The yeast (Saccharomyces) is unicellular, saprophytic, ascomycetous fungus, which occurs, abundantly in sugary exudate of fruits, such as, grapes, date palms, toddy, nectar, honey etc. There are many genera of yeasts but the sugar-fungus (Saccharomyces—sacckaron — sugar, myces= fungus) is well-known genus.
Structure of yeast:
The common yeast is the Saccharomyces cerevisiae (brewer’s yeast) which is used in alcoholic fermentation of sugars, for winemaking. It is unicellular. The cells are minute, microscopic (8-15 /*), oval or round. Each cell is differentiated into a distinct cell wall and the enclosed protoplast.
The cell wall is not composed of true cellulose but of chitin and polysaccharides (glucane, mannan). The protoplast is differentiated into a large nucleus and cytoplasm. The cytoplasm is granular and dense and contains reserve food in form of granules of glycogen, volutin granules, oil globules and proteins. Mitochondria are also present.
The nucleus is conspicuous and has a large vacuole, which occupies most of the middle part of the cell. According to Wager and Peniston (1910), the nuclear vacuole contains the chromatin, reticulum and a nucleolus is present on the anterior side of vacuole as a dense body. According to Lindegren (1952), there is a centrosome at the anterior end of nuclear vacuole. While nuclceolus is considered to be suspended in nuclear vacuole.
Modern view of Dougles (1957) based upon electron microscopy, is that vacuole is separate from nucleus although both are pressed together. Nucleus has a nucleolus, which is connected by a chromatin network to upper centrosome body.
Nutrition of yeast:
The yeast cells are not autotrophic as they lack chlorophyll. Their mode of nutrition is saprophytic as they grow in sugary media from which they get their energy. The cells secrete an enzyme called zymase, which converts insoluble starch and sugars into soluble forms. If the yeast is put in sugar solution, it decomposes the solution into CO2 and alcohol. The reaction is known as fermentation. The following chemical change occurs.
C2H12O4 ——————> 2C2H3OH + 2CO3 + Energy
(Sugar) zymase (alcohol)
In sucrose solution, the sucrose is first hydrolysed into glucose and fructose sugars by enzyme invertase, and later these sugars are fermented as above. The above reactions are important in wine industry and baking industry.
Reproduction of yeast:
In Yeast, reproduction takes place by three methods
1. Vegetative Reproduction
2. A sexual Reproduction,
3. Sexual Reproduction.
1. Vegetative Reproduction:
It occurs in yeast cells of both diploid and haploid phases of life cycle. This is the common method of multiplication, which takes place by budding and always under favorable conditions of food and temperature. When the yeast cells find proper sugary medium they swell well. The protoplast bulges out in form of a lateral outgrowth as a bud, which is smaller. Simultaneously, the nuclear division occurs and one of the daughter nuclei migrates into the bud. The bud Stages in budding; (E) Pseudomycellum (Budding In chain). Soon separates and behaves as a new individual. Sometimes a chain of buds and daughter buds is formed before being detached from the parent cell. The structure having a chain-like shape is known as pseudomycelium. As a result of budding along with the reproduction fermentation of sugar solution occurs.
In some yeast cells, the vegetative reproduction takes place by fission. The cell elongates and the nucleus divides into two nuclei, which move apart. Then a transverse partition develops in the middle of the cell, which divides the mother cell into two equal parts, each with a nucleus. The two parts separate after sometime and form two independent individuals. This type of reproduction is known as fission. It is seen is Schizosaccharomyces octosporus (= fission fungi).
A sexual Reproduction:
This method of reproduction takes place under unfavorable conditions by the formation of ascospores in diploid vegetative phase cells. It often occurs under shortage of food and abundance of oxygen. This diploid yeast cell secretes thick protective wall around. This acts as an ascus cell. The nucleus divides twice by meiosis and four haploid ascospores are formed. Each ascospore has its own wall. In some species, 8 ascospores may form by one more mitotic division. The yeast cell in this method acts as an asexual reproductive body, the ascus. When favorable conditions arrive, the ascus wall ruptures and liberates the ascospores, which change to haploid vegetative cells and multiply by the usual budding method. Ultimately, these haploid cells reproduce sexually by conjugation. In some cases, haploid ascospores without changing to vegetative cells directly behave as gametes and conjugate.
3. Sexual Reproduction:
In some species of yeast sexual reproduction by conjugation also occurs. It takes place between haploid vegetative cells of different (+ and -) sexual strains or between haploid ascospores directly. Each cell behaves as the gametangium. On one side of each cell a small outgrowth arises which meets the outgrowth of the other when two cells of different strains meet together. The two outgrowths form the small conjugation tube. By that time, the protoplasts of the two cells form gametes, which are identical and called isogametes. The isogametes fuse with each other in the middle and they produce a zygospore with a diploid nucleus. The zygospore functions as an ascus and produces four or eight ascospores by meiosis. The ascospores after liberation give rise to new individuals (haploid vegetative cells). In some cases, the diploid zygospore cell converts to diploid vegetative cells, which multiply by budding, but ultimately some of these cells behave as asci.