Table of Contents
Here we cover the main definition general Characters of Fungi and Economic importance of Fungi i.e Useful Activities Of Fungi and Harmful Activities of Fungi and also discuss about the Classification of Fungi Saccharomyces Habit/ Structure Reproduction /Economic Importance of Yeasts.
Do you really know about the fungus history if you don’t know about the history of fungi do check in below we are going to discuss now.
Definition of fungi
Thallophyta also includes a group of plants which unlike all others is heterotrophic. This is because these plants, placed together in Fungi, are achlorophyllous (without chlorophyll). These form a large group which includes more than 1,00,000 species. The branch of botany dealing with the study of fungi is known as mycology (mykes = mushroom, logos = discourse). The first systematic description of fungi was given by an Italian botanist Pier’ Antonio Micheli (1729) in his book Nova Plantarum Genera. He is, therefore, rightly called the Founder of Mycology’.
Who is the founder of mycology?
ans. Pier’ Antonio Micheli.
Although fungi have many characteristics common to chlorophyllous thallophyta (i.e., algae) but differ from them in many features Therefore, they have been placed in a separate group. Some distinctive features of fungi are as follows.
GENERAL CHARACTERS OF FUNGI
Characters of fungi is the main topic of fungus so now we are going to discuss about the general characters of fungus or fungi.
Thallus is called mycelium. It is made up of thread-like filaments known as hyphae (sing. = hypha). The hyphae are septate or aseptate. The cell wall is mostly made of chitin.
Fungi lack chlorophyll and, hence, cannot by manufacturing their own carbohydrate food by photosynthesis. Thus fungi are heterotrophic i.e., saprophytes (obtaining their food from dead organic matter) or parasites (obtaining food from living organisms). They store their food in the form of glycogen (glycogen being common in many animals, it is known as animal starch).
Fungi occur almost everywhere in nature. They grow in water, soil, air, on food, leather, cloth, on optical instruments, etc. They can also be seen on our hair, mouth, eyes and even intestine. The economic losses they cause are out of all proportion to their size and importance as plants.
Nutrition of Fungi
1. Modes of nutrition.
All fungi are characterised by the complete absence of photosynthetic pigment-the chlorophyll. Hence they lack the ability to manufacture their own organic food.They are, therefore, dependent upon living. They are organisms or dead organic matter for their nutrition. On the basis of food habits fungi can be grouped into following three classes.
Those fungi which obtain their food from other living organisms (plants or animals) are called parasites. The organism on which a parasite lives is known as the host. Many parasites grow only on the host surface and send out specialized absorbing organs called haustoria within the host tissue for food absorption. Such parasites are called ectoparasites or ectophytic parasites (e.g. Erysiphe, Mucor). Some of the other parasitic fungi which grow inside the host tiss (intracellular) are called endoparasites of endophytic parasites (e.g., Pythium, Puccinia).
Among fungi, different levels of parasitism may exist. These are briefly discussed below.
(i) Obligate (total or complete)
parasites. These fungi obtain their food from the host throughout their life without which they can not complete the life cycle; e.g., Albugo, Puccinia.
(ii) Facultative (partial by chance)
saprophytes. These fungi are normally parasites but under certain circumstances may obtain their food from dead organic matter, i.e., may lead saprophytic life; e.g., Ustilago.
Fungi which obtain their food from dead and decaying organic matter, are called saprophytes. Saprobes may be further divided into following two categories.
These fungi always obtain their food from dead matter throughout their life and do not require living organisms; e.g Mucor, Saprolegnia.
(ii) Facultative parasites.
These fungi are normally saprophytic but under certain conditions become parasites and obtain their food from a living host; e.g., Pestalotia.
It is an organic relationship between two individuals in which both the partners are benefited. Two common examples of symbiosis are (i) lichens and (ii) mycorrhiza. Lichen is a composite plant formed by the association of algae and fungi. The algal partner belongs to Chlorophyceae or Cyanophyceae and the fungal partner is a member of Ascomycetes or Basidiomycetes. Mycorrhiza is an association between fungal hyphae and roots of higher plants as in Pinus.
