientation of cellulose microfibrils limits and controls cell growth and shape.

4. The cell wall acts as a physical barrier protecting plant cells from invading pathogens like viruses, fungal spores, or bacteria. Thus they serve as a defense mechanism.