Yeast

Yeast—Saccharomyces cerevisiae

A Microscopy image of yeast, they are small ovular shaped cells, some with lumps within them or a smaller second ovular cell coming off of a side

Brewers (aka bakers yeast or commercial yeast), is the organism that is used to make bread rise and produce wine from the fruits of grape. It also is extremely important as a model organismin biology. It was the first eukaryote to have its entire genome sequenced and studies using S. cervisiae have been highly significant in developing our understanding of meiosis, mitosis and cancer. The translation of the scientific name is: saccharo = sugar, myces = fungus, cerevisiae = beer, reflecting its ability to make beer out of sugar water. It accomplishes this feat in a manner that several organisms can, by carrying out a processes termed fermentation, an anaerobic respiration process that releases carbon dioxide while converting six-carbon sugars (glucose and/or fructose) into ethanol. This same ability is important in bread making, not for the production of alcohol but for the production of carbon dioxide, which acts as a leavening agent, releasing carbon dioxide gas into a matrix of hydrated starch and protein molecules (bread dough), thereby producing a product with a light, aerated texture. In both bread/beer making and in scientific experimentation, what makes yeast particularly useful is the fact that it is easily culturedit can be readily grown (i.e. it is not fussy about growth conditions) and is easy to keep alive/viable (i.e. it is tough to kill) and actually can be kept viable under what is often considered harsh conditions cooling, freezing, drying.

Phylogeny and taxonomy

The name yeast is a morphological term referring to unicellular fungi. Used in this context it does not relate at all to phylogeny. Convergent evolution has resulted in unicellular fungi in several different groups including Zygomycetes (bread molds), Basidiomycetes (club fungi) and Ascomycetes (cup fungi). Most unicellular fungi (yeasts) are ascomycetes but even within this phylum there are yeasts that are not closely related. Commercial yeast (Saccaromyces cervisiae) is an ascomycete, as is fission yeast (Schizosaccharomyces pombe), another yeast used in brewing and also an important model organism with its entire genome sequenced. Although fission yeast and bakers yeast have a similar ecology and are in the same phylum they are not closely related, having diverged from each other over 300 – 1000 million years ago.

Structure

Yeasts in general are unicellular fungi and in form and size very similar to bacteria. Like all fungi, they have a cell wall composed of chitin and possess a nucleus and other organelles, in particular, mitochondria.  In many ways they represent fungi that have evolved to become bacteria-likein their form and ecology. Bakers yeast is typical of yeasts in generallythey typically are roughly spherical and around 5 um in diameter.

Sex and reproduction

An drawing that exemplifies two haploid yeast cells of opposite mating groups; then the two cells grow toward one another stretching their sides; then the two cells have merged together making an odd ovular shape with a depression in the middle

Brewers yeast primarily reproduces asexually, by budding, which is basically cell division but where the daughter cell starts as an outgrowth (bud) of the parent cell and eventually separates. Brewers (and fission) yeast are capable of sex when the diploid cells undergo meiosis, forming cells that can fuse with each other (i.e. serve as gametes) to restore the diploid condition. Both fission yeast and commercial yeast can occur as haploid or diploid cultures. Haploid cultures can be maintained by not bringing together different mating strains, while diploid cultures can be maintained because specific culture conditions (nitrogen starvation) are needed to bring about meiosis. When haploid cells of different mating strains encounter each other, chemical communication (pheromones) trigger the production of extensions (Shmoos) that allow cells to fuse with each other (plasmogamy). In the diagram to the left the blue astrain produces a mobile chemical (pheromone) designated by the blue circles and has receptors (red football goalposts) that can bind the pheromone (red squares) produced by red alphastrain. Similarly, the red alpha strain has receptors for the pheromone produced by the blue astrain. In both strains the binding of pheromones produced by compatible strains induces the production of a schmoo and the eventual creation of a diploid cell.

Matter and energy

Saccharomyces is a heterotroph with a sweet toothit prefers living off of simple sugars, although some strains can breakdown sugar polymers, e.g. starch, into simple sugars, thereby broadening its diet. Its mineral needs are obtained by absorbing small organic molecules (amino acids) and minerals like phosphate. Various strains differ in their ability to breakdown organic matter and absorb and metabolize nutrients; these features may be useful in genetic/cell biology experiments.

The chemical formula for alcoholic fermentation
Alcoholic fermentation: Two molecules of NAD+ are reduced to two NADH while a glucose is converted to two molecules of pyruvate. Each loses a carbon dioxide to form two acetaldehydes and then the acetaldehype is reduced to ethanol while NADH is oxidized back to NAD+, allowing the process to continue.

Interactions

Obviously yeast have significant interactions with humans in providing food products and beverages. The native habitatof both brewers yeast and fission yeast is the skins of sugar containing fruits such as grapes, apples and pears, making the inventionof wine-making relatively easy. In addition to ethanol, genetically engineered yeast are used to produce a variety of compounds including insulin. While brewers yeast is non-pathogenetic some yeast species can cause disease in humans and other organisms.

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Inanimate Life by George M. Briggs is licensed under a Creative Commons Attribution-ShareAlike 4.0 International License, except where otherwise noted.