2. Elements used by Fungi.
Using culture techniques, it has now been established that elements like hydrogen, carbon, Oxygen, phosphorus, potassium, manganese, sulphur, zinc, iron, copper, calcium and cobalt are required by almost all fungi. Besides, fungi also need vitamins and hormones for their growth and reproduction. Usually these are synthesized by fungi themselves but have to be absorbed from outside when deficient. Fungi store excess food material in the form of glycogen or oil.
Fungi show great variation in the shape and structure of thallus The thallus is usually simple consisting of either single cell or long slender filaments. Accordingly, they can be placed into the following two groups.
1. Unicellular fungi.
In lower groups of fungi, the thallus consists of only a single cell (e.g., Synchytrium). In some other non-mycelial (e.g..fungi called slime molds, the thallus is naked and multinucleated amoeboid mass of protoplasm. Such structure is called plasmodium, e.g., Plasmodiophora, Stemonites, etc.
2. Filamentous fungi.
In the majority of fungi the thallus is made of long and slender filaments called hyphae. The network of hyphae is known as mycelium. The hyphae are hyaline or coloured and may be aseptate or septate. Aseptate hyphae are long, branched and acellular with many nuclei embedded in the common protoplasm. This type of thallus is known as Coenocytic and is characteristic of Phycomycetes; e.g., Mucor.
The hyphae in Ascomycetes, Basidiomycetes And Deuteromycetes (Fungi Imperfecti) are septate. Septa in most cases possessS a central pore (perforated septa) or are non-porous.
In septate hyphae cells are either multinucleate (as in Ascomycetes) or uninucleate (as in Basidiomycetes). The uninucleate hyphae of Basidiomycetes are known as monokaryotic or homokaryotic. Sometimes monokaryotic cells of two different strains or genomes fuse and as a
result each cell of the hyphae may have two nuclei. Such a pair of nuclei is called dikaryon. These nuclei remain together without fusing and divide independently and simultaneously as new cells are formed. Such hyphae are termed as dikaryotic or heterokaryotic.
Modifications of Hyphal Structure
The hyphae of a mycelium are usually distinct. Under certain circumstances, the hyphal structures become modified into some specialized structures. Such modifications are usually adaptations to ecological or physiological factors. Some important modifications are described below.
It is loosely or . compactly woven mass of fungal hyphae. Plectenchymá is of the following two types.
In this type of plectenchyma the hyphae lie parallel to one another and are easily distinguishable.
The hyphae lie side by side and in a cross section look like oval or isodiametric parenchyma cells. In this type of tissue the fungal hyphae do not retain their individuality.
2. Sclerotium (plu. = sclerotia).
It is a tough and hard resting body resistant to unfavourable conditions. It may remain dormant for long periods and germinatesS under favourable conditions; e.g., Claviceps.
It is a thick, dark coloured strand of somatic hyphae running parallel to each other. The hyphae lose their individuality. The growing tip of the rhizomorph looks like a root tip, hence the name. These structures are resistant to unfavourable conditions and remain dormant until favourable Conditions return. Rhizomorphs also creep under bark of trees or in the soil, thus helping in the spread of fungus, e.g. Armillaria, Agaricus, etc.
Reproductive Process of Fungi
Fungi reproduce by three methods vegetative, asexual and sexual.
1. Vegetative reproduction of fungus or fungi.
Some common methods of vegetative multiplication are as follows.
Thallus or hypha breaks into a number of segments. Each segment grows into a new individual.
In fission a cell simply breaks into two daughter cells by constriction. It is common in unicellular fungi such as yeasts.
In this process a ‘bud’ or an outgrowth is produced by the parent cell. Meanwhile the nucleus divides and one of the two daughter nuclei migrates into the bud. Eventually the bud gets separated from the parent cell and grows into a new individual; e.g., yeast.
2. Asexual reproduction of fungi or fungus.
It occurs practically in all fungi. It usually takes place during unfavourable conditions by means of a wide variety of unicellular or multicellular spores. They are either motile or non-motile. There is also much variation in the form of spore bearing structures. Some common types of asexual spores are briefly described below.
These are unicellular motile, uninucleate and naked flagellate structures. Zoospores are formed endogenously within a sac-like structure called zoosporangium; e.g., Albugo, Synchytrium.
