It belongs to class Ascomycetes. It is reduced ascomycetes in which mycelium is reduced to single cell. In this plant zygote cell behaves as ascud and is simplest in structure with no specialization. Motile spores are absent.
Ascogenous hyphae and enveloping sheat are absent and thus ascus is naked.
Yeasts are saprophytic or parasitic fungus. They are rich in sugar solution but are found in the soil, in animal excreta, on vegetative parts of plants and in milk. Yeast is unicellular organism. The ells are more or less ovoid. Each cell has a delicate hyaline cell wall of clitin. Cytoplasm and a big nucleus bearing vacuole and chromatin filaments and there is small nucleus.
Economic importance:
Unicellular fungus yeast has property of breaking down sugar into Co2 and alcohol in aerobic condition and process is alcoholic fermentation.
(1) Useful activities of yeast:
(i) Production of alcohol: Alcohol is produced from sugar solution by fermentation activity of yeast.
(ii) Production of wine: Wine is also a form of alcohol produced by the activity of wild species of yeast. These yeasts ferment the fruits.
(iii) Production of beer: Beer is also a form of wine produced by fermentation activity of Saccharmonyces cerevesiae.
(iv) Backing industries: Co2 produced during fermentation process of sugar by yeasts is greatly used in backing industries.
(v) Medical value: Some yeasts are used in treatment of some skin and intestinal diseases. Yeasts are also used in the preparation of vitamins.
(vi) Food value: Yeast cells are rich in B group vitamins and enzymes hence their food value is high.
Harmful activities: They can spoil our food material like fruits, cheeses and beverages. Some species of yeast are responsible for serious human diseases like cryptoccosis and Moniliases.
Reproduction in Saccharmonyces (yeast):
Yeasts may reproduce by vegetative method and sexual method.
(1) Vegetative reproduction: In takes in favourable condition by following ways:
(i) By Budding: In this process a bud arises near a pole of cell. Bud increases in size to form a daughter cell and constricted off from the mother cell at the base. Nucleus divides into two daughter cell nuclei, one of which passes into the bud. Daughter cell still attached to parent cell may produce one or more buds and thus a simple or branched chain of cell is formed.
(ii) By Fission: Mother cell elongates and its nucleus divides into two. A transverse septum in the middle divides mother cell into two uni-nucleate daughter cells. Each cell begins new life.
(2) Sexual Reproduction: It takes place under unfavourable condition when there occurs deficiency of food. Sex organs are not specialized. Individuals come close to each other and the beak like outgrowth are given out from them, and they fuse with each other. Nuclei of both individuals come in these beaks, the wall of contact dissolves and the nuclei are fused with each other giving rise to zygote which soon converts into an oscas. Diploid nucleus of oscas divides thrice production eight nuclei. First reduction division brings haploid (x) condition again. Around each nucleus cytoplasm is deposited, they become walled and are called ascospores. On bursting the wall of oscus, the oscospores are liberated. On return of favourable conditions they germinate and new individuals are formed by budding.
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