Download Introduction to fungi: Yeast and mould

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 are 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 are 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 uses fungi in their research.
Structure
The body of the 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
Yeast is a unicellular fungus. Fungi can reproduce by sexual and asexual reproduction.
Asexually fungi reproduce by budding and transverse division. Sexually fungi divide 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 exhibit 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 forms exist on the plant while single celled forms exist in
outer environment.
Nutrition and metabolism
Generally fungi grow 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 release the extracellular hydrolytic enzyme which can digest the
organic food material. After digestion they absorb the digested soluble substances. Fungi are
chemoorganoheterotrophs. Fungi store 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 reproduce 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, needleshaped 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 possesses 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 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 is determined then after it is synthesized by the chemical methods.
Eg.: penicillin, ampicillin, amoxicillin, cephalosporin, etc.
Enzymes like protease, lipase and amylase are produced by the enzyme. Most of the textile
enzymes are produced by the fungi and bacteria.
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 hightech 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 bakers, 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.