In this article we will discuss about Yeast. After reading this article you will learn about: 1. Habit and Habitat of Yeast 2. Vegetative Structure of Yeast 3. Nutrition 4. Reproduction 5. Economic Importance.
Contents:
- Habit and Habitat of Yeast
- Vegetative Structure of Yeast
- Nutrition of Yeast
- Reproduction in Yeast
- Economic Importance of Yeast
Contents
1. Habit and Habitat of Yeast:
Saccliaromyces is the primitive representative of the class Ascomycetes. It is widely distributed and occurs saprophytically on substances rich in sugar like molasses, date palm, milk, surface of fruits (grapes, figs etc.), and in nector of the flowers. Yeasts are also found in milk, on the vegetative parts of the plants, in animal excreta and in other habitats.
Yeasts ferment carbohydrates, hence the name Saccharomyeetes (Gr. Saccharon = sugar + myketes = fungi) is applied to them. Because of this property they are used in baking and brewing and hence are called Baker’s yeast and Brewer’s yeast. Some yeasts are parasites e.g., Cryptococcus linguapilosae. It causes black tongue in human beings.
2. Vegetative Structure of Yeast:
Antony Von Leeuwenhoek (1680) was the first to describe the yeast cells. Its thallus is unicellular and non-mycelial. However, at the time of budding it rarely produces pseudo mycelium. The individual cells are polymorphic i.e., showing different shapes, even in the same culture, depending upon the nutrition available.
Generally, the shape of cells may vary from circular, spherical, oval, elliptical, elongated, rectangular, dumb-bell shaped to triangular. The cells are minute and range from 2 to 8 µ in diameter and 3 to 15 µ in length. Individually, the cells are hyaline (colourless) but its colonies appear white, cream-coloured or light brown. Each cell consists of a tiny mass of protoplast surrounded by a definite cell wall.
The Cell Wall:
The cell wall is double layered, thin, delicate and flexible. It is composed of two complex polysaccharides, mannan (30%) and glucan (30-40%) with smaller quantities of protein (6-8%), lipid (8.5 – 10-5%) and chitin (2%). Cellulose is absent.
The Protoplast:
Inner to cell wall is a cytoplasmic membrane or plasma membrane. It surrounds the cytoplasm and a nucleus. Under light microscope, a large hyaline structure, occupying a large portion of the cell and a deeply staining body associated on one side of it is seen.
These two structures have been interpreted in different manners by different workers (Fig. 1 A, B). According to Guilliermond (1920), Mundkur (1954), Roy an (1956) and Ganesan (1956) hyaline area represents a vacuole and nucleus is found just as its neighbour.
However, Wager and Peniston (1910), Srinath (1946), Subramanian (1948), Lindegren (1952), Thingarajan and Subramaniain (1954), opined that vacuole does not exist in the cytoplasm and it is occupied by nucleus (hyaline body) and nucleolus (deeply stained body Fig. 1 A).
Lindegren (1949, 1952) and Alexopoulos supported this view but they called nucleolus as centrosome (Fig. 1 B). Lindegren believed that vacuole has six pairs of chromosomes (Fig. 1 C).
Electron microscopic studies of ultra-thin sections of S. cerevisiae (Agar and Douglas, 1957; Hashimoto et al, 1959) and of S. octosporus (Conti and Naylor, 1959, I960) show that the nucleus is surrounded by a nuclear membrane and is distinct from the vacuole (Fig. 1 D).
The nuclear membrane has pores. The cytoplasm in addition to the various cell organelle (mitochondria, endoplasmic reticulum, ribosomes etc.) contains glycogen, proteins, oil and refractile volutin granules (an inorganic metaphosphate polymer) as reserve food materials.
3. Nutrition of Yeast:
Yeast is heterotrophic and saprophytic in nature.
Its protoplasm secretes two types of enzymes:
(i) Invertase:
Hydrolyses cane sugar to dextrose or inner sugar, which diffuses into cytoplasm through semipermeable membrane. It is used as a food.
(ii) Zymase:
Converts rest of the sugars into CO2 and alcohol by a process known as fermentation. Energy released in this process is used up by cell in various processes going on within the cell.
4. Reproduction in Yeast:
Yeast reproduces by vegetative, asexual and sexual methods.
