Yeast is a unicellular eukaryotic cell. It is a saprophytic fungus and is found in sugary mediums like the juice of sugarcane and sweet fruits, the nectar of flowers, etc. Yeast is larger than most bacteria. It is non-motile and does not consist of flagella or any other organ of locomotion.
Yeast reproduces by vegetative and sexual methods of reproduction. Vegetative reproduction occurs in yeast by budding or fission method. Sexual reproduction occurs in yeast by haplobiontic, diplobiontic, and haplodiplobiontic life cycle. Yeast is used mainly in the baking and brewing industry. Besides, it is used as a source of protein, biofuel, and commercially for producing different enzymes and organic compounds.
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Structure of Yeast
The structure of yeast is very simple since it is a unicellular cell. Sometimes this single cell form the pseudomycelium, where the cluster of yeast cells are seen in a chain. The shape of a yeast cell is elliptical, round, or spherical. Its size is 3-15 μm in length and 2-8 μm in diameter. It is in hyaline (pearl grey) color. It is surrounded by the cell wall, which is made up of chitin and mannan protein. Just beneath the cell wall, a cytoplasmic membrane is present in the yeast cell. This membrane surrounds the cytoplasm. The cytoplasm consists of different cell organelles like the Golgi apparatus, ribosomes, endoplasmic reticulum, mitochondria, sphareosome, nucleus, and vacuole. Volutin granules are also present in the cytoplasm, but chloroplast is absent in the yeast cell. Reserve food material is present in the form of glycogen or fats.
There are two types of reproduction in yeast. They are vegetative reproduction and sexual reproduction.
Vegetative reproduction occurs in the yeast cell in the favorable condition where there is sufficient nutrition, as in a sugary solution. It takes place either by budding or fission method.
The most common method of reproduction in yeast is budding. During the favorable condition, it produces the bud or the small outgrowth at its end. It enlarges gradually. Then the nucleus divides into two. One nucleus is present in the mother cell, and another moves to the bud. Then constriction appears between the mother cell and bud, separating them by a transverse wall. Sometimes pseudomycelium is also formed. Pseudomycelium is the chain of the temporary bud, which is not separated from the mother cell. It looks like the mycelium, which is long and unbranched. These cells of pseudomycelium later separate and form many vegetative cells.
Fission is a common method of vegetative reproduction in Schizosaccharomyces (fission yeast). During fission, the parental yeast cell elongates, and its nucleus divides into two same-sized daughter nuclei. A transverse wall is developed in the middle of the cell. Then the parent cell divides into the two daughter cells.
Sexual reproduction occurs in yeast cells by the ascospores formation. During unfavorable conditions, when there is high stress, the diploid cell can go through sporulation. During this process, meiosis division takes place, forming various haploid spores. These cells conjugate and again forms the diploid cells.
Based on the types of vegetative cells involved in sexual reproduction and the dominant phase in the life cycle (haploid or diploid phase), Gulliermond (1949) classified the life cycle of yeast into three types, i.e., haplobiontic, diplobiontic, and haplodiplobiontic lifecycle.
It occurs in fission yeast (Schizosaccharomyces octosporus). In this type of life cycle, a vegetative cell is only haploid, and the dominant phase in the life cycle is haploid. During unfavorable conditions, the two opposite strains (haploid vegetative) cells come in contact. Then it behaves as the gametangia. Then these strains develop short beak-like protuberances, which elongate gradually and come in contact with each other. It then forms a single conjugation tube by dissolving the contact wall between them. Then their nuclei fuse, forming the zygote. It grows and develops the ascus.
The diploid nucleus is present in the ascus. It divides and forms eight haploid nuclei. From the eight haploid nuclei, eight ascospores are formed. These ascospores have their cytoplasm and cell membrane. On maturation, there is a release of the ascospores from the ascus. Out of eight ascospores, four become the positive vegetative cell, and four become the negative vegetative cell. These vegetative cells also multiply by fission and produce many haploid vegetative cells.
It occurs in helobial yeast (Saccharomycodes ludwigii). In this type of life cycle, a vegetative cell is only diploid, and the dominant phase in the life cycle is diploid. During unfavorable conditions, the diploid vegetative cell acts as an ascus. Ascus consists of the diploid nucleus. Then by the process of meiosis, it forms four haploid nuclei. From the four nuclei, four ascospores are formed in the ascus. They have their cytoplasm and cell membrane. Inside the ascus, the opposite strain of ascospores develops the conjugation tube. The nuclei of opposite strain ascospores fuse at the conjugation tube. Thus, two diploid zygotes are formed within a single ascus. Each diploid zygote develops a germ tube that pierces through the ascus wall and divides to form a short sprout mycelium consisting of many bud cells. These bud cells separate later and form many diploid vegetative cells.
It occurs in budding yeast (Saccharomyces cerevisiae). In this type of life cycle, a vegetative cell is both haploid and diploid, and both phases are equally dominant in the life cycle. During the unfavorable condition, two opposite strains (haploid vegetative cells) come near each other, behaving as gametangia. From each cell, protuberances develop, which resemble a beak. Then these protuberances elongate and come in contact with each other. Then the conjugation tube is formed after dissolving the protuberances in between them. In the conjugation tube, two nuclei fuse, forming the zygote. These diploid zygotes directly change into diploid vegetative cells.
During favorable conditions, there is the multiplication of diploid vegetative cells by the budding process. During unfavorable conditions, the diploid vegetative cell directly changes into an ascus. The diploid nucleus of the ascus undergoes meiosis and forms four haploid nuclei. From the four haploid nuclei, four ascospores are formed in the ascus. These ascospores have their cytoplasm and cell membrane. After maturation, ascospores break the wall of the ascus and releases from it. Among these four ascospores, two ascospores form positive vegetative cells, and two ascospores later form negative vegetative cells. These vegetative cells also multiply by budding and produce many haploid vegetative cells.
Uses of Yeast
Among the various microorganisms, yeast has proven to be the most useful microorganisms to humankind. Its uses range from food industry to fuel industry. Some of its uses are as follows:
- Alcohol or brewery industry: Different species of yeasts like Saccharomyces cerevisiae and S. ellipsoidens are used for making alcoholic products like beer, wine, whisky, rum, gin, vodka, brandy, etc.
- Baking industry: The yeast used in baking is Saccharomyces cerevisiae which is used for making bread, doughnut, cakes, etc.
- Flavoring agent: Yeasts ferment the cocoa, which is used in making the chocolates. After the fermentation, it gives the flavor.
- Biofuel: Yeast is used to produce biofuel. It utilizes the sugar substrates and converts them into ethanol. It can be used in vehicles as a source of fuel. Saccharomyces cerevisiae has a high ethanol production capacity and can ferment a wide range of sugars. The use of biofuel helps in the reduction of crude oil consumption.
- Source of protein: Yeast is used as a source of protein, most as single cell protein. It supplements the protein in the diet like as milk and meat.
- Enzyme production : Saccharomyces cerevisiae helps in the production of the enzyme invertase on a commercial scale.
- Production of organic compounds: Saccharomyces cerevisiae helps in the production of different types of organic compounds like acetic acid, lactic acid, and glycerol.
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