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Introduction to fungi: Yeast and mould

Definition:

       Those microorganisms that are invariably nucleated, spore-bearing and do not possess chlorophyll, generally reproduce both asexually and sexually, and have somatic structural features that are essentially surrounded by cell walls comprising of polysaccharides, cellulose and/or chitin, mannan, and glucan.

General Introduction to fungi

The kingdom of organisms that includes yeast, molds, and mushrooms, is known  as fungi. Fungi can grow as single cells like yeast or as  multicellular filamentous colonies like molds and mushroom. Chlorophyll is absent in the fungi. That's why fungi is found to be saprophytic  or parasitic. Most of the fungi are not pathogenic in nature. Some fungi may cause the disease which are generally belongs to a group known as fungi imperfecti. Fungi are considered to be mostly saprophytic, making use of dead organic matter as a source of energy, vital natural organic decomposers, and destroyers of food stuffs. Certain fungi are closely associated with the manufacture of bread, beer, production of edible varieties of cheese, vitamins, and organic acids; and several ‘antibiotics’.  Geneticists and Biochemists exploit the fungi profusely by virtue of their extraordinarily unique reproductive cycles, but having a rather relatively simple metabolism.

Distribution:

Fungi are terrestrial organisms. Some fungi are also found in freshwater or marine. Some are causing diseases in plants and animals. Fungi also form symbiotic relation with other organisms like  mycorrhizae, lichens.

Importance

Fungi are harmful as well as beneficial to mankind.  Fungi primarily act as decomposers which is very important in mineral recycling. Fungi can degrade complex organic materials in the environment to simple organic compounds and inorganic molecules.  Such mineral recycling of carbon, nitrogen, phosphorus, and other important minerals from dead organisms are released and made available for other living organisms. Fungi are the major cause of plant diseases. They attack the important cash crops. Similarly fungi is also responsible for the many animal diseases.

Fungi also play major role in many industrial processes like fermentation. It is used in manufacturing of bread, wine, and beer. It has a role in the preparation of some cheeses, soy sauce, and in the production of many acids like citric acid , gallic acid. It is also used for the synthesis of some drugs like ergometrine, cortisone. First antibiotic prepared was penicllin that was a product of fungi. Similarly many other antibiotics like griseofulvin and the immunosuppressive drug cyclosporine are prepared by the fungi. It  is also very useful as a research tool. It is used in the study of many basic life processes. Life science scientist regularly use fungi in their research.

Structure

The body of a fungi is known as a thallus. Single celled fungi to multicellular structure of fungi are observed in nature. Macroscopic mushroom structures of fungi are also  observed in nature. Fungal cell is covered by cell wall. Cell wall of fungi is made up of chitin. Chitin is a polysaccharide made up of  N-acetyl glucosamine.

 

Structure of Yeast: A single celled fungi

A yeast is a unicellular fungi. Fungi can reproduces by sexual and asexual reproduction.  Asexually fungi reproduces by budding and transverse division. Sexually fungi divides through spore formation. Yeast cells are spherical or egg shaped. Their size is more than bacteria. They do not have flagella . Membrane bound organelles are present in the yeast cell.

 

Yeasts are characterized by a wide distribution in natural habitats. They are commonly found on the parts of plants like leaves , flowers. They are also found commonly in soil  and salt water.  They are also found on skin and alimentary canal of the animals. Common disease of yeast is Candidiasis. It  is

caused by the Candida albicans.

Structure of fungal mycelia

Fungal mold:  It is made up of  long, branched, threadlike filaments called hyphae which forms a mycelium structure. In some fungi hyphae is coencocytic means wall i.e. septa is absence in between the cell. Some fungi have cross walls called septa with either pore which facilitates cytoplasmic streaming. Fungal  hyphae with septa are known as septate. Hyphae contains outer cell wall and an inner lumen, which contains the cytosol and organelles. A plasma membrane surrounds the cytoplasm and lies next to the cell wall.

Dimorphic fungi: Some fungi exhibits dimorphic life. that is yeast form in the host and mycelial form in the environment. Such shift is due to the environmental factor. Such shift is called YM shift. In plants mycelial form exist on the plant while single celled form exist in outer environment.

Nutrition and metabolism

Generally fungi grows better in the dark condition and in moist environment. They grow on the environment where organic material found. They are saprophytes, obtain their food from dead  organic material. Fungi releases the extracellular hydrolytic enzyme which can digest the organic food material. After digestion they absorb the digested soluble substances. Fungi are chemoorganoheterotrophs. Fungi stores the glycogen as a storage polysaccharide. Fungi use carbohydrates and nitrogenous compounds to synthesize amino acids and proteins. They are  usually are aerobic. Yeast can carry out the fermentation. Obligate anaerobic fungi are observved in the rumen.

Fungi reproduces by sexual and asexual mode of reproduction.

Asexual Reproduction: The most common procedure of asexual reproduction is usually accomplished by the help of spores. Spores are generally found to be colourless , while a few of them are duly pigmented as green, yellow, red, orange, black or brown. In fact, their size may invariably range from small to large and their shape from round, oval, oblong, needle-shaped to helical. Variation in the spores of the fungi is used for the identification of fungi. Asexual reproduction may be borne particularly in a sac-like structure termed as sporangium ; and the spores being referred to as sporangiospores being called as conidia.

The simplest form of available fungal spore is known as the zoospore, which possess no rigid cell wall, and is duly propelled by flagella. Flagellum is usually found to be much more complex than that observed in bacteria.  Flagellum is made up of 11 parallel fibrils, of which 9 forming a cylinder and 2 placed centrally. Flagellum structure (9 + 2 fibrils) is usually found to be fairly consistent with that shown for other flagellated organisms. Sporangium designates the asexual reproductive structure pertaining to these aquatic fungi. In its early stages it is found to be loaded with nuclei and protoplasm. Cleavage takes place subsequently whereby the numerous sections invariably get developed into the corresponding uninucleate zoospores. Finally, following a motile phase, the resulting zoospore encysts, losing its flagellum, and rests quietly just prior to germination.

