Yeast – Structure, Reproduction, Life Cycle and Uses

What is Yeast?

• Yeast, a unicellular eukaryotic organism, belongs to the kingdom Fungi. Exhibiting saprophytic characteristics, it thrives in sugar-rich environments such as fruit juices, nectar, and sugary plant secretions. Distinctly larger than most bacteria, yeast is non-motile, lacking flagella or other locomotive structures.
• Reproduction in yeast manifests through both vegetative and sexual methods. Vegetative reproduction is primarily via budding or fission, while sexual reproduction encompasses haplobiontic, diplobiontic, and haplodiplobiontic life cycles. Industrially, yeast plays a pivotal role in baking and brewing, and also serves as a source for protein, biofuel production, and the commercial synthesis of various enzymes and organic compounds.
• Distinguished as eukaryotic microorganisms, yeasts encompass over 1,500 recognized species, forming about 1% of all described fungal species. Some species exhibit multicellularity, forming pseudohyphae or evolving into multicellular clusters with specialized cellular functions. Yeast sizes vary, typically measuring 3–4 µm in diameter, although some species can expand up to 40 µm. Asexual reproduction in yeasts predominantly occurs through mitosis, with many species employing budding. Their single-celled nature contrasts with mold’s multicellular hyphae growth. Yeasts like Saccharomyces cerevisiae metabolize carbohydrates into carbon dioxide and alcohols via fermentation, a process integral to baking and alcoholic beverage production. S. cerevisiae is also a critical model organism in cell biology research, aiding in the understanding of eukaryotic cell and human biology. Other yeasts, such as Candida albicans, are opportunistic pathogens in humans. Additionally, yeasts are harnessed in microbial fuel cells and ethanol production for biofuels.
• Taxonomically, yeasts do not represent a singular group but are found in both Ascomycota and Basidiomycota phyla. The term “yeast” is frequently synonymous with S. cerevisiae, though this does not reflect the full phylogenetic diversity of yeast species. Historically, the concept of yeast dates back to ancient civilizations, with evidence in Egyptian ruins and other archaeological sites indicating their use in food and beverage production. The first microscopic observation of yeast was by Anton van Leeuwenhoek in 1680, although their classification as fungi was established later by Theodor Schwann in 1837. Louis Pasteur’s research in the 19th century further elucidated yeast’s role in fermentation, highlighting its biological nature over chemical catalysis.
• Commercial exploitation of yeast began in the late 18th century, with strains like S. cerevisiae and S. pastorianus being identified for brewing. The development of refrigeration technology in the 19th century significantly impacted brewing and winemaking, liberating these industries from seasonal constraints and allowing for more controlled production environments.

Definition of Yeast Cell

A yeast cell is a single-celled, eukaryotic microorganism belonging to the fungus kingdom, characterized by its ability to ferment sugars into alcohol and carbon dioxide. It is widely used in baking, brewing, and biotechnological industries. Yeast cells vary in size and shape, reproduce mainly asexually through budding, and are found in diverse environments, particularly those rich in sugars.

Classification of Yeast Cell

Features of Yeast Cell

• Cellular Composition and Structure: Yeast, notably Saccharomyces cerevisiae, is often used synonymously with the term ‘yeast’, but it is just one of many species within the diverse Ascomycota and Basidiomycota phyla. Yeast cells demonstrate high polymorphism, meaning their shapes can vary considerably depending on the environment and the cell’s age. The structure of a yeast cell includes a distinct cell wall, granular cytoplasm, a prominent vacuole, and a nucleus. The cell wall, composed of chitin combined with other chemicals, forms a delicate and thin barrier.
• Vacuole and Cytoplasmic Features: The vacuole within a yeast cell, which can vary in size based on the cell’s activity, plays a significant role in cell metabolism and storage. The cytoplasm contains reserve materials like volutin, glycogen, and oil globules. The relationship between volutin content and cell metabolism is intricate, indicating a close link between these internal components and the cell’s overall functioning.
• Cell Wall Composition: The yeast cell wall is a complex structure comprising proteins, lipids, and at least two types of polysaccharides: mannan and glucan. This composition is crucial for the cell’s integrity and interaction with its environment.
• Cytoplasmic Membrane and Internal Organelles: Beneath the cell wall lies the cytoplasmic membrane, which consists of particles interspersed with fibrils, likely connected to the glucan fibrils in the cell wall. Within the cytoplasm, one finds essential organelles such as the endoplasmic reticulum, mitochondria, and deposits of fat and glycogen, all crucial for various cellular processes.
• Vacuolar Network and Dense Material: Within the large vacuole, surrounded by a single unit membrane, there sometimes exist strands and granules of dense material interconnected in a network. This structure indicates a complex internal organization within the vacuole, contributing to its multifunctional role in the yeast cell.
• Nuclear Structure and Function: The nucleus is encased in a double unit membrane, or nuclear membrane, perforated by pores. This configuration is consistent with other fungi, where the spindle apparatus is intra-nuclear, and the nuclear membrane remains intact during mitosis, highlighting the unique aspects of yeast cell division and genetic material management.

