Yeast


 * Yeast: //Saccharomyces// **

Figure 2.1: Yeast Cells Underneath an Electron Microscope

**1**. __**Classification/ Diagnostic Characteristics:**__

Yeast is a member of the Kingdom of Fungi and is part of the ascomycota and basidiomycota phylum.
 * Ascomycota: form the subkingdom Dikarya along with basidiomycota, characterized by their microscopic sac-like structure. In sexually reproducing ascomycota, t his structure is the site at which ascospores (sexually reproduced fungal spores) are formed.
 * Basidiomycota: filamentous fungi that reproduce by forming club-shaped end cells named basidia. Basidiomycota other than yeast have hyphae, the branches of filament which comprise the mycellium.
 *  Dikarya: fungi in this subkingdom produce a feature in their nuclei called dikaryon after plasmology in sexual reproduction, two compatible nuclei of two separate cells divide in synchrony.

This fungus is a Eukaryotic microbe, is single-celled, and oval-shaped. Most yeast are unicellular, although some can be multicellular. Yeast is a facultative anaerobe; it can utilize oxygen to create ATP when it is present, and will switch to fermentation in an anaerobic environment. Yeast's size can vary greatly depending on species, usually they are 3-4 µm in diameter, although some have been noted to reach over 40 µm in diameter.

Figure 2.2: Anatomy of a Budding Yeast Cell

**2**. __**Relationship to Humans:**__

Yeast is an indispensable factor in the fermentation process for many of the foods humans consume. Certain strains of yeast are used in alcoholic fermentation and conversion of sugars into carbon dioxide and ethanol.

The creation of alcoholic beverages includes the use of yeast. The energy source for yeast in the fermentation of alcohol is the breaking down of starches within germinated seeds. Using this energy, yeast can produce alcohol in environments without oxygen. Yeast is found on the outer layer of a grape’s skin. When grapes are compressed to make wine, the yeast on their skin converts the natural sugars into alcohol (ethanol). In anaerobic environments, yeast will produce more alcohol through fermentation than in aerobic environments. Yeast breaks down natural sugars to produce alcohol; the lesser the final sugar content there is an such a solution, the higher the alcoholic content as these two variables have an indirect relationship with one another.

Bread, a staple of the modern diet, is baked with yeast. The rising of leavened bread is due to the activity of yeast, the sugars found in flour feed the fungus which expels carbon dioxide as a result. Starch is found naturally in the seeds of wheat and is converted into sugar as water is added to a solution. The accumulation of carbon dioxide which forms bubbles, causes the bread to expand and rise. The yeast used in bread cannot thrive without oxygen <span style="font-family: Arial,Helvetica,sans-serif; font-size: 110%;">. <span style="font-family: Arial,Helvetica,sans-serif; font-size: 110%;">Yeast is used in industries other than the food industry, such as the biofuel industry to produce ethanol fuel. Yeast may as well be used to bioremediate, working as a agent to degrade harmful hydrocarbons such as palm oil, mill effluent, or TNT.

<span style="font-family: Arial,Helvetica,sans-serif; font-size: 110%;">Figure 2.3: Yeast Fermentation

<span style="font-family: Arial,Helvetica,sans-serif; font-size: 110%;">**3.** __**Habitat and Niche:**__

<span style="font-family: Arial,Helvetica,sans-serif; font-size: 110%;">Yeast can be found in various sources in nature, including plants, flowers, fruits, and soil. The skin and intestinal tract of warm-blooded organisms harbor yeast colonies. Many strains of yeast are also found in the stomachs of bees, an insect that is a member of the kingdom Animalia. A suitable environment for yeast includes sugar that the microbe uses to make the ATP needed to thrive in its niche. Yeast is extracted from natural sources and cultivated for its use in food production.

<span style="font-family: Arial,Helvetica,sans-serif;">A common integumentary infection, commonly referred to as a yeast infection, is caused by //Candida albicans//. This strain is prevalent in the gut of most people (80%) and has a commensalistic relationship with its host, though at abnormally high concentrations may cause such an infection.

