Saccharomyces (Yeast)

Saccharomyces cerevisiae

Commonly employed in bread making and beer brewing.
Hence popularly called the brewer’s or baker’s yeast

Thallus 
Thallus is non-mycelial
It is not made up of hyphae
Consists of a single minute oval or spherical cell.
In some species yeast cell is elongated, cylindrical or rectangular in shape.
In size ranges from 2 to 8µ in diameterby 3 to 15µ in length.
Individual the yeast cell appears hyaline in colour.

Cell structure
The cell wall is thin, delicate, frim.
It is made up to two polysaccharides, namely, glucans and mannanB in combination with traces of protein, lipid and chitin.
There is no cellulose.
The cytoplasm is differentiated into two portions, the outer and the inner.
The outer portion is a thin cytoplasmic membrane (ectoplasm).
The inner portion is dense or granular. It is endoplasm
The endoplasm contains a single, small dense nucleus.
Occupying much of the space in the yeast cell is a single large vacuole.
The vacuole contains a solution called volutin.
Volutin is a complex material consisting of RNA, lipoprotein and poly-phosphates.
The reserve food product are in form granules of glycogen, oil globules.

Nutrition
They are saprophytes.
Obtains nutrition in form of simple sugars in solution.
It gets it from bruised ripe fruits, fruit juices, sugar solutions or crushed moistened grains.
The yeast cells secrete enzymes called the zymase.
The zymase changes the starch or complex sugars into simple sugars.
The latter diffuse into the endoplasm through thin, delicate cell membrane.
A small percentage of the absorbed sugar is used as food and assimilated.
When growing in a well aerated medium the rest of the sugar is completely oxidized into carbodioxied and water.
In absence or poor supply of oxygen the major portion of sugar is converted into carbondioxide and ethyl alcohol.
This is called alcoholic fermentation.
The products of this reaction (carbondioxide and ethyl alcohol) diffuse out into the surrounding liquid.
This process is utilized in the production of industrial alcohol, in wine making, in brewing and in bread making.

Reproduction
Vegetative reproduction

Budding
Under favourable conditions yeast exclusively reproduce by this method.
A small portion of cell wall, at or near one pole softens and thins.
The protoplast in this region, covered by the thin softened membrane, bulges out in the form of an outgrowth or protuberance.
The protuberance gradually increases in size.
It is known as the bud.
As the bud is forming, the nucleus of the parent yeast cell divides .
One of these daughter nuclei migrates into the enlarging bud.
The bud grows and becomes constricted at the base.
A double cross wall form and the two cells separate leaving a scar on the both cells.
The detached bud grows and starts budding again.
A new bud arises on the surface of the parent cell at the opposite end to the first and not from the same area.
This is know as bipolar budding.
In presence of abundant food the process of budding is quickened.
It becomes so rapid that the buds often produce new buds before separation from the mother cell.
This process is repeated.
This results in the formation of branced or unbranced chains of cell constituting the pseudomycelium.
Soon the chain breaks and cells are separated from each other.

Fission
Vegetative reproduction by fission has also been reported but it occurs in some other yeasts and not in Saccharomyces. 
They are called the fission yeasts.
The mother cell elongates. 
The nucleus divides into two.
The daughter nuclei move apart.
Meanwhile a ring-like ingrowth appears at the wall of the yeast cell in the middle.
It grows inward toward the centre of the cell.
Finally it stretches across the cell forming a complete partition called the septum.
The septum thickens and then splits into two layers, one for each daughter cell before they separate.

Sexual reproduction
No sex organs are produced.
Sexual process is extremely simplified.
It consists of three phenomenoa characteristic of the sexual process, namely plasmogamy, karyogamy and meiosis.
Haplo-Diplobiontic life cycle.
In a culture of brewer’s yeast there occur intermixed two kinds of somatic cells, namely, small dwarf strain and large strain cells.
Dwarf strain cells are small haploid cells.
Under normal conditions these multilply by budding and increase in number.
When food supply is scarce they enter a dormant phase and become resistant.
Under normal conditions and presence of the small haploid cells of opposite mating strain they function as gametangia and resort to gametangial conjugation.

Gametangial conjugation
During conjugation process the + and – strain of haploid dwarf cells agglutinize and form clusters.
The two haploid cells of the opposite mating types bend towards each other and fuse to form a conjugation bridge.
Plasmogamy
The fusion between the protoplasts of the + and – strain takes place through the conjugation bridge. 
It is called plasmogamy and a dikaryon is formed.

Karyogamy
The two nuclei of the dikaryon finally fuse. 
The diploid nucleus is called the synkaryon and the cell containing it is the zygote.

Zygote
The zygote formed is in fact the large strain yeast cells.
They are diploid, larger in size.
Under favourable conditions they multiply by budding and increase there number.
During unfavourable conditions, the diploid large strain yeast cells (Zygotes) resort to ascospore formation.
They become spherical in shape and directly function as asci
Ascospore formation
The ascus mother cell is spherical in shape.
Diploid nucleus of the ascus mother cell undergoes meiosis and forms four haploid nuclei.
An envelope encloses each of the four nuclei together with some cytoplasm and form 4 globose ascospores.
Two of these are of + strain and two – strain.
On the onset of favourable conditions, the ascospores are released due to rupture of ascus wall.
The mature ascospore is a thick-walled, globose structure.
The ascospore wall is differentiated into 3 layers.
Ascospore germinates and form a new mycelium.
In this both haplophase and the diplophase are of equal importance.
Hence it is called as Haplo-diplobiontic life cycle.

Diplobiontic life cycle
Diploid vegetative cell directly functions as ascus mother cell.
In this the diploid nucleus undergoes meiosis and form 4 haploid nuclei.
These four haploid nuclei transforms into 4 ascospores.
Two of these of + strain and two are of - strain
Ascospores are not released from the ascus.
Sexual fusion take place within the ascus between the ascospores of opposite strain.
Consequently two diploid zygotes are produced.
Each zygote germinates in situ to form a small germ tube.
The germ tube grows and emerges through the ascal wall and functions as a sprout mycelium.
The cells of the sprout mycelium budd off diploid cells.
Soon the diploid sprout cells get severed from the parent cells.  
These diploid sprout cells functions as acus mother cells.
In this type, the haplophase (gametophyte) is extremely reduced and diplophase is prolong.
Hence, this type of life cycle is called diplobiontic life cycle.
Ex. S. ludwigii

Haplobiontic life cycle
In this the vegetative cells are elongated, uninucleate and haploid.
Each haploid somatic cell is a potential gametangiu.
At the time of sexual reproduction two somatic cells come to lie side by side.
A conjugation tube is formed between the two cells.
The nuclei of the two conjugating cells migrate into the conjugation tube and fuse.
The diploid nucleu or zygote functions directly as an ascus mother cell.
Its diploid nucleus immediately undergoes meiotic division followed by a mitotic division.
This results in formation of 8 eight haploid nuclei.
These 8 nuclei get transformed into 8 ascospores.
The ascus mother cell with eight ascospores is called an asucs.
The ascal wall ruptures and the ascospores are released.
The liberated ascospores on germination, enlarges and behaves like a somatic cells.
In this case the diplophase is very short.
It is represented only by a zygoe cell which undergoes meiosis immediately after karyogamy.                                                                 
The haplophase is long.
Hence this is called as Haplobiontic life cycle.
Ex. Schizosaccharomyces octosporus.