1. Vegetative Reproduction:
It takes place by two methods i.e.,
(i) Budding and
(ii) Fission.
(i) Budding:
This is the most common method of reproduction in the yeasts and takes place in favourable environmental conditions. In this process the protoplasm of the cell, covered by a thin membrane, pushes out of the cell wall in the form of a bud and forms a daughter cell.
The bud enlarges and receives the mitochondria which migrate from the parent mother cell much before the nucleus. Meanwhile, the nucleus also elongates and divides into two, and one of the daughter nculei passes into the bud (Fig. 2 A). The bud enlarges until it is separated from the mother cell by a constriction at the base (Fig. 2 B). The two cells are separated first by primary septum of chitin. Soon a secondary septum of glucan is formed.
Bud is separated from the parent cell leaving behind a bud scar (Fig. 2 D). Sometimes, the bud remains attached to the mother cell and it may produce further additional buds. This process is repeated several times and results in the formation of branched or un-branched chains of cells which gives the appearance of a short hypha and is termed as pesudomycelium (Fig. 2 C).
(ii) Fission:
Yeasts are also called fission yeasts because they reproduce by fission e.g., Schizosaccharomyces octosporus. Yeast cells undergo fission by transverse division (Fig. 3 A-C). The parent cell elongates, the nucleus divides and a transverse wall (septum) develops centripetally i.e., from periphery towards centre, thus separating the two daughter uninucleate cells (Fig. 3 D). The transverse wall thickens and then splits into two layers for each daughter cell before they separate (Fig. 3 D).
2. Asexual Reproduction:
Under starved conditions, the nucleus of the yeast cell divides mitotically into four nuclei. Cytoplasm gathers around the nucleus and each develops a thick wall. This structure is called endospore (Fig. 4). The endospores are capable of tiding over unfavorable period and under favourable conditions they again give rise to yeast cells.
3. Sexual Reproduction:
Yeasts reproduce sexually when conditions are un-favourable for vegetative reproduction. The sexual reproduction is very simple and the sexual reproductive organs i.e., the antheridia and oogonia are absent. Yeasts may be homo-or heterothallic and the sexual union takes place either between two somatic cells or between two ascospores which assume the function of copulating gametangia.
Three phenomena characteristic of the sexual process namely plasmogamy, (union of cytoplasmic material) karyogamy (union of nuclear material), and meiosis take place either between two somatic cells or between two gametangia, and produce a zygotic cell. This zygotic cell act as an ascus and produces 4 to 8 ascospores.
Yeasts exhibit following three types of life cycles:
(i) Haplobiontic life cycle e.g., Schizosaccharomyces octosporus.
(ii) Diplobiontic life cycle e.g., Saccharomycodes ludwigii.
(iii) Haplo-diplobiontic life cycle e.g., Saccharomyces cerevisiae.
(i) Haplobiontic life cycle:
The cells of Schizosaccharomyces octosporus are haploid (x), and diploid (2x) phase is very short in the life cycle. The diploid phase is represented only by the zygotic cell. The cells are uninucleate, elongated, homothallic and act as gametangium at maturity (Fig. 5 A).
Two such cells come closer and protrude out short narrow beak-like processes, which meet and the intervening walls between them dissolve to form a conjugation tube or copulation tube (conjugation bridge). The nuclei and the cytoplasmic contents migrate into the conjugation tube where the two fuse to form the zygote (diploid phase).
It directly function as ascus mother cell. The zygote nucleus undergoes a meiotic division followed by a mitotic division forming eight haploid nuclei (Fig. 5 E-I). Meanwhile, the conjugation bridge broadens and the whole structure becomes barrel shaped and is known as ascus (Fig. 5 H).
Thus, the parent cells directly transformed into the ascus. Cytoplasm gathers around each nucleus and each of them develops into an ascospore. Thus, eight haploid ascospores are formed in the ascus (Fig. 5g).Which are liberated by the breaking of the ascus wall. These ascospores behave as independent somatic cells. They enlarge and form daughter cells by fission.
(ii) Oiplobiontic Life Cycle:
The cells of Saccharomycodes ludwigii are diploid (2x), and haploid (x) phase is represented only by ascospores eventually enlarges to function as an ascus (Fig. 6). During sexual reproduction the diploid nucleus divides meiotically forming 4 haploid ascospores (Fig. 6). Two adjacent ascospores of opposite mating type (‘+’ and ‘-‘) fuse (plasmogamy and karyogamy) within the ascus and form a diploid cell.
