YEASTby Clara Mao
Yeast.jpg
(SL) (15)



Table of Contents:
1. Classification/Diagnostic Characteristics
2. Relationship to Humans
3. Habitat and Niche
4. Predator Avoidance
5. Nutrient Acquisition
6. Reproduction and Life Cycle
7. Growth and Development
8. Integument
9. Movement
10. Sensing the Environment
11. Gas Exchange
12. Waste Removal
13. Environmental Physiology
14. Internal Circulation
15. Chemical Control
16. Review Questions
17. References


Classification/Diagnostic Characteristics

Domain: Eukaryota
Kingdom: Fungi
Phylum: Ascomycota
Class: Saccharomycetes
Order: Saccharomycetales
Family: Saccharomycetaceae
Genus: Saccharomyces
(16) (SL)

Yeasts are eukaryotic, unicellular, free-living fungi, meaning that they have membrane bound organelles and digest their food outside their bodies. The term "yeast" does not refer to a specific taxonomic group but rather to the behavioral characteristic of yeasts to live in moist or liquid environments and to absorb nutrients directly across their cell surfaces. As such, there are over 1,000 yeast species that fit this description. The diameter of a yeast cell is anywhere from 2 µm to 10 µm. Yeast can live either in haploid or diploid state, depending on the environmental conditions. In unfavorable conditions, diploid yeast cells will undergo sporulation to produce haploid cells. Yeast are facultative anaerobes, meaning that they can survive in environments with or without oxygen. The species that humans tend to associate with the word yeast is Saccharomyces cerevisae, or baker's yeast, which is used in baking and beer brewing. (3)

Yeasts are eukaryotes characterized by their enclosed nucleus with a double DNA strand, as well as organelles that perform different functions. They are also single-celled, heterotrophic, fungi organisms, meaning that they cannot fix their own carbon and must obtain it through autotrophic organisms. (7) Fungi are characterized by their mix of traits that belong to plants and animals. Fungi consume organic matter and have an enclosed nuclei like both animals and plants but don't have chlorophyll like plants or the ability to move like animals. (8) (RG)


Relationship to Humans

The most commonly know yeast is Saccharomyces cerevisiae, or baker's yeast. These yeasts are the most important domesticated fungi to humans because we use them so often in our daily lives to bake and brew alcohol. In the case of Saccaromyces cerevisiae, the yeast metabolizes glucose from the environment into ethanol and carbon dioxide through a process called fermentation. This carbon dioxide and the bubbles it forms is what makes bread so light. Although the ethanol and carbon dioxide produced by yeast during baking is released during the process, they are retained in the process of fermenting grain into beer. (3)

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Yeast is important in the production of bread and beer. (22)

Humans have also been able to use yeast as eukaryotic hosts for recombinant DNA studies. Although yeasts are unicellular, they are eukaryotic, meaning that they are more closely related to humans than bacteria. Because they divide rapidly, grow easily, and have a relatively small genome size, yeasts are considered model organisms that are studied extensively in laboratory work. (3)

One negative impact of yeast on humans is yeast infection, which can be caused by many species of yeast, the most commonly being Candida albicans. In women, a vaginal yeast infection leads to irritation and discharge that can usually be treated easily. (4)

Yeast's efficiency in producing ethanol has made it a huge asset in the development of biofuels, which offer an alternative to oil and natural gas for powering machines like cars. Yeast cultures are usually given concentrated samples of sugars derived from grain or sugar cane, and produce ethanol that is up to 96% pure. Some yeast has actually been genetically modified to be able to ferment xylose, which is a common sugar found in many human waste products like paper waste and wood chips, making yeast-produced ethanol even more competitive in the biofuel market. (14) (YR)




(SF)(19)(20)

Habitat and Niche

Yeast live in moist, liquid environments and are important in their environments because of their production of carbon dioxide when they break down sugars. (3)

Yeast are found in nature, cultures, and grocery stores. Yeast can grow naturally on decaying fruits that provide sugars for it to feed on. Yeast is also found in an on animals and plants or wherever they can receive the proper nutrients. It's also found on skin surfaces and in intestines of warm-blooded animals in a symbiotic or parasitic relationship with its host. (1) (LK)