<span style="font-family: Arial,Helvetica,sans-serif; font-size: 110%;">**4.** __**Predator Avoidance:**__

<span style="font-family: Arial,Helvetica,sans-serif; font-size: 110%;">As a member of the Kingdom Fungi, yeast has cell walls made of chitin and polysaccharides. The glucose in substances that yeast utilize for energy are used to build the walls of chitin which protect the fungal cell. As a member of the subkingdom Dikarya, yeast do not have flagellum and thus lack the structural components necessary for movement. Yeast are immotile and lack any defense mechanisms, making them especially susceptible to predation.

<span style="font-family: Arial,Helvetica,sans-serif; font-size: 110%;">**5**. __**Nutrient Acquisition:**__

<span style="font-family: Arial,Helvetica,sans-serif; font-size: 110%;">The nutrients yeast needs are absorbed directly across the cell’s surface. Yeast are absorptive heterotrophs, releasing digestive enzymes to break down sugars in their environment and taking in the monomers back into the plasma membrane. The lipid bilayer of yeast are selectively permeable to ions and molecules, with proteins scattered about to allow for substances to be transported through channels. Lipids are soluble to other lipids and repel water, thus lipids may pass with ease through the cell membrane.

<span style="font-family: Arial,Helvetica,sans-serif; font-size: 110%;">Using sugars, including maltose and glucose, the mitochondrial complex of the yeast cell is responsible for converting them into ethanol and carbon dioxide. Yeast depend on its environment to supply the nutrients that are essential to its function. The surface to volume ratio which is critical in nutrient intake, is large in yeast, allowing for greater absorption. In regards to nutrient intake, surface area and volume have an indirect relationship with one another, for the greater the nutrient intake, the greater the surface area and smaller the volume.

<span style="font-family: Arial,Helvetica,sans-serif; font-size: 110%;">**6.** __**Reproduction and Life Cycle:**__

<span style="font-family: Arial,Helvetica,sans-serif; font-size: 110%;">As fungi, yeast may reproduce in either an asexual or sexual manner.

<span style="font-family: Arial,Helvetica,sans-serif; font-size: 110%;">Asexual reproduction for yeast includes two possibilities. <span style="font-family: Arial,Helvetica,sans-serif; font-size: 110%; line-height: 1.5;"> Reproduction through budding entails that a daughter cell grows from the parent and eventually splits off, along with an identical copy of its genetic material. The budding cell along with the parent cell grow and undergo a checkpoint ensuring that it is ready for genome replication. In the “S” phase of the cell cycle, the genome is replicated. In G2 phase, the nucleus for the daughter cell emerges from the parent cell’s nucleus, and shifts to the budding cell. “M” phase concludes the process of budding, as the two cells become separated from one another.

<span style="font-family: Arial,Helvetica,sans-serif; font-size: 110%;">Through fission, a yeast cell produces two identical daughter cells. The DNA molecule of the parent is replicated, and each respective copy moves toward a different part of the cell from which it pulls away, separating the chromosome of the daughter cells and that of the parent.

<span style="font-family: Arial,Helvetica,sans-serif; font-size: 110%;">Sexual reproduction in fungi is uncommon. Two or more mating types, which are genetically determined by the expression of genes at the mating locus, may fertilize one another. Individuals of the same mating type cannot mate with another, making self-fertilization impossible. Meiosis is triggered by environmental stressors, when this comes into effect, a dipolid yeast cell will split into two a type cells, and two <span style="background-color: #ffffff; color: #444444; font-family: arial,sans-serif; font-size: small;">α <span style="font-family: Arial,Helvetica,sans-serif; font-size: 1.066em; line-height: 1.5;">type cells.



