Yeast : Intro ,Reproduction and Economic Importance

Yeast is a saprophytic fungus.

Classification

Kingdom: Fungi

Division: Mycophyta

Class: Ascomycetes

Order: Endomycetales

Family: Saccharomycetaceae

Genus: Saccharomyces

Species: cerevisiae

Occurrence

Yeast is a saprophytic fungus. Saccharomyces cerevisiae is found in sugary rich substances like rotten fruits, rotten juice and other organic substances containing sugar. It is commonly known as baker's yeast and brewer's yeast because in the baking industry it is used to make the bread soft and porous and it is used in the brewing industry for the preparation of ethyl alcohol.

Alcoholic fermentation

It is the incomplete breakdown of glucose into ethyl alcohol and carbon dioxide in an anaerobic condition in the presence of enzymes invertase and zymase.

C₆H₁₂O₆ → 2 C₂H₅OH + 2 CO + heat energy

Structure

Saccharomyces cerevisiae is a unicellular eukaryotic fungus. The cell structure is round or oval in shape. The cell wall is made up of chitin. It contains all double membrane-bound cell organelles like ER, Golgi body, mitochondria, ribosomes etc. There is the presence of centrally located large vacuole covered by tonoplast. Reserved food material is glycogen, volutin granule, and oil droplets.

Reproduction in Yeast

Yeast reproduces both by vegetative, asexual and sexual methods

Vegetative reproduction

Vegetative reproduction takes place by budding and fission.

Budding

Saccharomyces cerevisiae reproduces by budding process in favourable condition.In this process parent cell develop short outgrowth known as bud and nucleus divides in two nuclei. One of them moves towards the bud. When bud enlarges in size their become large constriction between bud and parent cell due to which bud gets detached from parent cell and live as a new yeast cell.

In most favourable conditions, budding process takes place very fast as a result parent cell develops one bud which again develop another bud without detaching from parent cell and a chain like structure is formed which is called pseudo mycelium

Fission

In Schizosaccharomycesfission takes place in a favourable condition. In this process parent cell become elongated and transverse wall is formed at centre due to which parent cell divides into daughter cell and each daughter cell live at new yeast cell.

Asexual reproduction

It takes place by the formation of endospores during unfavourable conditions. During this process, the protoplast of the vegetative cells divides into four parts. Each part later becomes surrounded by a thick wall, and act as an endospore.It remains dormant to withstand the adverse conditions. On the return of favourable conditions, the endospores germinate to produce chains of yeast cells.

Sexual reproduction

Haploid diplobiontic life cycle

In Saccharomyces cerevical haploid diplobiontic life cycle is found. In this lifecycle, haploid and diploid phases are equally represented so, this lifecycle is called haploid diplobiontic lifecycle.

A haploid somatic cell can act as gametangia under the scarcity of water and food supply. Fusion of gametangia takes place and a diploid zygote is formed. Zygote changes into a diploid somatic cell and can reproduce by the budding process. Some diploid somatic cells can act as mother cell in which meiosis cell division takes place and four haploid nuclei are formed. This four haploid nuclei changes into Ascospores. Ascospores come out by bursting ascus wall and change into a haploid somatic cell. A haploid somatic cell can reproduce by the budding process and some haploid cell again act as gametangia. In this process, haploid and diploid phase is equally dominant or represented.

Haplobiontic lifecycle

Haplobiontic lifecycle is found in Yeast(Schizosaccharomyces endosporous).In this lifecycle, the haploid phase is more represented than the diploid phase.

Diplobiontic lifecycle

Diplobiontic lifecycle is found in yeast(Saccharomyces ludwigii). In this lifecycle, the diploid phase is more represented than the haploid phase.

Economic Importance of Yeast

Yeast exhibits both beneficial and harmful activities to human beings.

Advantages of Yeast

• Yeast takes part in the alcoholic fermentation of sugar molasses to alcohol and carbon dioxide.
• They are used in baking industries to make bread.
• They are used in the synthesis of protein, fat, vitamins and enzymes.
• Yeast Vite tablet is used as food value.
• Along with bacteria, the yeast gives characteristic flavour in coca beans.
• Some yeast cells are used in the treatment of skin diseases.

Disadvantages of Yeast

• Yeast spoils food, vegetables and fruits.
• They cause many plant and animal diseases.

Economic Importance of Fungi

It also posses both useful and harmful activities to the human beings.

Food:

Agaricus composters, Amanita vaginata are used as nutrient-rich food.

Used in baking industries, brewing industries:

Saccharomyces cerevisiae is used in baking industries for making the bread soft and in making alcohol.

Enzymes production:

Saccharomyces cerevisiae is used for the production of Invertase zymase.

Aspergillus niger? Protease, Amylase

Antibiotic preparation:

Penicillium notatum? Penicillin

Aspergillusproliferans? Proliferin

Organic acid preparation:

Citric acid? Aspergillus niger

Oxalic acid? Penicillium glaucas

In hormone production:

One of the most important plants growth hormone, gibberellin is produced by the fungus Gibberella fljikuroi.

In study:

Some fungi like Neurospora are exclusively used in different genetical studies.

Harmful effect

In spoilage of food

Several fungi are responsible for food spoilage in nature. Eg; Rhizopus nigricans spoils stored sweet potatoes. Yeast spoils the tomato.

Causes plant diseases

Several fungi destroy many agricultural crops, fruits, nut plants ornamental plants and shade trees.

Mushroom poisoning

Sometimes poisonous mushrooms like Amanita phalloides, A.vernaetc are mistaken as edible mushrooms and as a consequence, the person pops off.

A variety of fungi causes several diseases of man and animals. The names of the diseases are named after the affected organs, causal agents, discoverers, localities of occurrence etc.