(b) Conidium (pl.= conidia).
Conidia are non-motile spores borne either singly (e.g., Phyto-phthora) or in chains (e.g., Penicillium Aspergillus) at the tip of the hyphae. The hypha bearing conidia at its tip is called conidiophore. Conidia are usually unicellular but septate conidia are found in some fungi; e.g., Alternaria.
(c) Oidium (pl. = oidia).
These are small, thin walled free cells formed due to the fragmentation of a somatic hyphae into cells. The hyphal branch forming oidia is called conidiophore; it cuts off oidia apically in succession. Each oidium behaves as a spore; e.g.. Coprinus, Mucor.
These are unicellular,thick walled, non-motile resting spores formed by the rounding off of the contents of the hyphal cells. Chlamydospores may be terminal or intercalary in position; e.g., Mucor, Peziza,
(e) Aplanospores or sporangiospores.
These are non motile unicellular spores which are formed within a sac-like sporangium. The hypha bearing terminal sporangium is called sporangiophore; e.g., Mucor.
In some fungi the entire thallus is converted into one or more reproductive structures (sexual or asexual). Therefore, the vegetative and reproductive phases do not occur together at the same time in the same individual. Such fungi are called holocarpic, e.g., Synchytrium. However, in the majority of fungi only a part of the thallus produces the reproductive structures and the rest of the thallus remains vegetative. Such fungi are called eucarpic; e.g., Mucor
3. Sexual reproduction of fungus or fungi.
Sexual reproduction in fungi, as in other organisms, also involves fungi fusion of two compatible nuclei. Some fungi develop distinct male and female sex organs whereas others (the advanced forms) do not develop them. In such forms the somatic hyphae take part in sexual reproduction.
The sexual process involves following three steps.
This is the fusion of the protoplasm of two sexually compatible elements such as the spores, gametes or vegetative cells. In this process the haploid nuclei come to lie in a single cell but they do not fuse.
This is the fusion of two nuclei brought together by plasmogamy. These nuclei are haploid and fuse to form a diploid nucleus. Karyogamy may take place either immediately after plasmogamy or there is sufficient interval between these two steps.
Karyogamy, sooner or later, is followed by the reduction division of the diploid nucleus. In fungi the plasmogamy and subsequent stages of sexual process may be brought about by any one of the following types
(i) Gametic copulation.
It may be any one of the following three types
It involves fusion of two morphologically similar and motile gametes
Here both the fusing gametes are motile but the male gamete is smaller than the female gamete.
In this type of copulation, female gamete 1S larger and non-motile while the male gamete is smaller and motile due to flagella Primitive fungi usually have isogamous plano gametes whereas in advanced forms the male anisogamous Fusing gametes is motile and small, and female is non-motile Synchytrium, large, e.g and relatively Monoblepharella.
(ii) Gametangial contact.
In this process gametes are not released from the gametangia, instead the two gametangia of opposite sex come in contact and the gametes are directly transferred from one gametangium to the other through a tube called fertilization tube ; e.g., Albugo, Pythium.
(iii) Gametangial copulation.
It involves fusion of the entire contents of two gametangia. Two gametangia of different sex come close together and the common wall between the two gametangia dissolves. As a result a common cell is formed where the protoplasm and the nuclei of the two gametangia fuse; e.g., Mucor.
Some fungi (Basidiomycetes and Ascomycetes) produce numerous minute, uninucleate, spore-like male gametes called spermatia (sing spermatium). These are non-motile and are carried by wind, insects or water to respective female hyphae. A pore develops at the point of contact of spermatium and receptive hyphae through which nucleus of the spermatium is transferred to the receptive hypha; e.g Puccinia, Podospora
In many higher fungi sex organs are not produced and instead somatic cells directly function as gametes; e.g., Peniophora.
Economic Importance of Fungi
Fungi are important to us as they affect our daily life directly or indirectly. Some of them are friendly and provide us with food, life saving drugs and other items of human use while others cause diseases in man and plants and spoil food and other articles.
Useful and Harmful Activities of Fungi or Fungus
We going to discuss about the Useful and Harmful Activities of Fungi, so keep tune with us, we are cover all Useful and Harmful Activities of Fungi with examples.
