Introduction:

Fungi are the eukaryotic spore bearing organisms which lack chlorophyll and usually reproduce both asexually and sexually. They are of great practical and scientific significance. One of the reasons for this is that most of the fungi are of microscopic cellular dimensions. Fungi encompass a diversity of morphological appearances based on the species. Fungi include the mushrooms, molds and yeasts. Molds are multicellular and filamentous whereas yeasts are unicellular. They are broadly distributed and found wherever moisture is present. They are of great significance to man in both valuable and hurtful ways.

Definition of Fungi:

Fungi are the eukaryotic spore bearing organisms which lack chlorophyll and usually reproduce both asexually and sexually.

Distinguishing features of Fungi:

They are big, diverse and wide-spread group of organisms, the molds, yeasts and mushrooms.

1) Fungi belong to Eukaryotic. They are members of the domain Eucarya. 

2) They have a membrane-enclosed nucleus and some other organelles. 

3) They have no chlorophyll.

4) The body of the fungi is termed as thallus. 

5) They are chemo organotrophic organisms. 

6) The thallus might comprise of a single cell as found in yeasts. 

7) The thallus might comprise of filaments, 5 to 10µm across which are generally branched as found in molds. 

8) The yeast cell or mold filament is bounded by a true cell wall (exemption is the slime mould which encompass a thallus comprising of a naked amoeboid mass of protoplasm).

9) Some of the fungi are dimorphic, that is they exist in two forms. A few pathogenic fungi of humans and other animals encompass a unicellular and yeast like form in their host however when growing saprobically in soil or on a laboratory medium they encompass a filamentous mold form.

10) Habitat allocation of fungi is diverse. Some of them are aquatic, living mainly in fresh water and some marine fungi are terrestrial. They live in soil and dead plant.

Structure and Forms of Fungi:

The body or vegetative structure of the fungus is termed as thallus. It differs in complexity and size ranging from the single cell microscopic yeasts to the multicellular moulds and mushrooms. The fungal cell is generally enclosed in a cell-wall of chitin.

Yeasts:

a) They are unicellular fungi which encompass a single nucleus.

b) They are generally egg-shaped however some are elongated and some spherical. Yeasts comprise no flagella or other organelles of locomotion.

c) They have most of the other eukaryotic organelles. 

d) Yeast cells are bigger than most of the bacteria. Yeasts differ considerably in size ranging from 1 to 5µm in width and from 5 to 30µm or more in length.

e) They reproduce asexually through budding and traverse division or sexually via spore formation.

Molds:

The thallus of a mold comprises of long branched threadlike filaments of cells termed as hyphae. Such hyphae form a mycelium that is a tangled mass or tissue like aggregation of hyphae.

Hyphae:

a) Each hypha is around 5 to 10µm wide. Hyphae comprised of an outer tube like wall surrounding a cavity the Lumen that is filled or lined by protoplasm. Among the protoplasm and the wall is the plasmalemma, a double layer membrane that surrounds the protoplasm.

b) The hyphal wall comprises of microfibrils comprised of hemicelluloses or chitin. True cellulose takes place only in the walls of the lower fungi.

c) Wall matrix material in which the microfibrils are embedded comprises of lipids, proteins and other substances. Growth of a hypha is distal close to the tip. 

The Mycelium:                                                     

a) The mycelium is a complex of some filaments termed as hyphae (singular, hypha). New hyphae usually occur from a germinated spore. The germinated spore puts out a germ tube or tubes that get longer to form hyphae.

These hyphae make a tangled mass or tissue similar to aggregation.

In some of the fungi, protoplasm streams via hyphae uninterrupted by cross walls, such hyphae are termed as aseptate or coenocytic.

The hyphae of others contain cross walls termed as septa (s. septum) having either single pore or multiple pores which lets cytoplasmic streaming. Such hyphae are known as septate.

