Classification
Kingdom-Fungi
Phylum- Ascomycotina
Order-Saccharomycetales
Family- Saccharomycetaceae
Genus- Saccharomyces
Occurrence:
They are found in growing saprophytically on substrates that contain sugar, such as decaying vegetables, ripe fruits, grains, nectar of flowers, soil containing abundance of humus, decaying organic matter and milk products.
Vegetative structure:
The vegetative body not composed of hyphae. It is a unicellular fungi, but the cells may often be united in chains, thus forming filaments or pseudo mycelium. They are polymorphic, colourless, but when grown on solid media, colonies are formed by yeast cells which may be white, cream coloured, or tinged with brownish pigments. They are spherical, oval or shortly cylindrical in shape. The cells are small ranges from 5-10 micron in diameter. The cell wall is thin, delicate and is composed of chitin in combination of other compounds such as carbohydrates, mannan and glucan. Cytoplasm enclosed within the cell wall, it is differentiated into outer ectoplasm and inner endoplasm. Mitochondria, endoplasmic reticulum, ribosomes are embedded in the cytoplasm. The reserve food material in the cell are glycogen, oil globules and protein particles. The yeast cell contains a large central hyaline area and a small deeply staining body on one side of it.
Reproduction:
Yeast reproduce by two methods
1) Asexual Reproduction
Asexually, they reproduce by budding, or by fission.
Budding: When food supply is abundant, the yeast cells reproduce rapidly by a peculiar process, known as budding. During this process, the nucleus of mother cell divides mitotically. Soon, a small protuberance appears on the surface of the vegetative cell in the form of a 'bud', and into this, passes one of the nuclei. It is then separated from the mother cell by formation of a wall between the two cells, thus forming a new individual, and the process may be repeatedly. The chains of bud cells which are attached to parent cell are known as pseudo mycelium.
Fission: In some yeasts, during reproduction the parental cell elongates and its nucleus divides to produce two daughter nuclei and gradually a partition wall is formed just in the middle of cell, dividing it into equal or nearly equal halves results in the formation of two daughter nuclei, these nuclei remain together and repeat the process or separate and divide.
Endospore formation: Under unfavourable condition spores are formed from the mother cells. During the process, the protoplasm of vegetative cell divides, usually into four portions, each of which later becomes surrounded by a comparatively thicker wall. These structures are called as endospores. These spores can withstand unfavourable condition on approach of favourable condition germinate to produce chains of cells.
Sexual Reproduction
The sexual reproduction takes place when the food is insufficient. For the purpose of sexual reproduction no specialised sex organs are produced. It occurs by conjugation of two similar or dissimilar haploid vegetative cells or gametangia this process is known as hologamy. This results in the formation of diploid zygote cell which function as an ascus and produces ascospores. In some yeasts, the copulation occurs in between the mother cell and its bud this process is called pedogamy. Yeast may be homothallic or heterothallic.