Useful Activities of Fungi
In this paragraph we will discuss about the useful activities of fungi.
Five useful activities of fungi with example.
1. Fungi as food
2. Fungi and nitrogen fixation
3. Soil fertility
4. Fungi in the control of plant diseases
5. Fungi in biological research
[I] Fungi provide us with food that is rich in proteins.
Dried yeasts contain about 50% protein. Besides, they are rich in vitamins and B-complex. Being rich in protein, it is commonly used as single cell protein (SCP).
These are members of Basidiomycetes. Fruiting bodies of about 105 saprophytic mushrooms are edible; they are preferred for both their taste and food value. Different mushrooms have 21 to 30% protein and the proportion of fats and carbohydrates is relatively less. Therefore, they are considered to be a good food for diabetics and heart patients. World production of mushrooms is more than 300,000 metric tonnes per year.
These are highly prized edible fungi belonging to the genus Morchella of class Ascomycetes. They grow abundantly in apple and peach orchards of northern India.
[II] Fungi and nitrogen fixation
Biological nitrogen is the commonest and the largest cause of soil fertility. It is mainly achieved through symbiotic bacteria present in the root nodules of legumes. A small amount of atmospheric nitrogen is also fixed by non symbiotic fungi such as Rhodotorula and Saccharomyces
[III] Soil fertility
Some fungi like Aspergillus, Cladosporium, Rhizopus, Penicillium, etc., have soil binding properties. This is achieved by the secretion of mucilaginous substances. Many saprophytic fungi along with bacteria help in the decomposition of dead organic matter.As a result the amount of humus increases in the soil and it becomes more fertile.
[IV] Fungi in the control of plant diseases
Many fungi are helpful in controlling plant diseases caused by soil borne pathogens such as nematodes, insects, viruses and other fungi. These fungi compete with pathogens for essential nutrients or destroy them.
[V] Fungi in biological research
1. In Biological assay.
Use of microorganisms in determining the potency of drugs, detection and estimation of various chemicals in given samples and other similar purposes is known as biological assay. Amongst fungi, Aspergillus niger is used to detect very minute quantities of Zn, Ca, Pb, Mn, Mo, Cu, etc., in given samples. Similarly, Neurospora crassa is used as a test organism to detect the presence and quantity of vitamin B-complex in any sample.
2. In genetic experiments.
Neurospora is an ideal material for genetic and biochemical studies. It is popularly known as “Drosophila of Plant Kingdom”, because of its suitability in the studies of heredity. Besides this, Physarum polycephalum is also a good material for the study of mitotic cycle and DNA synthesis.
[VI] Fungi as a source of hormones
Gibberellin or gibberellic acid is a growth obtained from promoting hormone an ascomycetous fungus Gibberella fujikuroi. Besides stimulating growth in genetically dwarf plants, it also accelerates flowering and breaks dormancy of certain resting bodies such as potato tubers.
It is a symbiotic association between fungal hyphae and roots of higher plants. Several fungal genera such as Amanita, Boletus, Tricholoma and as Scleroderma cover the roots of evergreen trees. These fungi decompose the dead organic matter and thus make mineral nutrients available to the roots.
[VIII] Fungi in industry
1. In Baking industry.
Saccharomyces cerevisiae (yeast) popularly known as baker’s yeast is widely used in the baking industry.
2. In production of alcoholic beverages.
beverages. Preparation of alcoholic beverages involves fermentation of sugar or salt solution by yeast. cerevisiae particularly Saccharomyces and S.ellipsoideus. The enzyme zymase present in yeast cells converts hexose sugars into alcohol and CO²
Besides this, yeast also has two more enzymes- sucrase and maltase which facilitate the conversion of other sugars into hexose sugar.
3. In acid production.
Several fungi are helpful in the commercial production of many organic acids. A list of industrially important acids produced by fungi is given in
4. In enzyme production.
Many fungi produce enzymes which have industrial uses. Table 5 gives a list of some enzymes, along with their Sources.
5. In cheese Penicillium Making.
Camembert and P. roqueforti are used in cheese making. These molds add a special flavour to the cheese.