Figure 2.4: Yeast Asexual Reproduction

<span style="font-family: Arial,Helvetica,sans-serif; font-size: 110%;">**7**. __**Growth and Development:**__

====<span style="font-family: Arial,Helvetica,sans-serif;">As haploids, yeast cells may live as either of two mating types, a or α. Haploid yeast cells have the capability to function independently, though when the opposite mating types come into contact, they may fuse together to form a diploid cell. ====

====<span style="font-family: Arial,Helvetica,sans-serif;">Yeast cells grow during the process of fermentation through the usage of ATP synthesized from nutrients it takes in. Yeast cells will grow due to an abundance of nutrients and will reproduce fruitfully. Throughout the life cycle of a yeast cell, the daughter cells may become as large as their predecessors and eventually replicate on their own. ====

====<span style="font-family: Arial,Helvetica,sans-serif;">The yeast //Candida// become dimorphic because it grows in an oval budding shape but under certain circumstances it will continue to elongate to create these structurally long filament like strands called pseudohyphae or true hyphae which are shorter strands that aren't all linked together. These structures help to invade and delve deeper in tissue after it has colonized the epithelium layer. ====

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 * <span style="font-family: Arial,Helvetica,sans-serif; font-size: 110%; line-height: 1.5;">8 **<span style="font-family: Arial,Helvetica,sans-serif; font-size: 110%; line-height: 1.5;">. __<span style="font-family: Arial,Helvetica,sans-serif; font-size: 110%; line-height: 1.5;">**Integument:** __

<span style="font-family: Arial,Helvetica,sans-serif; font-size: 110%;">The cell walls of yeast, which are about 100 to 200 nanometers thick, are made of chitin and other polysaccharides. Glucan, the main polysaccharide found in yeast cell walls, is responsible for the rigid characteristic of the cell wall. Mannans and <span style="background-color: #ffffff; display: block; font-family: Arial,Helvetica,sans-serif; line-height: 1.5; text-align: justify;">N-acetylglucosamine <span style="display: block; font-family: Arial,Helvetica,sans-serif; font-size: 110%; line-height: 1.5; text-align: justify;">, other types of polysaccharides, are also present in chitin. Proteins, lipids, and inorganic phosphate are present in smaller quantities in the cell walls. The cell membrane of a yeast cell, which is about 7 nanometers thick, is made up of hydrophobic heads and hydrophillic tails. The membrane is a lipid bilayer interspersed with a variety of proteins that act as cytoskeleton anchors, transport proteins, and receptor proteins.

Figure 2.5: Fungal Cell Membrane

<span style="font-family: Arial,Helvetica,sans-serif; font-size: 110%;">**9.** __**Movement:**__

<span style="font-family: Arial,Helvetica,sans-serif; font-size: 110%;">Yeast cells are incapable of moving on their own, barring them from a scientific classification as a living organism. Lacking the organelles essential to projectile motion, namely flagella and cilia, yeast cells cannot move without an external force.

<span style="font-family: Arial,Helvetica,sans-serif; font-size: 110%;">**10.** __**Sensing the Environment:**__

<span style="font-family: Arial,Helvetica,sans-serif; font-size: 110%;">The environment of a yeast cell plays a significant role in its functioning. In a stressful environment, diploid yeast cells will respond by creating haploid spores and sexually reproducing. A dearth of nutrients in the environment will set off a sensor in the yeast cell to signal for meiosis through a process called sporulation. The resulting haploid spores can float to a new location, hopefully to one with more nutrients and mate, reforming the diploid.

<span style="font-family: Arial,Helvetica,sans-serif; font-size: 110%;">**11.** __**Gas Exchange:**__

<span style="font-family: Arial,Helvetica,sans-serif; font-size: 110%;">A product of fermentation by yeast is carbon dioxide. This gas is diffused directly through the thin and porous membrane into the surrounding environment. When yeast cells use oxygen, this gas enters the cell from the surrounding environment through the membrane, which has a large surface area to volume ratio to allow for greater gas exchange.

<span style="font-family: Arial,Helvetica,sans-serif; font-size: 110%;">**12.** __**Waste Removal:**__

<span style="font-family: Arial,Helvetica,sans-serif; font-size: 110%;">The potential waste products of yeast fermentation, carbon dioxide, water, and ethanol, are secreted out of the cell in packaged vacuoles. These vacuoles diffuse through the cell membrane and enter the external environment. The vacuoles are lipid-soluble and can readily pass through the fungal cell's lipid bilayer. Through exocytosis, in which the vacuole's membrane fuses with that of the cell, the contents of the vacuole are released into the surrounding environment.