Structure of Yeast

1. Basic Structure and Form: Yeast, as a unicellular organism, exhibits a simple structural form. It sometimes forms a pseudomycelium, where cells cluster in a chain. Yeast cells are typically elliptical, round, or spherical in shape, measuring 3-15 µm in length and 2-8 µm in diameter. The cells display a hyaline (pearl grey) color.
2. Cell Wall Composition: The cell wall of yeast is primarily composed of chitin and mannan protein, providing structural support and protection. Directly beneath the cell wall lies the cytoplasmic membrane, enclosing the cell’s cytoplasm.
3. Cytoplasm and Organelles: Within the cytoplasm, yeast cells house various organelles, including the Golgi apparatus, ribosomes, endoplasmic reticulum, mitochondria, sphareosome, nucleus, and a large central vacuole. Notably, yeast cells do not contain chloroplasts. Volutin granules are present within the cytoplasm, and reserve food materials are stored in the form of glycogen or fats.
4. Distribution and Diversity: Yeasts are omnipresent eukaryotic organisms within the fungus kingdom, estimated to have originated hundreds of millions of years ago. Approximately 1% of the total fungal population consists of yeast, with around 1500 known species, including Saccharomyces cerevisiae and Candida albicans.
5. Cell Wall and Vacuoles: The yeast cell wall is constructed of glycoproteins and polysaccharides, including chitin and mannoproteins. Vacuoles, which occupy about 20% of the cell volume, play a crucial role in protein degradation, nutrient storage, and maintaining cellular homeostasis.
6. Mitochondria Functionality: Yeast mitochondria are integral for energy production, respiration, and homeostasis, generating ATP through oxidative phosphorylation, similar to their role in plant and animal cells.
7. Endomembrane System: This system encompasses ribosomes, the Golgi apparatus, and the endoplasmic reticulum, and is involved in the sorting, packaging, and transport of molecules within the cell.
8. Physical Appearance and Size Variation: Saccharomyces cerevisiae cells typically have a smooth, moist, and either flat or glistening appearance, with a creamish color. Yeast cells are generally larger than most bacteria, with variations in size and shape across different species. They lack flagella and other locomotive organs.
9. Protoplasm and Internal Structure: The protoplasm is enclosed by a cell membrane, containing typical eukaryotic organelles like ribosomes, mitochondria, and the endoplasmic reticulum. The vacuole is usually singular, large, and centrally located within the cell.

Genome structure of Yeast

The genome structure of Saccharomyces cerevisiae, a model organism in genetics and molecular biology, exhibits unique characteristics that have been extensively studied and documented. This expository piece aims to outline the key features of the yeast’s genome structure, emphasizing its complexity and relevance to broader genetic research.

• Chromosomal Composition: The Saccharomyces cerevisiae genome consists of 16 linear chromosomes. These chromosomes are composed of double-stranded DNA, which is a common characteristic of eukaryotic organisms. Each chromosome in this yeast species contains a single, continuous molecule of DNA.
• Genome Size and Gene Content: The total size of the S. cerevisiae genome is approximately 12.1 million base pairs. This genome encodes around 6,275 genes, of which about 5,800 are believed to be functional. These genes encode for various proteins, RNA molecules, and other functional genetic elements.
• Introns and Gene Density: A notable aspect of the yeast genome is the low frequency of introns, with less than 5% of the sequences containing these non-coding regions. This results in a relatively compact genome, where genes are densely packed. The limited number of introns contributes to the efficiency of gene expression and regulation in yeast.
• Ribosomal DNA Repeats: The genome includes regions that encode ribosomal RNA (rRNA), which are essential for ribosome assembly and protein synthesis. These rRNA-encoding sequences are some of the few repetitive elements found in the yeast genome, unlike the genomes of many other eukaryotes which often contain a higher proportion of repetitive DNA.
• Homology with Human Genes: Research has shown that approximately 31% of S. cerevisiae genes have homologous counterparts in humans. This significant level of homology underscores the relevance of yeast as a model for understanding human genetics and molecular biology.
• Genetic Nomenclature and Databases: The genes in the S. cerevisiae genome are classified using specific gene symbols or systematic names, facilitating research and reference. The Saccharomyces Genome Database and the Munich Information Center for Protein Sequences are key resources that maintain and update this genomic information.
• Application in Research: The genome structure of S. cerevisiae has been pivotal in various genetic research areas. Studies utilizing this yeast have contributed to understanding fundamental processes such as gene expression, ribosome assembly, and the functional implications of genetic mutations.

