In this article we will discuss about the cell wall and its structure.
In plants a cell is always bounded by a wall known as cell wall. It is primarily composed of cellulose. There are also hemicellulose, pectin, lignin, cutin and mineral deposits involved in building up of the cell wall at different stages of development. The cell wall of a young cell has a thickness of 1-3 microns. During the cell growth the wall increases both in length and girth.
The primary wall stops growing after some time and then a secondary wall is laid inside it. The thickness of the secondary wall ranges from 5-10 microns and unlike the primary flexible wall, it is rigid. Both the walls are secretions productions of the cell. Occasionally a tertiary wall may also be found present.
In multicellular organism adjacent cells are cemented together by an intercellular substance or a middle lamella which is made up of calcium and magnesium pectate (In real sense middle lamella is nothing but itself a primary cell wall).
Animal cells lack a cell wall. Some protozoa and metazoa have a thin cell wall which is referred as to Pellicle. The outermost layer of animal cell is plasma membrane (plasma lemma) or cell membrane.
Structure of Cell Wall:
Electron microscopic studies have revealed a wealth of knowledge on the cell wall structure which is as follows:
(a) Middle Lamella:
The middle lamella is formed between adjacent cell walls during division. In other words the primary walls of different cells are joined by middle lamella. It consists of comparatively thin layer of intercellular material. It is a viscous and jelly like substance and acts as a cementing material between the primary cell walls of adjacent cells.
It is composed mainly of pectin, calcium, magnesium and polymers of various type. Pectin is a hydrophilic colloidal substance and is really hydrolysed by the enzyme pectinase as well as by strong acids.
Adhesion or attachment of the different walls of the cells is basically dependent on the presence of Ca++ and Mg++ ions in the middle lamella. A removal of these ions result in the separation of cells from each other. Cellulose is absent from middle lamella.
Middle lamella is the first to develop following cell division. During telophase three types of membraneous structures deposit at the equatorial plate of cell. First one is spherical vesicles of 20 µm diameter, second, small cisterns of endoplasmic reticulum origin and last third one phragmosomes of about 250 µm diameter.
These membraneous structures are well arranged in a layer. The vesicles and cisterns fuse with each other and form cell plate. The 20 µm vesicles are filled with pectin which is accumulated in between the two newly formed membranes. This pectin ultimately gives rise to middle lamella.
Phragmosomes relatively contain more dense substance which is probably a polysaccharide. It is used in the formation of plasma membrane (= plasma lemma or ectoplast). Once the cell plate is formed, phragmosomes disappear. The source of production of vesicles are Golgi bodies.
(b) Primary Cell Wall:
It is found in all plant cells and is formed during the early stage of growth and development. It is composed chiefly of hemicellulose (53%), cellulose (30%), pectin (5%), and lipid (7%).
The primary cell wall is elastic and undergoes extension with the growth of the cell. In many roots, fleshy stems, fruits and leaves the cells contain only the primary cell wall and middle lamella. Cellulose molecules are nothing but long chains of about 3000 glucose units.
In the cell wall these molecules are arranged parallel to one another to form micro fibrils. Each micro fibril contains about 2000 cellulose molecules and has a diameter of 100-250 angstrom (Å) and a length of several microns. Cellulose micro fibrils are the units of cell wall structural organisation. A layer of pectin is also seen in between two layers of micro fibrils.
According to Setterfield and Barley (1961), cell wall is made up of following constituents:
Water- 60%, Cellulose- 10-15%, Hemicellulose- 5-15%, Pectin- 2-8% , Protein- 1-2%, Lipids- 0.5%—3%.
(c) Secondary Cell Wall:
Secondary cell wall is laid down inside the primary wall when it has finished its growth. It is the innermost layer of the wall. It is well composed mainly of cellulose but in many tissues it contains suberin, lignin and in some cases cutin.
In secondary walls, cellulose micro fibrils are more closely packed and they are arranged more or less parallel to each other. Several micro fibrils unite to form a macro fibril, a structural unit of secondary cell walls. It is 5-10 µm thick.
After the formation of secondary cell wall the protoplasm disappears in many cell types. The secondary cell wall commonly has three layers. These are known as outer layer, the middle layer and the inner layer. The succeeding layers are laid down at different angles to one another.
(d) Tertiary Cell 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 chemical substances xylan instead of cellulose.
Cell wall gives a:
(i) Definite shape and
(ii) Mechanical strength to the cells.
It is permeable and hydrophilic (water-loving) and contains enzymes which are concerned with the synthesis, transport and degradation of cell wall, macromolecules, digestion and modification of extra cellular metabolites and other metabolic functions.