6. In vitamin extraction.
Some fungi are a rich source of vitamins.
7. In antibiotic production.
Antibiotics are substances produced by microorganisms which
inhibit or kill other microorganisms. At present more than 700 fungal species are known to secrete these antifungal and antibacterial substances. The first antibiotic penicillin was extracted from Penicillium notatum by Sir Alexander Fleming in 1928, for which he was awarded the Nobel prize in 1945. Some important antibiotics and their sources are given in.
HARMFUL ACTIVITIES OF FUNGI
If you think fungus activities are good in nature sorry dear there are so many harmful activities found in fungi so are you curious about the harmful activities of fungi? we are listing six harmful activities with examples below.
[I] Deterioration of articles
Saprophytic fungi(e.g Rhizopus,Mucor, Aspergillus, etc.) grow on food articles such as bread, jam, pickles, meat, etc., and make them inedible. They also destroy leather articles. Damage of library books by cellulose eating fungi is quite frequent. In addition, rubber, wool and painted surfaces are damaged by species of Aspergillus, Penicillium, Cladosporium, Alternaria and Rhizopus. Etching and blurring of microscopes and camera lenses is due to Aspergillus and Helminthosporium.
[II] Decay of wood
The biological decomposition Decay is organic matter. In India commercial timber yielding plants such as sal, teak, sisham, deodar, etc, are destroyed by species of Fomes, Polyporus, secrete fungi These etc.Ganoderma, cellulose and lignin decomposing enzymes (e.g, cellulose and phenolic oxidase) and cause heart rot.
[III] Staining of wood
Some fungi feed on sap wood. Although these fungi do not destroy the wood but stain it. Penicillium sp. give yellow stain to the wood, Fusarium negundi red and Trula ligniperda stains green.
[IV] Fungal toxins
Mushrooms like Amanita phalloides, A. virosa and A. muscaria are poisonous and are quite a menace to common people. Poisoning by these mushrooms causes abdominal pains with vomiting, cold sweats, diarrhoea, excessive thirst and eventually death. fungus Claviceps purpurea, a parasitic (causing ergot of rye) contains a powerful poison. When eaten, it causes gangrenes and convulsions. LSD (Lysergic acid diethylamide), a hallucinogenic and hypnotic compound, is also obtained from Claviceps.
Besides this, some fungi secrete a group of toxic compounds called aflatoxins. Aspergillus flavus, an important toxin producing fungus, frequently infects ground nuts. Animals eating such nuts fall sick.
[V] Plant diseases
A disease may be defined as a disturbance brought about by a living organism or an environmental factor which interferes with manufacture, translocation or utilization of nutrients, yield loss and morphological changes. A disease causing agent is known as pathogen and the branch of botany that deals with all aspects of plant diseases is known as Phytopathology (phyton = plant, pathos= sufferings, logos = knowledge).
Fungi infect many economically important plants and minimise the yield of food grains considerably. In 1945 late blight of potato (caused by Phytophthora infestans) destroyed millions of acres of potato crop and caused famine in Ireland. It resulted in the death of about a million people and almost the same number of people migrated to other continents. Similarly, the 1942 Bengal famine which resulted in the death of two million people was due to destruction of rice crop by brown leaf spot disease caused by Helminthosporium oryzae.
[VI] Human and animal diseases
A good number of human and animal diseases are caused by fungi. Fungal infections are either restricted to the skin and its appendages like hair, nails, hooves, horns, etc., (superficial mycosis) or may infect internal organs like lungs, intestine, deep skin, brain, etc., (deep mycosis).
CLASSIFICATION OF FUNGI OR FUNCTIONING OF FUNGI
We are going to discuss the fungus / fungi classification so do read with us.. to know about the classification of fungus or fungi.
Various classifications of fungi have been given by different scientists. One of them, which is the most practical, has been briefly described below. In this system fungi have been classified into five classes on the basis of thallus structure and types of sexual reproduction.
Class 1. Myxomycetes. These fungi are also known as slime molds. The body (thallus) consists of a multinucleate structure called plasmodium.