Nutrition and Metabolism of Fungi:

Mostly, the fungi are saprobes, securing their nutrients from the dead organic matters. They discharge hydrolytic exo-enzymes which digest external substrates and absorb the soluble products.

They are as well chemoorganoheterotrophs, that is, they utilize organic compounds as a source of carbon, electrons and energy.

Fungi are generally aerobic; though, some yeast is facultatively anaerobic and can get their energy through fermentation. Necessarily anaerobic fungi are found in the rumen of cattle.

Reproduction in Fungi:

Reproduction in fungi can either be sexual or asexual.

Asexual Reproduction:

Asexual reproduction is a kind of reproduction comprising just one parent which generates genetically similar offspring by budding or by the division of a single cell or the whole organism into two or more parts.

Asexual reproduction, as well termed as somatic or vegetative reproduction is achieved in many ways and doesn't comprise the fusion/union of nuclei, sex cells or sex organs. It might be achieved by:

a) Fission of the somatic cells yielding two similar daughter cells.

b) Budding each and every bud a small outgrowth of the parent cell builds up into a new individual

c) Fragmentation or disjointing of the hyphal cells, each and every fragment becoming a new organism

d) Spore formation.

Sexual Reproduction:

Sexual reproduction is a kind of reproduction in which two parents give rise to offspring which contain unique combinations of genes inherited from the gametes of the two parents.

It is taken out by fusion of the compatible nuclei of two parent cells. The procedure of sexual reproduction starts with the joining of two cells and fusion of their protoplast (that is, plasmogamy) therefore enabling the two haploid nuclei of two mating kinds to fuse altogether (that is, karyogamy) to form a diploid nucleus. This is followed by meiosis, which again decreases the number of chromosomes to the haploid number.

The sex organelles of fungi if present are termed as gametangia. They might form differentiated sex cells termed as gametes or might have rather one or more gamete nuclei. When the male and female gametangia are morphologically dissimilar, the male gametangium is termed as the antheridium (plural antheridia) and the female gamentangium is termed as the Oogonium (Oogonia).

Process of sexual reproduction comprises: 

a) Gametic copulation: This is the fusion of naked gametes, one or both of which are motile.

b) Gamete-gametangial copulation: The two gametangia came into contact however do not fuse; the male nucleus migrate via a pore or fertilization to be into the female gamentangium.

c) Gametangial copulation:  Two gamentangia or their protoplast fuse and give mount to a zygote which builds up into a resting spore.

d) Somatic copulation: Fusion of the somatic or vegetative cells.

e) Spermatization: The union of a special male structure termed as a spermatium (plural spermatia) having a female receptive structure. The spermatum unfilled its content into the female in plasmogamy.

Physiology of Fungi:

1) Fungi are better capable to survive in certain extreme environments than other microorganisms. They can stand more acidic conditions than the other microbes. A few kinds of yeasts are facultative; that is, they can grow under both aerobic and anaerobic conditions.

2) Fungi grow over a broad range of temperature. The optimum temperature for most saprobic species is around 22 to 300C, whereas pathogenic fungi encompass a higher temperature optimum of around 30 to 370C.

3) Some of the fungi will grow at or near 00C and therefore can cause spoilage of meat and/or vegetables in the cold storage.

Importance of Fungi:

1) Around 90,000 fungal species have been explained according to literature. Though, some estimates propose that 1.5 million species might exist. Fungi are significant to humans in both beneficial and harmful manners.

2) Usefully, fungi act as decomposers. They degrade complex organic materials in the atmosphere and discharge simple organic and inorganic molecules such as nitrogen, carbon, phosphorus required by other living organisms.

3) Moulds and yeasts are employed in most of the industrial processes comprising fermentation to produce beer, cheese soy-sauce, wine and bread, organic acids and lots of antibiotics.

4) They are vital research tools in the study of basic processes like genetics, cytology, biochemistry and microbiology.

5) On the other hand, fungi cause lots of diseases of animals, plants and humans. Around 20 new human fungal pathogens are documented every year.

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