The importance of fungi

The importance of fungi

Being the heterotrophic organisms, Fungi require organic compounds for their nutrition. They feed on the dead organic matter so they are also known as saprophytes. Saprophytes decompose remains of complex plant and animal , changing them down into simpler chemical substances that are finally returned to the soil, thus enhancing the soil's fertility which makes them quite useful to humans in farming. But they can also be harmful as they decompose timber, textiles, food, as well as other materials.

Fungus with the saprophytic mode of nutrition are also important in industrial fermentations: for example, the brewing of beer, the making of wine, and the production of antibiotics like penicillin. The leavening of the dough as well as, the ripening of some cheese also depends on fungal activity.

In the other hand, as parasites (i.e., when living in or on another organism), these organisms cause diseases in plants, humans, and other animals. Although fungal diseases are encountered less frequently than bacterial or virus diseases in humans and other animals, they play the significantly important role in causing diseases of plants.

Quite separate from the applied fields of the study of fungi, these microorganisms are to be studied in their own groups as biological entities. The science of the study of fungi is called mycology. Fungi have also become an essential tool for the physiologist, biophysicist, geneticist, and biochemist, who use them as highly suitable subjects for the study of some biological processes.

Distinguishing characteristics of fungi

Fungi are eukaryotic and chemoorganotrophic organisms possessing no chlorophyll. The body or so called the thallus of a fungus may contain a single cell as in the yeasts; more commonly the thallus consists of filaments, 5 to 10 across, which are typically branched. The yeast cell or mold filament is entirely covered by a true cell wall (with some exceptions like slime molds, which have a thallus consisting of a naked amoeboid mass of protoplasm).

Some fungi can even be dimorphic; as they exist in two forms. Some of the pathogenic fungus of humans and other animals exist in a unicellular and yeast-like form in their host, but while growing outside the host body saprophytically in soil or on a laboratory medium possess a filamentous mold form. The laboratory detection and the identification of such fungal pathogens are often dependent on the demonstration of dimorphism in their living forms. The opposite dimorphic process exists in some plant pathogens. In the case of Taormina (which causes peach leaf curl) or in smuts (which causes diseases of cereal crops), the mycelial form exists in the host whereas the unicellular yeast-like form occurs in laboratory culture.

Thus a fungal colony can be called as a mass of yeast cells (not like a bacterial colony except for surface texture), or it may be called as a filamentous mat of mold.

Morphology

Yeast cells are generally larger than the bacterial cells. Yeasts have variations in their size, ranging from 1 to 5 in width and from 5 to 30 or more in length. They are regularly egg-shaped, whereas some are elongated and some spherical. Each species is characterized by its specific shape, but even in the case of pure culture, there is considerable variation in size and shape of individual cells, that depends on age and environment. Yeasts contain no flagella or other organelles of locomotion.

The thallus of mold consists essentially is made up of two parts: the mycelium and the spores (resistant, resting, or dormant cells). The diversity of these spores will be detailed later. The mycelium is a complex structure made up of several filaments called hyphae. New hyphae emergefrom a spore which puts out a germ tube or tubes on germination . Later on, these germ tubes elongate and branch to form the new hyphae.

Each hypha measures about 5 to 10 in width, as compared with a bacterial cell which measures about 1 in diameter. Hyphae consists of an outer tube-like the wall which surrounds a cavity, called the lumen, that is filled or lined with protoplasm. plasmalemma is situated in between the protoplasm and the wall as a double-layered membrane which surrounds the protoplasm. The hyphal wall is enriched with microfibrils composed for the most part of hemicelluloses or chitin whereas true cellulose is present only in the walls of lower fungi. Wall matrix material in which the microfibrils are embedded contain proteins, lipids, and other substances.

The hypha grows in the distal position, near the tip. The major sector of elongation is situated in the region just behind the tip. Crosswalks which are formed by centripetal invagination (inward growth) from the existing cell wall could even divide the young hypha into cells. These crosswalks after constricting the plasmalemma, grow inward to form generally an incomplete septum (plural, septa) that consists of central pore which further enhances the protoplasmic streaming. Even cell to cell migration of nuclei could occur in the hypha.

Hyphae occur in three major forms:

1. Nonseptate, or coenocytic: These hyphae do not contain septa.
2. Septate with uninucleate cells.
3. Septate with multinucleate cells. Each consisting of more than one nucleus in each compartment.

Mycelia can exist in both vegetative as well as reproductive forms.In order to obtain nutrients, some hypha of vegetative mycelium penetrates into the medium. Soluble nutrients are absorbed through the walls whereas insoluble nutrients are first digested externally by secreted enzymes. Reproductive mycelia account for the spore production and usually, extend from the medium into the air. The mycelium of a mold could be loosely woven network or it may even be an organized, compact structure, as seen in thecase of mushrooms.

References

Arvind, Keshari K. and Kamal K Adhikari. A Textbook of Biology. Vidyarthi Pustak Bhander.

Michael J.Pleczar JR, Chan E.C.S. and Noel R. Krieg. Microbiology. Tata Mc GrawHill, 1993.

Powar. and Daginawala. General Microbiology.

Rangaswami and Bagyaraj D.J. Agricultural Microbiology.