They do not have a definite cell wall and instead The outer protoplast (ectoplasm) is thick and tough. Thus the thallus resembles amoeba, hence, these fungi are also termed as myxamoeba. Like amoeba they move by means of pseudopodia. These are holocarpic. Asexual reproduction takes place by fission and cysts. During sexual reproduction haploid spores are formed. The sporangium has a characteristic network of protoplasmic threads, called capillitium. Gametes are biflagellate and the sexual reproduction is isogamous. Examples: Physarun, Reticularia, etc.
Class 2. Phycomycetes. The thallus of these fungi is non-mycelial or aseptate, filamentous and coenocytic. Many of the hyphae Phycomycetes are aquatic, hence these fungi are also known as algal fungi. Aquatic members are isogamous, anisogamous 0gamous but Or terrestrial members are mostly isogamous with non-motile gametes.
The class Phycomycetes has been further classified into the following three sub-classes.
Sub-class 1. Archimycetes. Thallus consists of a large single cell and mycelium is absent.
Examples: Synchytrium, Protomyces.
Sub-class 2. Oomycetes. Mycelium is well developed. Sexual reproduction is by gametic
contact or gametic copulation. Gametic fusion results in the formation of oospore.
Sub-class 3. Zygomycetes. Mycelium is well developed; sexual reproduction is by gametangial
Examples: Mucor, Rhizopus.
Class 3. Ascomycetes. These are also known as sac fungi. Thallus is non-mycelial (e.g. Saccharomyces) or profusely branched septate mycelium (e.g., Aspergillus, Penicillium). Asexual reproduction occurs by means of conidia or budding. Sexual reproduction takes place by means of antheridia and ascogonia and their fusion results in the formation asci (sing. = ascus). The asci are formed endogenously within a fruiting body called ascocarp. Each ascus has eight ascospores. The ascospores on germination give rise to haploid mycelium.
The class Ascomycetes has further classes. classified into two sub- classes
Sub class 1. Hemiascomycetes: Ascogenous hyphae and ascocarp are absent.
Sub class 2. Euascomycetes: Ascogenous hyphae and ascocarp are present.
Class 4. Basidiomycetes. These are also known as club fungi. Mycelium is septate and uni-, bi-, or multinucleate. Asexual reproduction takes place by budding and conidia but no special Sex organs are formed. Sexual reproduction takes place by conjugation of hyphae of two different strains, resulting in the formation of dikaryotic hyphae. Two nuclei of one of the dikaryotic cells fuse to form diploid nucleus (this cell is now called basidium) which after meiosis exogenously forms four haploid basidiospores. Thus basidium always has definite number of (i.e., four) basidiospores
The Class Basidiomycetes has been classified into two subclasses:
Sub -class 1. Holobasidiomycetes: Basidium is not septate (called holobasidium).
Sub class 2. Heterobasidiomycetes: Basidium septate and deeply divided (called phragmobasidium).
Examples: Ustilago, Puccinia.
Class 5. Deuteromycetes. This class is also known as Fungi imperfecti. Mycelium is septate and well branched. The class includes fungi in which stages of sexual reproduction are absent and reproduction occurs by asxeual methods only. Some of the asexual structures are synnema,pycnidia, acervuli, sporodochium, etc.
Examples: Alternaria, Helminthosporium, Colletotrichum, etc.
Saccharomyces is a saprophytic fungus that grows rapidly in media containing sugar, such as sugar- cane juice, date palm, grapes, etc. lt also occurs in the nectariferous glands of flowers, ripe and decaying fruits and fermented food material. In fact, name Saccharomyces comes from its great affinity to the medium containing sugar (Saccharin = sugar, Oomycetes = fungi, i.e., sugar containing fungi or Saccharomyces)
Structure of Fungi
At first we know that what is fungi mean what is the definition of fungus and here now we are going to discuss about the structure of fungi so keep reading with us and know about the full structure of fungus or fungi.
Yeast was first described by Antony von Leeuwenhoek in 1680. It is a non-mycelial unicellular fungus. The eukaryotic cells are round, oval or elliptical in shape. Occasionally, the newly
formed cells may adhere to one another in a chain forming a pseudomycelium. The cells are microscopic, generally varying from 5 30u in length and 1 5 u in width. They possess a definite thin cell wall. The cell wall is mainly composed of two polysaccharides glucan (30-40%) and mannan (30%). Besides, proteins (6-8%), lipids (8.5 – 10.5%) and chitin (2%) are also present.
