Specimen #11: Yeast

Figure 1.  Saccharomyces cerevisiae which is commonly known as Baker's yeast or Brewer's yeast.

Name: Saccharomyces cerevisiae

Common Name:  Baker’s Yeast/ Brewer’s Yeast

Family:  Saccharomycetaceae

Collection Date: October 14, 2011

Habitat: Saccharomyces when translated means “sugar fungus”. That is what this yeast uses for food. They are found in the wild growing on the skins of grapes and other fruits.

Location: Grayhouse in Westlake, Ohio

Figure 2. Diagram of Kingdom Fungi

Means for Classification: Saccharomyces cerevisiae is in the fungi kingdom. The reasons for this classification are because it has a cell wall made of chitin, it has no peptidoglycan in its cell walls, and its lipids are ester linked. It also uses DNA template for protein synthesis and it has larger ribosomes. It is then considered a yeast because it is a unicellular organism so it cannot form a fruiting body; like other fungi.

Volk, T., & Galbraith, A. (2002, December 01). Tom volk's fungus of the month for december 2002. Retrieved from http;//tomvolkfungi.net/

Adaptations: Saccharomyces cerevisiae has adapted in several important ways. One is the fact that they are able break down their food through both aerobic respiration and anaerobic fermentation. They can survive in an oxygen deficient environment for a period. Another adaptation they have is their ability to have both sexual and asexual reproduction. Very few other Ascomycota can do both processes. And very few organisms can do all four of these processes. This allows this species to live in many different environments (Madigan & Martinko, 2006, p. 457).

Madigan, M. T., & Martinko, J. M. (2006). Biology of microorganisms. (11 ed., Vol. 1, p. 457). Upper Saddle River, NJ: Pearson Education.

Nutrition: Saccharomyces cerevisiae gets its energy from glucose.

Life Cycle: Saccharomyces cerevisiae has both asexual and sexual reproduction.

In asexual reproduction the haploid of the yeast under goes mitosis and forms more haploid yeasts. There is an a and ά strain of these haploids. Then these haploid yeasts, one from each strain, can fuse together and become on cell. Then the nuclei of both cell fuses together and this cell is now the zygote. These diploid cells can go through mitosis, which they call budding, and four more zygotes or they can undergo meiosis and from an ascus which will split into four ascospores. These haploids can then undergo germination and become haploid yeast again (Madigan & Martinko, 2006, p. 457).

Madigan, M. T., & Martinko, J. M. (2006). Biology of microorganisms. (11 ed., Vol. 1, p. 457). Upper Saddle River, NJ: Pearson Education.

Figure 3. Saccharomyces cerevisiae. Note the budding in the yeast which is an example of asexual reproduction.

Importance: Saccharomyces cerevisiae is one of the most important fungi in the history of the world. This yeast is responsible for the production of ethanol in alcoholic drinks and is the reasons your mother’s bread dough rises in the pan. That is where the names brewer’s and baker’s yeast come from. The process in which it produces ethanol is one way this yeast converts glucose into energy. There are two ways Saccharomyces cerevisiae breaks down glucose. One way is through aerobic respiration. This process requires the presence of oxygen. When oxygen is not present the yeast will then go through anaerobic fermentation. The net result of this is two ATP, and it also produces two by products; carbon dioxide and ethanol. So if this yeast is allowed to grow in a container lacking oxygen it will produce ethanol (alcohol). Humans have been isolating this process since the beginning of history. The yeast helps in the rising of bread with its other by-product carbon dioxide.  The gas that is produce inside the dough causes it to rise and expand. Both of these processes use the haploid of this yeast for this process. In industry they isolate one strain, either a or ά, of the haploid to keep them from undergoing mating (Madigan & Martinko, 2006, p. 457). In the baker’s yeast, they have a strain were the production of carbon dioxide is more prevalent then ethanol and vice versa for brewing (Volk & Galbraith, 2002).  

Madigan, M. T., & Martinko, J. M. (2006). Biology of microorganisms. (11 ed., Vol. 1, p. 457). Upper Saddle River, NJ: Pearson Education.

Volk, T., & Galbraith, A. (2002, December 01). Tom volk's fungus of the month for december 2002. Retrieved from http;//tomvolkfungi.net/

Figure 4. Saccharomyces cerevisiae is commonly used in the brewing of beer, thus the common name, Brewer's yeast.

Figure 5. Saccharomyces cerevisiae is used in the baking of many types of bread, thus the name Baker's yeast.