Thus, the ascospores directly function as gametangia and two diploid cells are produced within the ascus (Fig. 6). The zygote germinates in situ forming tubular outgrowth or germ tube. It pierces its way through the ascus wall and functions as sprout mycelium and produces diploid yeast cells by budding. These buds are separated from the mother cells and repeat the process.
(iii) Haplo-Diplobiontic Life Cycle:
This type of life cycle is represented by Saccharomyces cerevisiae. In this life cycle both haploid (x) and diploid (2x) phases are of fairly long duration because both the haploid and diploid cells are capable of multiplication by budding (Fig. 7 B, G). There are two types of somatic cells in S. cerevisae called ‘Dwarf strain’ and ‘large strain’ cells.
Dwarf strain cells. In haploid phase, the cells are small, spherical and belong to two mating types (‘+’ and “-‘) strains. Due to their small size they are known as ‘Dwarf strain’ yeast cells. The haploid cells function as gametangia (Fig. 7 A-C). They come in contact and finally the gametes of ‘+’ and ‘-‘ strain fuse to form a dikaryon (Plasmogamy). The two nuclei fuse (karyogamy) and the diploid fusion nucleus is called a synkaryon, and the cell containing it is called zygote (Fig. 7 D, E).
Large strain Cells:
The zygote is large in size and lives like vegetative cells. It multiplies by budding (Fig. 7 F, G). The cells produced by budding are diploid, ellipsoidal and larger in size than the ‘dwarf strain’ cells. These cells are known as ‘large strain’ cells and represent diploid phase in the life cycle (Fig. 7 G).
Under suitable conditions these cells divide by budding to increase the number of diploid somatic cells of ‘large strain’ of yeast.
Under unfavorable conditions the diploid cell directly functions as ascus mother cell (Fig. 7 H) and its nucleus divides mitotically to produce four ascospores. Two of these are of ‘+’ strain and the other two are of ‘-‘ strain (Fig. 7 J-K).
Under suitable conditions the ascospores are liberated by breaking of ascus wall. Each ascospore is globose in shape and germinates into a small somatic cell of ‘dwarf strain’ (Fig. 7 L. A). These cells multiply by budding to increase the number of haploid phase, thus completing the life cycle.
Some Other Methods of Copulation in Yeasts:
(1) Adelphogamy:
In Schizosaccharomyces mellacei and S. pombe the copulation takes place between two adjoining sister cells. This is isogamous and the cells destined to copulate do not separate after ‘fission’ but unite to form short chains.
(2) Pedogamy:
In yeasts like Debaromyces, Nadsotiia copulation takes place between the mother and the daughter cell formed by ‘budding’. It is anisogamous type of copulation in which the small daughter cell behaves as the male and the large mother cell acts as the receiving cell.
(3) Parthenogamy:
In species of Zygosaccharomyces, Saccharomyces and Schizosaccharomyces asci are formed without copulation.
5. Economic Importance of Yeasts:
A. Useful Activities:
(i) In Alcoholic Fermentation:
Schwann and pasteur discovered the fermenting quality of yeasts.
Growing yeasts on sugary media produce alcohol and carbon dioxide:
Many species of Saccharomyces used in the production of different types of alcoholic beverages are given in table 1:
(ii) In Baking Industry:
Baker’s yeast (Saccharomyces cercvisiae) is added in the lump of dough (of wheat flour). The fermentation liberates CO2 which forms bubbles and gives the bread its spongy nature. This bread is then baked in oven.
(iii) Vitamin Source:
Yeast cells have high contents of certain vitamins for example:
Vitamin B-complex Saccharomyces cerevisiae
Riboflavin Ashbya gossypii
Vitamin A Rhodotorula gracilis
(iv) Protein Source:
Toridopsis utilis are rich in protein. Some yeasts like Rhodotorula rubra contain 56% protein. So, now-a-days tablets of yeasts are sold for food and stomach disorders.
B. Harmful Activities:
(i) Food spoilage:
Many species of yeast (e.g., Geotrichium candidium) spoil food and produce foul ‘yeasty’ odour.
(ii) Diseases:
Many parasitic species of yeasts causes disease in human and other animals (Table 2).