Sexual Reproduction: Importantly, the sexual reproduction is characterized by the strategical union of two compatible nuclei ; and the entire phenomenon may be distinctly divided into three phases, namely : Phase I : The union of the gametangia (i.e., sex-organs) brings the nuclei into close proximity within the same protoplast. It is also referred to as plasmogamy. Phase II : It is known as karyogamy, which takes place with the fusion of two nuclei. It has been duly observed that in the lower fungi the said two processes may take place in immediate sequence ; whereas, in the higher fungi they do occur at two altogether different time periods in the course of their life-cycle. Phase III : It is known as meiosis that essentially takes care of the nuclear fusion whereby the actual number of the chromosomes is distinctly and significantly reduced to its original haploid state.

Industrial Importance of fungi

There are several vital and important industrial importance of fungi, which shall be enumerated

briefly as under :

Production of Wines and Beer

Natural yeasts have been employed over the centuries in Italy and France, to ferment fruit juices (wines) or cereal products viz., malt (silent alcohol) in the commercial production of various types of world-class whiskies, rums, vodkas, brandies, gins, and the like. The high-tech industrial manufacturers of today largely make use of the critical and effective pasteurization of the yeast Saccharomyces cerevisiae. In the production of wine and beer, the lower temperature favours the fermentation of yeast. Thus, the fermentation invariably takes place under the anaerobic conditions thereby giving rise to the production of alcohol (i.e., ethanol).

Examples : Following are certain typical examples of alcohols commonly used in the manufacture

of ‘alcoholic beverages’, such as : Silent Spirits — Spirits obtained by the fractional distilation of alcohol produced by fruit or cereal fermentation. Brandy — obtained from wine. Whisky — obtained from malted cereals (Barley). Rum — obtained from fermented molasses (i.e., a by product from sugar-industry containing unrecoverable sugar upto 8–10%).

Production of Bakery Products

The baker, strain of Saccharomyces cerevisiae are meticulously selected for their specific high production of CO2 under the aerobic parameters. In actual practice, the Baker’s Yeast is particularly

manufactured for bread-making, and is available commonly as ‘dried yeast’ or ‘compressed yeast’. These also find their abundant use as a food supplement by virtue of the fact that are fairly rich in

Vitamin B variants.

Production of Cheeses

There are certain typical fungi which are specifically important in the manufacture of cheeses. Example : The mould Penicillium roqueforti is usually employed in the production of the blueveined

cheeses. In actual practice, the spores of the fungus are normally used to inoculate the cheese, that is subsequently ‘ripened’ at 9°C in order to discourage the very growth of organisms other than the

Penicillium. Because, the moulds happen to be of aerobic nature, adequate perforations are carefully made in the main bulk of the cheese so as to allow the passage of air to gain entry. However, the decomposition of fat takes place to impart these cheeses a characteristic flavour. Interestingly, the mould Penicillium comemberti grows very much on the surface of the cheese, and develops inwards producing the characteristic liquefaction and softening of the surface, i.e., in contrast to the aforesaid P. roqueforti that grows within the body of the cheese.

Production of antibiotics and enzymes.

Antibiotics are produced from fungi and bacteria. Many antibiotics are primarily produced by the fungi. Actually it is the secondary metabolite of the fungi. Generally once the structure of the antibiotics are determined then after it is synthesized by the chemical methods.

Enzymes like protease, lipase and amylase are produced by the enzyme. Most of the textile enzymes are produced by the fungi and bacteria.

 

 

Question : 1 Which of the following is not a characteristic of the fungi?

1: They are all absorptive heterotrophs
2: They have cell walls made of chitin
3: Mitosis takes place within the nuclear membrane
4: They require inorganic compounds for nutrition

Question : 2 Mycelium of fungi is

1: A specialized reproductive structure of a fungus
2: A mass of connected fungal hyphae
3: A mutualistic relationship between a fungus and a plant
4: A partition between the cells of fungal hyphae

Question : 3 A basidium is typically found in

1: Bread mold
2: Gilled mushroom
3: Lichen
4: Yeast

Question : 4 A lichen is a mutualistic symbiosis between an Ascomycetes fungi and -------------------.

1: Rice plant
2: Archeabacterium
3: Green alga
4: Angiosperm root

Question : 5 An Ascomycete is different from other fungi because

1: These are mainly diploid
2: Dikaryotic phase is absent
3: Eight sexual spores are present in an ascus
4: Of the presence of gills on the mycelium

Question : 6 Staining of the fungi is done by

1: Safranin
2: Glycerine
3: Cotton blue
4: None of the above

Question : 7 Which of the following is a predatory fungus?

1: Fusarium
2: Alternaria
3: Puccinia
4: Arthrobotrys

Question : 8 Perfect fungi are

1: Perfectly healthy
2: The fungi producing perfect resting spores
3: The fungi reproducing sexually
4: The fungi with both an asexual and a sexual life cycle

Question : 9 When infected by a Synchytrium(class Chytridiomycetes) zoospore, the host cell reacts by undergoing

1: Hypertrophy
2: Hyperplasia
3: Repeated cell division resulting in wart
4: None of these

Question : 10 Which of the following is false regarding fungi

1: Mycorrhiza is beneficial to the host plant
2: Haustoria are special hyphal branches that penetrate host cell
3: Agaricus belongs to class Ascomycetes
4: Aspergillus and Penicillium belongs to class Deuteromycetes