Predator Avoidance

Without the ability to move, yeast has no particular predator avoidance tactics. However, yeast cells have tough outer shells composed of protein and cellulose. These shells are several layers thick and make a barrier to protect against invaders and foreign materials. (6) (SP)

Although yeast lack an effective predator avoidance strategies, they are able to survive in great numbers because they reproduce so easily and at such a high rate. (3)

One of the predators of yeast is paramecium. The video below shows paramecium consuming yeast for nutrient acquisition. The yeast has been digested when the red particles turn blue. (MT)





Nutrient Acquisition

Yeasts, like all fungi, digest their food outside their bodies by using digestive enzymes to break down large food particles. Once the food is broken down, the yeasts can then absorb their nutrients directly across their cell membrane because they are unicellular creatures in a process called absorptive heterotrophy. Because they digest their foods outside of their body, yeasts are often found on fruits such as figs and grapes to gain nutrients or in the guts of insects to help the insects break down materials that are difficult to digest, all the while gaining their own nutrients. (3)

Because there are thousands of species of yeast, some yeasts are saprobes, which absorb material from dead organic matter, while others are parasites, which absorb nutrients from living hosts, or mutualists, which live in a relationship with other organisms to benefit both partners. (3)

The enzymes that saprophytes use to break down organic molecules from dead organic matter are hydrolytic enzymes, meaning that they use the process of hydrolysis to break chemical bonds by the addition of water. After the originally large organic molecules are broken down into smaller molecules, the yeast can now absorb the smaller molecules. This allows yeast and other fungi to act as decomposers, as they break down large molecules from dead organisms into smaller molecules. (21) (RS)


Reproduction and Life Cycle


Yeast can reproduce asexually or sexually. Sexual reproduction occurs when two adjacent haploid cells, or cells that have only one set of chromosomes, of opposite mating types fuse. Depending on the species of yeast, the resulting zygote, or fertilized cell, buds and forms a diploid cell population in which the cells contain two sets of chromosomes. In other species, the zygote nucleus will undergo meiosis so that the cell becomes an ascus, which is a sac containing sexually produced haploid spores called ascospores. Each ascus generally contains four to eight ascospores, and these ascospores germinate, or grow, to become haploid cells. (3)


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Yeast cells can exist in both haploid and diploid states and yeast cells can undergo both asexual and sexual reproduction. Haploid cells exist in two mating types, a and alpha. (30)

In asexual reproduction, yeasts undergo either budding or binary fission, both through mitosis. During budding, a small bud, or daughter cell, is formed on the edge of the parent cell, and because the parent cell has gone through through mitosis, the genetic material in the nucleus of the parent cell has been duplicated and migrates to the daughter cell. During the process, the bud continues to grow until it is large enough to be separated from the parent cell, and a new haploid daughter cell is produced. What separates budding from binary fission is that in budding, the daughter cell is generally much smaller than the parent cell, while in binary fission, the result is two identically sized cells. (30)

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Process of budding in a yeast cell. (29)

The life cycle of yeast is advantageous to laboratory work because it takes only 2-8 hours for yeast to divide.


Growth and Development


The growth and developmental rate of yeast is dependent on available nutrients, important ones including sugars, amino acids, and nitrogen compounds. In response to the environment, Yeast can alter their growth rate by adjusting the rate of their cell cycle, and also have a number of different "developmental programs," depending on conditions. For example, in a fecund environment that is nutrient rich, yeast will rapidly undergo mitosis, while in nutrient deficient environments, they will elongate under filamentous growth, which enables the organism to search food. Yeast will also stop growing, and then divide into many spores during cases of total nutrient depletion. (SS) (5)

Yeasts colonies can also synchronize their growth. Key nutrients decide the developmental programs and growth rates of yeast (12). Yeasts have signalling networks that are able to assess the quality and quantity of carbon and nitrogen sources available. These networks can have an influence on the transcriptional, metabolic, and cellular level. (13) (SM)




(SM) (11)