Reproduction of Yeast

Yeast cells employ two primary modes of reproduction: vegetative and sexual. Each mode is adapted to specific environmental conditions and nutritional availability.

1. Vegetative Reproduction:
   • Budding (Gemmation): This is the most prevalent method of yeast reproduction. Under favorable conditions, a yeast cell produces a bud or small outgrowth. This bud enlarges as the nucleus divides, with one nucleus remaining in the mother cell and the other moving to the bud. A constriction then forms between the mother cell and the bud, leading to separation by a transverse wall. Occasionally, pseudomycelium forms, a temporary chain of buds not detached from the mother cell, resembling mycelium. These cells eventually separate to form individual vegetative cells.
   • Fission: In certain yeasts like Schizosaccharomyces (fission yeast), vegetative reproduction occurs via fission. Here, the parental cell elongates, and its nucleus divides into two equal daughter nuclei. A transverse wall then forms, dividing the parent cell into two daughter cells.
2. Sexual Reproduction: Sexual reproduction in yeasts typically involves the formation of ascospores. Under unfavorable conditions, a diploid cell may undergo sporulation, wherein meiotic division produces various haploid spores. These spores can conjugate to form diploid cells again.
3. Classification of Yeast Life Cycles: Based on the vegetative cells involved and the dominant life cycle phase, yeast life cycles are categorized into haplobiontic, diplobiontic, and haplodiplobiontic types.
   • Haplobiontic Lifecycle: Common in fission yeast like Schizosaccharomyces octosporus, the vegetative cell is haploid, and the dominant life cycle phase is also haploid. Under unfavorable conditions, haploid cells conjugate, forming a zygote that develops into an ascus containing a diploid nucleus. This nucleus divides to form eight haploid nuclei, which become ascospores. These spores mature, release, and form new haploid vegetative cells.
   • Diplobiontic Lifecycle: In yeasts like Saccharomycodes ludwigii, the vegetative cell is diploid, and the life cycle is predominantly diploid. Under stress, the diploid cell forms an ascus with a diploid nucleus. Meiosis in the ascus produces four haploid nuclei and subsequently four ascospores. Fusion of nuclei from opposite strain ascospores within the ascus leads to the formation of diploid zygotes, which then develop into diploid vegetative cells.
   • Haplodiplobiontic Lifecycle: This occurs in yeasts such as Saccharomyces cerevisiae, where both haploid and diploid phases are dominant. During unfavorable conditions, haploid cells conjugate to form a zygote, which then develops into a diploid vegetative cell. Under favorable conditions, these diploid cells reproduce by budding. In stressful conditions, they can form an ascus, undergoing meiosis to produce haploid nuclei and ascospores, which eventually lead to new haploid vegetative cells.

life cycle of Saccharomyces cerevisiae – Haplodiplobiontic life cycle

Saccharomyces cerevisiae, a species of yeast, undergoes a haplodiplobiontic life cycle. This cycle is characterized by the existence of both haploid and diploid phases, with each phase playing a significant role in the organism’s life cycle.

