A layer of structural materials composed of polysaccharides, glycoprotein, and phenolic compounds found external to protoplasm is called the cell wall. It defines the shape and size of the cell.
In this article, the structure and composition of cell wall will be discussed explicitly.
Gross Structure of cell wall
The cell wall is complex and usually consists of 3 layers-
- Primary wall
- Middle lamella
- Secondary wall
These three walls are described below:
The primary wall is about 0.1 μm thick normally but the wall of collenchyma may be 10μm thick. the cuticular epidermal cell may be thickened. The thick cellulosic primary wall may be observed in the endodermis. They are plastic and reversible in nature. The compounds of the primary wall are deposited on both sides of the middle lamella. At maturity, the middle lamella and the two primary walls of adjacent cells become confluent and become too difficult to define a practice boundary between them. Example- They are found in the meristematic cell, parenchyma cell, collenchyma, and root hairs.
The layer that lies in between the 2 primary walls of adjacent cells is called middle lamella. It is the connecting material that fastens cells to the neighbor. Therefore, they are held together by the primary walls. They are very thin (less than 30nm thick) and thickest at the corners. Pectic substance, possibly in association with calcium and others compose the middle lamella. It originates at the end of the nuclear division and creates a new boundary between the two nuclei which had previously shared the same cytoplasm.
The secondary wall is one whose polysaccharide components are deposited over the primary wall. In contrast to the primary wall, the secondary wall is formed after the cessation of surface growth. Therefore, it causes growth in thickness only. Secondary wall materials are deposited outside the plasma membrane, thus is present internal to the primary wall surrounding the cell lumen. In addition to cellulose, hemicellulose, and other polysaccharides, the secondary wall contains lignin. Example- They are found in those cells which are devoid of protoplasm.
Therefore, cells of the secondary wall consist of 5 layers–
- A 3 layered secondary wall (from inner S1, S2, and S3)
- A layered primary wall
- A layered middle lamella
Ultrastructure of cell wall
The cell wall is a biphasic structure consisting of cellulose microfibril embedded in a gel-like cellulose matrix. The microfibrillar phase consists of cellulose (β-1,4-glucan) only. The main component of the cell wall is cellulose and ultrastructure is based on it.
Cellulose molecules unite with each other to form a bundle. They arrange in a parallel way to form a strand-like structure called micelle or elementary fibril (3.5nm in diameter). Primary microfibril or elementary strand unite and form a microfibril bundle
They are radially visible in an electron microscope and are crystalline. This phase is composed of microfibril, which is a long, thin structure with oval and circular in cross-section and has uniform width (10nm in higher plants). At some regions, microfibrils are arranged in a definite 3-dimensional crystalline lallicis termed micelles.
They are about 0.4μm wide and can be visible under an electron microscope. It is suggested to be composed of 500000 cellulose molecules in transition. Several macro fibrils are combined to form a cell wall.
Chemical composition of cell wall
It is associated with various substances, most often with other compound carbohydrates. They are as follows-
- Cellulose: The most common unit of the cell wall, which is a hydrophilic, crystalline compound.
- Non-cellulosic polysaccharides: e.g. Mannans, galactans, xylans, arabans, etc.
- Pectic substances: Amorphous colloidal substances, plastic, and highly hydrophilic. Helps in the binding of cells.
- Gums and mucilages: They exude as a result of physiological disturbances. Has the property of swelling.
- Lignin: Organic compound with large carbohydrate constituents.
- Cutin suberin and waxes: Fatty substances in the cell wall. Cutin and suberin are not melted but waxes are. They protect plants from the leaching effects of rain and penetration by potential parasites.
- Mineral substances: Silica, calcium carbonate, and some organic substances like tannins resins, etc. may be impregnated in the cell wall.
- Class Lecture of Parveen Rashid, PhD
Professor, Department of Botany, University of Dhaka.
- Plant Anatomy- B.P.Panday (page-71)
- Saifun Nahar Smriti on 16 February, 2021.