Integument


To protect themselves from damage, yeast cells contain cell walls constructed of three sugars that are bonded covalently: glucose, mannose, and N-acetylglucosamine. Furthermore, the yeast walls are composed of chitin, a polymer of N-acetylglucosamine, which are crucial for bodily functions to occur. This allows yeast cells to maintain their structural integrity and persist independently without the direct aid of other organisms. (17) (SR)

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Structure of chitin. (26)


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The cell wall of yeast is an important feature that protects the cell. (25)


Movement


Yeast do not have a mechanism like flagellum or cilia to move independently. Because they live in moist or liquid environments, yeast move passively only as the liquid is moving. (3)


Sensing the Environment


In stressful environments of nutrient starvation, diploid cells can respond by sporulating, entering meiosis and producing haploid spores. (SM) (3)

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Sporulating yeast

Additionally, Saccharomyces cerevisiae contains a glucose-sensing mechanism that regulates expression of glucose transporter genes, Snf3 and Rgt2. Extracellular glucose causes these sensors to generate an intracellular signal that induces genes that encode glucose transporters. (SM) (2)


Gas Exchange


As single-celled organisms, yeast can exchange gas directly through the cell membrane. (3)

Yeast is a facultative anaerobe, meaning that it is able to perform anaerobic methods of respiration as well as aerobic respiration. It performs fermentation when respiring anaerobically, and this is what is used in brewing. (JM) (9)

Yeast can use both aerobic respiration and a form of anaerobic respiration to produce energy. When there is enough oxygen available, yeast undergo aerobic respiration to generate ATP, but when oxygen is not available, they can also go through alcoholic respiration, which allows yeast to continue to produce energy, just not as much as it would be able to during aerobic respiration. The ability to switch between aerobic and anaerobic respiration makes yeast a facultative anaerobe. (23)

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Alcoholic fermentation allows yeast to continue to produce ATP for energy. This process produces carbon dioxide and ethanol, which are both used in the making of bread and beer. (24)



Waste Removal


Because they are unicellular, any waste produced by yeast can be removed directly through the cell membrane. Waste products (specifically alcohol from fermentation) are stored in vacuoles, membrane bound organelles that hold excretory materials, and are then released through the semipermeable membrane. (SL) (3)


Environmental Physiology


Yeasts have a range of temperatures at which they can survive adequately. In the optimal range of 10 to 37 degrees Celsius, yeasts are able to multiple and reproduce quickly, but they are also able to survive outside of that range. Yeasts can minimally survive without death or reproduction and live in dormancy at temperatures as low as 0 degrees Celsius. However, this ability is not applicable in higher temperatures, in which yeast cells experience stress and degeneration. Beyond 50 degrees Celsius, yeast cells die off. (AC) (10)

Osmoregulation is an important feature in cells to maintain homeostasis in yeast. In particular, the osmoregulatory of the yeast species Saccharomyces cerevisae has been studied, and scientists have found that the solute glycerol plays a role in maintaining levels of water in yeast cells. The vacuole of yeast cells also aids in osmoregulation because it plays a role in storing and removing excess water from cells. (27)

To adjust to hypertonic environments that contain higher concentrations of salt than the yeast cell, the cell produces large amounts of compatible solutes in an effort to balance osmotic pressure inside and outside the cell so that the cell does not become dehydrated. In similarly stressful conditions, yeast cells accumulate osmolytes, water soluble particles, to maintain cell volume, as well as amino acids, fatty acids, and glycerol. (28)


Internal Circulation


Unicellular by nature, yeasts do not require an internal circulatory system. As unicellular organisms yeast cells maintain a high enough surface area to volume ration that they are easily able to transfer materials through their semipermeable membrane, as well as small enough so that substances can easily diffuse throughout the cell without requiring any circulation systems. (SL)


Chemical Control


As unicellular organisms with immensely simple anatomies, yeast do not have internal systems to regulate chemicals from the environment to the body. Rather, some plasma membrane proteins dot the cell membrane of yeast for facilitated diffusion of nutrients. (18) (NU)


Review Questions


1. Explain the process called budding that yeast sometimes use during asexual reproduction. (SM)
2. What kind of cell is a yeast cell? Explain how the yeast cell is able to aqcuire necessary nutrients. (SL)
3. What occurs if yeast live outside of their optimal temperature range? Can they survive, and if the answer is yes, how well do they function? (AG)
4. Why is yeast considered a model organism?
5. How do yeast cells react and respond to stressful environmental conditions?