1. Somatic Cell Forms:
   • In this life cycle, the somatic cells of the yeast exist in two forms: as haploid dwarf cells and as diploid large cells.
   • The haploid cells are categorized into two mating types, known as “a” and “α”.
2. Reproduction in Haploid Cells:
   • Under favorable conditions, each haploid cell reproduces asexually by budding.
   • These haploid cells lead independent lives, multiplying solely through the budding process.
3. Sexual Reproduction and Gametangia Formation:
   • When the two mating types, “a” and “α”, come into contact, they form structures known as gametangia and initiate the sexual reproduction process.
   • The fusion of these two haploid cells results in the formation of a large fusion cell, a process termed plasmogamy.
4. Formation of the Zygote:
   • Following plasmogamy, the nuclei of the fused cells also merge through a process known as karyogamy, resulting in the formation of a zygote.
   • This zygote then multiplies by budding, producing several diploid cells.
5. Characteristics of Diploid Cells:
   • These diploid cells are larger than their haploid counterparts.
   • Similar to haploid cells, the diploid cells also live independently and reproduce through the process of budding.
6. Response to Unfavorable Conditions:
   • When environmental conditions become unfavorable, the diploid large cells undergo a transformation, becoming spherical and directly acting as ascus mother cells.
7. Formation of Ascospores:
   • The nucleus of the ascus mother cell divides by meiosis, resulting in the formation of four haploid nuclei.
   • Of these nuclei, two belong to mating type “a” and two to mating type “α”.
   • Each nucleus accumulates some cytoplasm around it, forming an ascospore.
8. Ascospore Maturation and Release:
   • Each ascospore within the ascus is a globular, thick-walled structure.
   • These ascospores are eventually released through the rupture of the ascus wall.
9. Germination of Ascospores:
   • Upon release, the ascospores germinate, giving rise to new haploid dwarf cells, thus completing the cycle.

life cycle of Schizosaccharomyces – Haplobiontic life cycle

The haplobiontic life cycle is characteristic of the yeast genus Schizosaccharomyces. This life cycle is predominantly haploid, where the somatic cells exist and multiply in the haploid state.

1. Haploid Cell Multiplication:
   • In this cycle, somatic cells, which are haploid, reproduce asexually by fission. This process results in the production of numerous haploid cells, maintaining the haploid state throughout their vegetative phase.
2. Initiation of Sexual Reproduction:
   • Sexual reproduction is initiated when two sister haploid cells, functioning as gametangia, come into contact. These cells develop beak-like structures as part of the reproductive process.
3. Formation of the Conjugation Tube:
   • The beaks of the two gametangia fuse together, and the wall at the point of contact dissolves. This dissolution forms a canal known as the conjugation tube, facilitating the fusion of the two cells.
4. Zygote Formation:
   • The nuclei of the two haploid cells move towards each other within the conjugation tube and fuse to form a diploid zygote.
   • Subsequently, this zygote functions directly as the ascus mother cell.
5. Meiotic Division in Ascus Mother Cell:
   • The nucleus of the ascus mother cell undergoes meiosis, resulting in the formation of four haploid nuclei. These nuclei then divide by mitosis, producing a total of eight haploid cells.
6. Ascospore Development:
   • Each of the eight haploid nuclei gathers cytoplasm around itself, developing into an ascospore.
   • These ascospores are thick-walled structures formed within the ascus.
7. Release of Ascospores:
   • The ascospores are released through the rupture of the ascus wall, completing the sexual phase of the life cycle.
8. Germination of Ascospores:
   • Following their release, the ascospores germinate to produce new haploid cells. These cells then continue the life cycle, predominantly in the haploid state.

Life cycle of Saccharomyces ludwigii – Diplobiontic life cycle

The life cycle of Saccharomyces ludwigii is characterized as diplobiontic, where the predominant phase of the organism’s life cycle is diploid.

1. Diploid Somatic Cell Multiplication:
   • In this cycle, the somatic cells, which are diploid, primarily multiply through the processes of budding and fission.
   • This method of reproduction ensures the maintenance of the diploid state during the vegetative phase of the organism.
2. Formation of Ascus Mother Cell:
   • Under specific conditions, these diploid somatic cells transition to function as ascus mother cells.
   • The nucleus of each ascus mother cell undergoes meiosis, resulting in the formation of four haploid nuclei.
3. Development of Ascospores:
   • These haploid nuclei accumulate cytoplasm around themselves, transforming into ascospores.
   • Each ascus typically contains four ascospores.
4. Mating Types of Ascospores:
   • Among these four ascospores, two are of mating type “A1” and the other two are of mating type “A2”.
5. Fusion and Formation of Diploid Zygote:
   • Within the ascus, the two mating types of ascospores fuse to form a diploid zygote.
   • This fusion represents the sexual phase of the life cycle.
6. Germination of the Diploid Zygote:
   • The diploid zygote then germinates within the ascus, producing a germ tube.
   • This germ tube eventually breaks through the ascus wall.
7. Formation of Diploid Sprout Mycelium:
   • Once outside the ascus, the germ tube functions as diploid sprout mycelium.
   • This sprout mycelium is critical for the next phase of growth and development.
8. Budding of Sprout Mycelium:
   • The diploid sprout mycelium gives rise to sprout diploid cells through the process of budding.
   • These sprout diploid cells then develop into new diploid somatic cells, continuing the life cycle.

