Saccharomyces cerevisiae

Saccharomyces cerevisiae is one of today’s most widely used yeasts. This unicellular eukaryote plays a key role in the fermentation processes which produce alcoholic beverages and bread. For this reason, Saccharomyces cerevisiae is otherwise referred to as Brewer’s yeast and Baker’s yeast. Nutritional yeast is composed of the deactivated form of this species, which is also the main component of yeast extract. These nutritional supplements are rich in protein and B-complex vitamins. Nutritional yeast is a complete protein, as it contains substantial amounts of all of the essential amino acids. This fungal species provides quite substantial nourishment in these products, but it also exists in smaller concentrations in many common everyday products. S. cerevisiae is a component of beer and wine, as its fermentation produces these liquids. Bread and other leavened baked products contain this yeast, without which rising wouldn’t occur.

Because S. cerevisiae has been used so widely, scientists have studied it closely. This single-celled organism is abundant in nature, as it grows on the skin of grapes and other fruits. It can grow and reproduce on the surface of many sugars, which makes it easy to culture. The ease of attaining and culturing this species allows for its vast studies which have been conducted. Due to its thorough investigation, this yeast was the first eukaryote to have its genome completely sequenced.

Genome Structure

Saccharomyces cerevisiae was the first eukaryotic genome to be completely sequenced. This genome sequence was originally published on April 24, 1996. It consists of about 12,156,677 base pairs, and approximately 6,275 genes. This genetic material is compiled onto 16 chromosomes. Of the genes discovered, only about 5,800 are believed to be functional genes. Since the genome sequence‘s first publication, it has been updated regularly on the Saccharomyces Genome Database (SGD) when new findings arise.^[1]

Cell Structure and Metabolism

Saccharomyces cerevisiae is a single-celled eukaryotic organism. This yeast consists of small elliptical cells that can appear round or ovoid in shape, which are about 5-10 micrometers in diameter. Enclosed by a two-layered cell wall, the cell’s most prominent structures are the nucleus and a large storage vacuole. Other visible structures are mitochondria and the endoplasmic reticulum. The cell wall is composed of proteins, polysaccharides, and chitin, and can be up to 300 nanometers thick. The cell surface appears smooth, except for the altered contour of bud scars. A bud scar forms on the end of the mother cell opposite to the location of the budding daughter cell.

An important characteristic of this yeast is its ability to utilize two different metabolic pathways. It is considered heterotrophic because it gains energy from the breakdown of carbohydrates. When oxygen is present in its environment, S. cerevisiae carries out aerobic respiration to gain ATP. When oxygen is absent, the yeast carries out anaerobic fermentation of its sugar substrate to gain ATP. The by-products of fermentation include carbon dioxide and ethanol. Carbon dioxide is what causes bread to rise during baking, as bubbles of it get trapped in the dough. CO2 is also what produces the carbonation in beer and champagne. Ethanol, or alcohol, is what makes beer and wine alcoholic beverages.

Saccharomyces cerevisiae can also reproduce sexually or asexually. Asexual reproduction occurs more often, in a process known as budding. During budding, a haploid cell undergoes mitosis to form more haploid cells, which “bud” off of the mother cell. Two different types of haploid cells form, which are known as a haploids and ά haploids. During sexual reproduction, these two differing haploid strains mate to form a diploid cell. The diploid cell undergoes mitosis to form zygotes. When using Saccharomyces cerevisiae for brewing or baking, the haploid form of the yeast is used. To ensure the inhibition of sexual reproduction, an isolated form of either haploid is used. Certain forms favor either carbon dioxide or ethanol as a product, which determines what process they will be used for. Strains that favor CO2 formation are used more in the baking of bread, while strains that favor ethanol formation are used for the brewing process. Because Saccharomyces cerevisiae can survive with or without oxygen, and can reproduce sexually or asexually, this species can thrive in many varying environments.