Inner to the cell wall is the cell membrane which surrounds the protoplasm. The cell membrane is characterised by shallow invaginations at certain points. It is made of lipids and proteins. Tc Protoplasm is differentiated into cytoplasm and nucleus. The cytoplasm includes endoplasmic reticulum, ribosomes, mitochondria, golgi apparatus and lipid granules
The mitochondria, besides respiratory enzymes, also contains RNA and DNA. Ribosomes give a granular appearance to the cytoplasm. The centre of the cell is occupied by a large hyaline area consisting of a nucleus and vacuole. There is however, controversy whether the vacuole is a part of the cell nucleus or is a separate entity.
1. Colourless structure in the centre of the cell is the nucleus. According to Wager and Peniston (1910), the hyaline structure situated in the centre of the cell is in fact a vacuolated nucleus. A similar view has been given by Alexopolous (1958). Vacuole is filled with chromatin threads. At one end of the nucleus is present a small nucleolus. Later, Lindgren described the nucleolus as centrosome.
2. Structure situated in the centre of the cell is vacuole. According to Guilliermond (1905) the central hyaline area is a vacuole and the nucleus lies close to it. Nagel (1946) also confirmed this on the basis of staining techniques.
Agar and Douglas (1957) made electron microscopic studies of ultra thin sections of yeast cells. They confirmed that the nucleus is surrounded by a membrane of its own and is distinct from the vacuole. The nuclear envelope is composed of two unit membranes perforated by pores, whereas the vacuole is surrounded by a single unit membrane without pores. The vacuole has many hydrolytic enzymes Such as proteases, ribonucleases and esterases.
Chromosome number of a diploid yeast cell is 8.
Reproduction System Of Fungi
Here it’s time to know about the reproduction of fungus or fungi so before we know the reproduction of fungi we have to know one thing that Saccharomyces reproduce by vegetative and sexual methods.
Vegetative Reproduction of fungi
Vegetative reproduction takes place by budding and fission. Accordingly, yeasts have been grouped into budding yeasts (e.g., S. cerevisiae) and fission yeasts (e.g.. S. octosporus).
1. Budding. It is the most common method of multiplication in Saccharomyces. Budding usually occurs during favourable conditions when food material is available in plenty. During the formation of a bud one or more outgrowths are formed at or near one pole of the cell. The bud enlarges in size.
In the meantime the nucleus divides mitotically, one daughter nucleus passing into the bud and the other remaining in the parent cell.
The bud eventually separates from the parent cell by a constriction. It leaves a circular scar with a convex surface on the parent cell wall. This is called bud scar.
The process of budding is very rapid. Usually a bud starts forming new buds before its separation from the parent cell. This frequently results in the formation of branched or unbranched chains of buds, called pseudomycelium
2. Fission. Fission involves division of a cell into two by transverse wall. During this process the parent cell elongates and the nucleus divides mitotically into two daughter nuclei. A transverse wall now divides the mother cell into two daughter cells each containing a daughter nucleus Later both the cells separate, cach leading an independent life.
Sexual Reproduction of Fungi
Sexual reproduction in yeasts takes place during unfavourable conditions, particularly when there is less amount of food. Yeasts are homothallic (e.g..S. octosporus) or heterothallic (e.g., S. cerevisiae). They lack definite sex organs, instead somatic cells or ascospores function as copulating gametangia. Their fusion results in the formation of a diploid zygote cell.
The following three life cycle patterns are found in different species of Saccharomyces.
(1) Haplobiontic life cycle (e.g., Saccharomyces octosporus)
(2) Diplobiontic life cycle (e.g., S. ludwigii)
(3) Laplo Diplobiontic life cycle (e.g., S. cerevisiae).
1. Haplobiontic life cycle.
This type of life cycle is shown by S.octosporus, a homothalliC species. Here the somatic cells are haploid. Two such haploid cells come together and form conjugation tubes. The common wall where the two conjugation tubes meet dissolves and the nuclei of two conjugating cells fuse.