Uses of Yeast

• Significance in the Alcohol or Brewery Industry:
  • Yeast, particularly species such as Saccharomyces cerevisiae and S. ellipsoidens, plays a pivotal role in the production of alcoholic beverages. These yeasts are employed in the fermentation process to produce drinks like beer, wine, whiskey, rum, gin, vodka, and brandy.
  • The yeast cells metabolize sugars to produce ethanol, which is the primary alcohol in these beverages, and also contribute to the flavor and aroma profiles of these drinks.
• Application in the Baking Industry:
  • Saccharomyces cerevisiae is also the primary yeast used in baking. Its role is to ferment sugars present in dough, producing carbon dioxide gas and ethanol.
  • This fermentation process causes the dough to rise, resulting in the soft and fluffy texture of baked goods such as bread, doughnuts, and cakes.
• Role as a Flavoring Agent:
  • Yeast is crucial in the fermentation of cocoa, which is a key ingredient in chocolate production. The fermentation process enhances the flavor profile of the chocolate, making it more appealing to consumers.
• Biofuel Production:
  • Yeasts, particularly Saccharomyces cerevisiae, are utilized to produce biofuel. They convert sugar substrates into ethanol, which can be used as a renewable fuel source in vehicles.
  • The use of yeast in biofuel production contributes to the reduction of crude oil consumption, making it a more sustainable and environmentally friendly energy option.
• Source of Protein:
  • Yeast serves as a significant source of protein, often referred to as single-cell protein. It supplements dietary protein needs, offering an alternative to traditional sources like milk and meat.
  • This aspect of yeast is particularly valuable in providing nutritional supplements in areas where conventional protein sources may be scarce or expensive.
• Enzyme Production:
  • Saccharomyces cerevisiae is instrumental in the commercial production of various enzymes, such as invertase. These enzymes have numerous applications in different industries, including food processing and pharmaceuticals.
• Production of Organic Compounds:
  • Beyond its role in food and energy, Saccharomyces cerevisiae is also used for synthesizing organic compounds like acetic acid, lactic acid, and glycerol. These compounds have diverse applications in various industries, ranging from food additives to cosmetics.

Examples of Yeasts

1. Saccharomyces cerevisiae: Commonly known as baker’s or brewer’s yeast, it is widely used in the baking and brewing industries due to its ability to ferment sugars, producing carbon dioxide and ethanol. It’s also a model organism in molecular and cellular biology studies.
2. Candida albicans: This species is part of the normal flora in human mucous membranes but can cause infections, known as candidiasis, especially in immunocompromised individuals. It’s studied extensively in medical mycology.
3. Schizosaccharomyces pombe: Also known as “fission yeast,” this species is important in molecular biology and genetics research. Unlike Saccharomyces cerevisiae, which divides by budding, it divides by fission, making it a valuable model for studying cell division and genetics.
4. Saccharomyces pastorianus: This yeast is used in brewing, particularly in the production of lager beers. It’s a hybrid species known for its cold-tolerant fermentation properties.
5. Pichia pastoris: Often used in biotechnology, this yeast is significant in the production of recombinant proteins. It’s known for its ability to grow to high cell densities and is used in the expression of genetically engineered proteins.
6. Kluyveromyces lactis: This yeast is used in dairy fermentation and is notable for its ability to ferment lactose. It’s also used in genetic studies and biotechnological applications, such as enzyme production.
7. Cryptococcus neoformans: This species is a pathogenic yeast known for causing cryptococcosis, particularly in immunocompromised individuals, including those with HIV/AIDS. It’s significant in medical research focused on fungal pathogens.
8. Brettanomyces bruxellensis: Known for its role in the wine industry, this yeast can impact the flavor of wines, often producing distinctive and sometimes undesirable flavors.
9. Rhodotorula: A genus of pigmented yeasts, Rhodotorula species are found in various environments and can be involved in food spoilage but also have potential in biotechnological applications like bioremediation.

References

• Monroy Salazar, Humberto & A.Z.M., Salem, & Kholif, Ahmed & Monroy, H. & Pérez, L.S. & Zamora, J.L. & Gutiérez, A.. (2016). YEAST: DESCRIPTION AND STRUCTURE.
• https://www.studysmarter.co.uk/explanations/biology/microbiology/yeast-cell/
• https://milnepublishing.geneseo.edu/botany/chapter/yeast/#