References:


1. "Baker's and Brewer's Yeast." BandB's Yeast Habitat. N.p., n.d. Web. 18 Nov. 2013.
2. Kruckleberg, Al. "How Do Yeast Cells Sense Glucose?" National Center for Biotechnology Information. U.S. National Library of Medicine, 20 Dec. 1998. Web. 21 Dec. 2012. <http://www.ncbi.nlm.nih.gov/pubmed/10048296>.
3. Hillis, David M., David Sadava, H. C. Heller, and Mary V. Price. Principles of Life. High School ed. Sunderland: Sinauer Associates, 2012. Print.
4. Staff, Mayo Clinic. "Yeast Infection." Mayo Clinic. Mayo Foundation for Medical Education and Research, 01 Nov. 2012. Web. 28 Nov. 2013.
5.http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3430547/
6."Enzymes & Bacteria / Yeast / Parasites." Enzymes & Bacteria / Yeast / Parasites. Enzyme Stuff, 25 Aug. 2005. Web. 17 Nov. 2013.
7. "Baker's and Brewer's Yeast." BandB's Yeast Classification. N.p., n.d. Web. 18 Nov. 2013.
8. "Fungi Classification - What Are Fungi." Fungi Classification - What Are Fungi. N.p., n.d. Web. 18 Nov. 2013.
9. http://www.ncbi.nlm.nih.gov/pubmed/20087724
10. Wassenaar, Trudy. Yeast and Temperature. June 2012. Ask a Scientist. November 18, 2013. ‹http://www.newton.dep.anl.gov/askasci/bio99/bio99693.htm›.
11. "Yeast Growth." YouTube. YouTube, n.d. Web. 21 Nov. 2013.
12. Broach, James R. "Genetics." Nutritional Control of Growth and Development in Yeast. N.p., n.d. Web. 24 Nov. 2013.
13. Z, Palkova, and Forstova J. "Yeast Colonies Synchronise Their Growth and Development." NCBI. U.S. National Library of Medicine, n.d. Web. 24 Nov. 2013.
14. http://aem.asm.org/content/75/8/2304.long
15. http://scienceforkids.kidipede.com/biology/cells/pictures/yeastbuds.jpg
16. http://www.uniprot.org/taxonomy/4932
17. http://www.jbc.org/content/276/23/19679.long
18. http://www.ncbi.nlm.nih.gov/pubmed/13678596
19. "Expanding Yeast's Diet to Improve Biofuel Production." //YouTube//. YouTube, 19 Jan. 2012. Web. 25 Nov. 2013.
20. "QB3." //QB3//. N.p., n.d. Web. 25 Nov. 2013.
21. "Fungi." Microbiology. The Encyclopedia of Earth, 23 Dec. 2008. Web. 25 Nov. 2013.
22. http://www.myrecipes.com/recipe/sweet-beer-bread-10000000478107/
23. "Facultative Anaerobe." Biology-Online.org. N.p., n.d. Web. 26 Nov. 2013.
24. http://www.uic.edu/classes/bios/bios100/lectures/respiration.htm
25. http://www.sigmaaldrich.com/technical-documents/articles/biofiles/antifungals.html
26. http://www.omicsonline.org/2155-9872/2155-9872-3-145.php
27. Hohmann, S., M. Krantz, and B. Nordlander. "Yeast Osmoregulation." National Center for Biotechnology Information. U.S. National Library of Medicine, n.d. Web. 01 Dec. 2013.
28. Nasser A, Abdel, and El-Moghaz. "Comparative Study of Salt Tolerance in Saccharomyces Cerevisiae and Pichia Pastoris Yeast Strains." Advances in Bioresearch 1 (2010): n. pag. Web.
29. http://mowse.blogspot.com/2013/07/yeast-autolysis-process-and-benefits.html
30. "Budding Yeast: Saccharomyces Cerevisiae." //Budding Yeast//. N.p., n.d. Web. 01 Dec. 2013.