Figure 2.6: Yeast Cell and Vacuole

<span style="font-family: Arial,Helvetica,sans-serif; font-size: 110%;">**13.** __**Environmental Physiology:**__

<span style="font-family: Arial,Helvetica,sans-serif; font-size: 110%;">As facultative anaerobes, yeast may respire in environments that have oxygen, and switch seamlessly to fermentation without oxygen. The environment determines the way in which yeast cells will reproduce and create ATP for energy.

<span style="font-family: Arial,Helvetica,sans-serif; font-size: 110%;">Fermentation is a temperature-sensitive process, occurring at cooler temperatures between 10 and 37 degrees celsius. Below this range, yeast cells do not have enough thermal energy to carry out chemical processes in a timely fashion, and will fail to reproduce. Above this temperature range, the structure of yeast cells will decompose and the cells will cease to function.

<span style="font-family: Arial,Helvetica,sans-serif; font-size: 110%;">Yeast cells are incredibly resilient to environmental stresses, including pollutants, salt and osmotic imbalances. Fungi as a kingdom are noted to withstand immense levels of pollutants and may be used to bioremediate.

<span style="font-family: Arial,Helvetica,sans-serif; font-size: 110%;">**14**. __**Internal Circulation:**__

<span style="font-family: Arial,Helvetica,sans-serif; font-size: 110%;">Yeast do not have internal circulation systems. The small size of a yeast cell allows for nutrients to move throughout the cell without any barriers that would necessitate a circulation system. After the yeast cells cease to perform their function, the waste products are released via vacuoles into the surrounding environment.


 * <span style="font-family: Arial,Helvetica,sans-serif; font-size: 110%;">15. __Chemical Control:__ **

<span style="font-family: Arial,Helvetica,sans-serif; font-size: 110%;"> As yeast is a single celled organism, it has no endocrine system, though the substances which enter and exit the cell are regulated by its cellular membrane. Yeast cell receptors on the membrane signal to the cell body the movement of substances throughout.


 * <span style="font-family: Arial,Helvetica,sans-serif; font-size: 110%;">16. __Review Questions:__ **


 * 1) <span style="font-family: Arial,Helvetica,sans-serif; font-size: 110%;">What are the functions of yeast in the processes of making alcoholic drinks and bread?
 * 2) <span style="font-family: Arial,Helvetica,sans-serif; font-size: 110%;">How is yeast able to consume and dispose of important nutrients and bodily byproducts?
 * 3) <span style="font-family: Arial,Helvetica,sans-serif; font-size: 110%;">Explain fermentation. Is fermentation aerobic or anaerobic? Explain.
 * 4) <span style="font-family: Arial,Helvetica,sans-serif; font-size: 110%;">What activity in yeast is useful for baking?
 * 5) Could fermentation occur in an environment that is 8 degrees Celsius? why or why not, what factor(s) influence this?

<span style="font-family: Arial,Helvetica,sans-serif; font-size: 110%;">__**Sources**__**:**

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 * 5) <span style="font-family: Arial,Helvetica,sans-serif; font-size: 110%; line-height: 1.5;">http://faculty.ccbcmd.edu/courses/bio141/lecguide/unit3/fungi/yeast.html
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 * 9) <span style="background-color: #ffffff; font-family: 'Times New Roman',Times,serif; font-size: 16px; line-height: 1.5;">Hillis, David M., David Sadava, H. C. Heller, and Mary V. Price. //<span style="background-color: #ffffff; font-family: 'Times New Roman',Times,serif; font-size: 16px; line-height: 1.5;">Principles of Life //<span style="background-color: #ffffff; font-family: 'Times New Roman',Times,serif; font-size: 16px; line-height: 1.5;">. High School ed. Sunderland: Sinauer Associates, 2012. Print.
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