The following points highlight the top three types of cell wall. The types are: 1. Prokaryotic Cell Wall 2. Plant Cell Wall 3. Animal Cell Glycocalyx.
Type # 1. Prokaryotic Cell Wall:
Some bacteria contain a capsule that surrounds their cell wall, and protects them from phagocytosis. The capsule layer is loose, gel like structure and can be observed by negative staining. It is composed of polysaccharides.
Certain bacterial species have both encapsulated and non-capsulated strains, e.g., Streptococcus pneumoniae. In general, the encapsulated strains are pathogenic, while those without capsule are non-pathogenic.
b. Cell Wall:
Certain bacteria like Mycoplasmas are not enclosed by a cell wall, but most bacteria contain cell wall.
Cell walls of different bacterial species may show different staining reactions (e.g., with crystal violet) ; on this basis, bacteria are classified as
(1) Gram-positive and
The cell wall is many times thicker in gram-positive bacteria (3-100 nm thickness) than in gram-negative bacteria (3-8 nm) (Fig. 2.3).
The bacterial cell wall consists of a polysaccharide complex called peptidoglycan. The gram-negative bacteria contain an ‘outer membrane’ surrounding the cell wall. It protects the bacteria from antibiotics like penicillin.
The outer membrane contains ‘lipopolysaccharides’ which are made up of polysaccharides and fatty acids, and constitute half of the mass of the outer membrane. The polysaccharides are hydrophilic while the fatty acids are hydrophobic in nature.
c. Periplasmic Space:
Between the cell wall and the plasma membrane, there lies a space called ‘periplasmic space’ (Fig. 2.3). The periplasmic space contains hydrolytic enzymes, such as, ribonucleases, deoxyribonucleases, alkaline phosphatases and phosphodiesterase. Nutrient macromolecules taken in from the environment are degraded by these hydrolytic enzymes.
Type # 2. Plant Cell Wall:
Eukaryotic plant cells are enclosed by a cell wall external to the plasma membrane. The cell wall provides rigidity, mechanical support to tissues and organs and gives a definite shape to the cell. Meristematic cells are thin-walled. Cell differentiation is accompanied with cell enlargement and cell wall thickening. Deposition of different substances on the inner surface causes thickening of the wall.
The cell wall of differentiated cells may have 3 to 4 component parts:
(a) Middle lamella,
(b) Primary wall,
(c) Secondary wall and
(d) Tertiary wall.
a. Middle Lamella:
Plant cells contain middle lamella which lies between the cell walls of two neighbouring cells and holds the cells together. It is made of pectins which are polymers of poly-galacturonic acid; it also contains Ca2+ and Mg2+ ions. The formation of middle lamella occurs as follows.
After cell division, small cisternae of ER (endoplasmic reticulum) origin, spherical vesicles containing pectin and phragmosomes accumulate at the equatorial plate. The cell plate is formed by the fusion of ER cisternae, whose membranes give rise to the plasma lemma of the two daughter cells. The vesicles filled with pectin are deposited between the two plasma lemma so formed, these give rise to the middle lamella.
Cell walls are made of cellulose and are multilayered. They are called primary, secondary and tertiary cell walls (Fig. 2.4). The proportion of cellulose varies in the walls of the different cell types. The cotton fibre cell walls contain 100% cellulose, while in woody plants, the cell walls have only 50% cellulose. Young seedlings usually have only 1% cellulose in their cell walls.
b. Primary Cell Wall:
The formation of primary wall occurs after the formation of middle lamella. It is composed of a loosely textured net-work of cellulose fibrils (Fig. 2.5). Several thousand β-glucose units are linked to form a long cellulose molecule (molecular weight, MW > 500,000). Several individual cellulose molecules are organised parallel to each other to form ‘micro-fibrils’ of 10-25 nm diameter.
These micro-fibrils are the units of the structure of the cell wall. Primary wall also contains glycoprotein molecules, lipids, pectin and hemicellulose. Hemicelluloses are formed by polymerization of 5- and 6-carbon sugars, e.g., galactose, arabinose and xylose etc.
c. Secondary Cell Wall:
Secondary wall is deposited on the inner surface of the primary wall; it occurs after the completion of the growth (by expansion) of a cell. The secondary wall may be 3-4 layered (Fig. 2.4). It is made up of cellulose ‘micro-fibrils’ which are densely packed in bundles and have parallel arrangement (Fig. 2.5).
Each layer of the secondary wall is organized in such a way that its micro-fibrils make an angle with those of the adjacent layers. In many tissues, the secondary wall also contains suberin and lignin. Lignin is made from polymerization of certain aromatic alcohols ; it is found in the secondary walls of woody plants. Mechanical strength is provided by the secondary wall.
d. Tertiary Wall:
It is the innermost layer of the cell wall (Fig. 2.4) and is relatively thin; it is not found in all the cells. Typically, it does not contain any cellulose micro-fibrils.
They are small openings through which the cytoplasm of adjacent cells come in direct contact with each other. There is no deposition of the cell wall in the vicinity of plasmodesmata (Fig. 2.4).
Type # 3. Animal Cell Glycocalyx:
Animal cells are not enclosed by a cell wall. Instead they contain polysaccharide layers called glycocalyx on their surface. The functions of glycocalyces are adhesion, protection, permeability and cell recognition. Animal cells secrete some substances which are deposited in the intercellular spaces.
These molecules form an extracellular matrix which is found mainly in connective tissues. The matrix contains a mixture of collagens, proteoglycans and non-collagen glycoproteins.