Diploid cell, thus formed, now functions as an ascus. The diploid nucleus undergoes a meiotic division followed by a mitotic division forming eight haploid nuclei. Each of these eventually Thus eight transforms into an ascospore. ascospores are formed within the ascus. The wall of the ascus ruptures at maturity and ascospores are set free. Each ascospore now develops into a somatic cell.
In this type of life cycle the zygote is the only diploid cell. Other stages, forming the major part of the life cycle, are haploid. On this basis the life cycle is called haplobiontic.
2. Diplobiontic life cycle.
This type of life cycle is shown by S. ludwigii.The somatic cells are diploid. Each cell directly functions as ascus. The diploid nucleus of ascus divides meiotically forming four haploid ascospores. These spores now behave as gametes and fuse in pairs within the ascus. Thus two diploid cells are formed from two pairs. subsequently, these cells germinate by a germ tube which pushes through
the ascus wall. It is multicellular and functions as diploid sprout-mycelium. The cells of the mycelium form many diploid cells by budding.
In this type of life-cycle the haploid stage is represented only by ascospores and all other stages
are diploid. Hence it is called diplobiontic.
3. Laplo Diplobiontic life cycle.
It is shown by S. cerevisiae, a heterothallic species. Here both haploid and diploid phases are equally developed. Diploid somatic cells multiply by budding, forming a large number of diploid cells. Some of the diploid cells function as ascii. They divide mechanically, forming four haploid ascospores. These upon liberation, form a number of haploid somatic cells by budding which are smaller than the diploid cells. Fusion also takes place between haploid cells to form diploid somatic cells.
In this type of life cycle both haploid and diploid phases are capable of reproducing independently, thus both are equally important. Hence, it is called haplodiplobionte.
ECONOMIC IMPORTANCE OF YEASTS
Useful Activities of Yeasts
[I] In baking
In bakery, CO² from fermentation by yeast makes
The dough rises in the baking oven and gives the bread its spongy nature. Flavour and quality
bread depends upon the proper selection of different strains of yeast. Commercial yeast is
available in the market as dried yeast cakes and powder
[II] Production of alcohol
Carbohydrate can be converted into ethyl alcohol by fermenting them with yeasts.
C6H1206 ——-→ 2C²H⁵OH + 2C02
(glucose) (ethyl alcohol) (carbon dioxide)
Many species of Saccharomyces used in the production of different types of alcoholic beverages are given below
Yeasts used in breweries are grouped into two classes.
1. Top yeast. These cells ferment while floating on the surface of the beverage. They have greater fermented power.
2. Bottom yeast. Such cells settle down at the base of the beverage where oxygen supply is poor.
[III] Protein source
Dried yeast cells contain about 40-50% Besides, their protein. growth is very rapid. Therefore, in a given area and time, protein yield from yeast can be 10 to 15 per cent more than Soybean and 25 to 50 per cent more than maize.
[IV] Source of vitamins
Yeast cells are a rich Source of vitamin B complex. They contain Thiamine, Riboflavin Riboflavin, Nicotinic acid, Pyridoxine, Pantothenic acid, Folie acid, Biotin, P – aminobenzoic acid, Choline and Inositol. It is due to these substances that the Yeast cells are useful in stomach disorders.
Harmful Activities of Yeasts
(1) Fermentation of fruits and fruit juices by yeast cells makes their taste unpleasant.
(2) Parasitic species of yeast cause diseases in tomato, cotton, etc.
(3) Parasitic yeasts cause diseases in human beings (e.g., cryptococcosis, blastomycosis and torulopsis).
Some Important Questions about FUNGI
Long answer type questions
1. Describe various types of life cycles found in yeast.
2. Describe briefly the life history of yeast and state its usefulness.
3. Describe briefly the structure, reproduction and economic importance of yeast.
Write short notes on the following each with 3 to 5 important points
1. Give differences between algae and fungi.
2 Obligate parasites.
3. Facultative parasites.
4. Obligate saprophytes.
5. Facultative saprophytes.
8. Parasite and saprophyte.
9. Gametangial contact.
10. Gametangial copulation.
11. Vegetative reproduction in yeast.