1.4 Cell wall composition
Do you
remember that although fungi have certain affinities with animal cells, why are placed in the plant kingdom? Simply we may say that because they have
much more similar characteristics to plants than animals. Possessing a cell wall
is one of the significant characteristics of fungi to be placed under the plant kingdom.
Fungal cell wall structure and its position are very specific and
important. Each cell has a cell wall
outer to the cell membrane and it protects the cell and provides rigidity. The
common cell wall constituents in each division of fungi are given in Table
1. Now, let us see the cell wall composition of fungi-
a. We mentioned that the fungal cell wall is composed of chitin which is a fibrous substance consisting of polysaccharides. It is composed of N-acetyl glucosamine instead of glucose as in cellulose, and the glucans are synthesized at the plasma membrane. Cell wall glycoproteins are synthesized in the endoplasmic reticulum and carried through the Golgi to the plasma membrane where vesicles release the glycoproteins to the cell wall,
b. Basically fungal cell walls contain fibrillar materials bound together by sugars, proteins, lipids, and a variety of polysaccharides,
c. While the fibrillar component of the wall is largely inert, the composition of the wall changes with time and the growth of the organism,
d. The fibrillar structure of the cell wall in most fungi is built on chitin, chitosan, ᵦ-glucans, and a variety of heteropolysaccharides. The fibers are contained in a complex gel-like matrix,
e. Protein constitutes only 20% of the cell wall materials and often as glycoproteins. Not all proteins found in the cell wall have a structural role. Proteins are involved in mating recognition, wall modifications, communication, etc.,
f. Lipids are found in the cell walls usually very in very small concentration. They appear to regulate the movement of water, especially in the prevention of the desiccation of cells,
g. Fungal cell walls also contain pigments and salts. Melanin is the most dominant pigment found in the walls. It is important in protecting the hyphae, and spores, and helps in pathogenesis, attachment to surfaces, and also in the stabilization of organic carbon in the soil.
Table 1: Cell wall constituents of different divisions
of fungi
(From Gooday in Gow and Gadd, 1995).
|
Division |
Fibrous components |
Gel-like polymers |
|
Ascomycota |
Chitin ᵦ(1-3), ᵦ(1-6) Glucan |
Galactomannoproteins α(1-3) Glucan |
|
Basidiomycota |
Chitin ᵦ(1-3), ᵦ(1-6) Glucan |
Xylomannoproteins α(1-3) Glucan |
|
Chytridiomycota |
Chitin Glucan |
Glucan |
|
Zygomycota |
Chitin Chitosan |
Polygluchoronic acid Glucurosomannoproteins Polyphosphate |
Check your
progress
a.
What
are the similarities in the cell wall composition of fungi with animal cells?
b.
What
are the similarities of fungal cell walls with other plant cells?
c.
What are the functions of proteins present in the
fungal cell wall?
d.
What
are the functions of lipids present in the fungal cell wall?
e. How chitin is different than cellulose?