Yeast was first described by Antony von Leeuwenhoek in 1680. It is a unicellular fungus. It is a saprophyte and grows rapidly in media containing sugar, decaying fruit and fermented food materials.

Structure

The cells are round, oval or elliptical shape. It has a definite cell membrane surrounding the protoplasm is differentiated into cytoplasm and nucleus. The cytoplasm includes endoplasmic reticulum, ribosome, mitochondria, Golgi apparatus, etc. The center of the cell is occupied by a large hyaline area consisting of a nucleus and vacuole.

Reproduction- It is reproduced by both sexual and vegetative methods.

(I) Vegetative reproduction:

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It takes place by budding and fission. Budding is the most common method of multiplication. Budding takes place during favourable conditions when food material is available in plenty. The buds are developed on their mother cells. The bud eventually separates from the parent cell by constriction.

Fission- fission involves division of a cell into two by a transverse wall. During this process, the parent cells elongate the nucleus divides mitotically into daughter nuclei. Later both the cells separate, each leading an independent life.

(II) Sexual reproduction:

Sexual reproduction takes place during unfavourable conditions. Yeasts are hamothallic or heterothallic. They lack definite sex organs. The somatic cells or ascospores function as copulating gametangia. The gamentagia are fused to form a diploid zygote cell.

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Three types of life cycle pattern are found in yeast.(1) Haplobiontic life cycle, (2) Diplobiontic life cycle, (3) Haplodiplobiontic life cycle.

(1) Haplobiontic life cycle:

Yhe type of life cycle is shown by hamothallic species. Here the somatic cells come together and form conjugation tubes which diddolve and the nuclei of two conjugating cells fuse.Now diploid cell functions as an ascus. The diploid nucleus undergoes meiotic division following a mitotic division forming eight haploid nucei known as ascospores. Each ascospores now develops into a somatic cell.

In this type the zygote is only diploid cell. Other stages are haploid in the life cycle, so it is called haplobiontic.

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(2) Diplobiontic life cycle:

Here the somatic cells are diploid. Each cell directly functions as ascus. The diploid nucleus of ascus divides meiotically, forming 4 haploid ascospores. The spores behave as gametes is fuse in pairs within the ascus, forming two diploid cells. Subsequently these two diploid cells germinate and function as sprout mycelium. The cells of mycelium from 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 diploibantic.

(3) Haplodiplobiontic life cycle:

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It is seen in heterothallic species. Here the diploid somatic cell multiplies by budding forming large number of diploid cells. Some of tdiploid cells function as asci. They divide meiotically forming four haploid ascospores. These upon liberation, form a number of haploid somatic cells by budding, which are smaller than the diploid cells. Fusion takes place bethaploid cells to form diploid somatic cells.

It this type life cycle both haploid and independently, thus both are equally important. Hence, it is called haplodiplobiontic.

Economic Importance of Yeast:

(A) Useful activities:

(i) In bakery, yeast is used for the preparation of bread. It becomes spongy due to fermentation by yeast.

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(ii) Production of alcohol by fermentation. Yeast is also used in breweries.

(iii) Dried yeast cells contain 40 to 50% protein.

(iv) Yeast cells are rich source of Vitamin B complex. They contain Thymine, Riboflavine, Nicotinic acid, Pantothellic acid, Folic acid, Biotin, Inositol, etc. Due to these substances, yeast is useful in stomach disorders.

(B) Harmful activities:

  1. Fermentation of fruit juice makes the taste unpleasant.
  2. Parasitic species of yeast cause disease in tomato, cotton, etc.
  3. Parasitic yeasts cause disease in human beings.