= Saccharomyces cerevisiae
{c}
{wiki}
= S. cerevisiae
{c}
{synonym}
<Saccharomyces cerevisiae> is a particular <species> of <yeast>, used in baking, brewing and laboratory research. During ordinary vegetative growth it reproduces by <budding>: a smaller daughter grows from the mother before separation. Laboratory strains can be maintained as either <haploid> or <diploid> <biological cells>. This makes it possible to investigate <genes> in a single <eukaryote>, then use controlled crosses and <meiosis> to connect a <genotype> to a <phenotype>. See https://smburgess.faculty.ucdavis.edu/wp-content/uploads/sites/33/2019/08/Burgess-Powers-Mell-eLS.pdf[Burgess, Powers and Mell, Budding Yeast Saccharomyces cerevisiae as a Model Genetic Organism (2017), pp. 1–2].
The practical value of <Saccharomyces cerevisiae> as a <model organism> is the combination of easy laboratory cultivation, accessible <genetics>, and cellular functions shared with other <eukaryotes>. A <haploid> strain has one set of <chromosomes>, so a changed <allele> is not accompanied by a second copy at that <genetic locus>. But a <mutation> need not produce an observable <phenotype> under every condition. The <yeast> is also a whole <organism>, not a miniature human: transferring a result requires evidence that the relevant function is shared in the target system. See https://pmc.ncbi.nlm.nih.gov/articles/PMC4012490/[Duina, Miller and Keeney, Budding Yeast for Budding Geneticists (2014): cultivation, genetic tools and conserved functions].
For a concrete example, Hartwell, Culotti and Reid's 1970 work followed temperature-sensitive <Saccharomyces cerevisiae> strains with successive microscope photographs. Failure to complete a particular part of the <cell cycle> helped identify required <gene> functions. Their distinction between when a function is required and where a cell finally stops is especially instructive: the visible endpoint need not be the time of the underlying defect. See https://pubmed.ncbi.nlm.nih.gov/5271168/[Hartwell, Culotti and Reid, Genetic control of the cell-division cycle in yeast. I. Detection of mutants (1970): primary abstract]. This is a useful way to turn an observable change in a <biological cell> into a question about <genes> and causal order.
In <alcoholic fermentation>, <Saccharomyces cerevisiae> converts <sugars> into <ethanol> and <carbon dioxide>. The <carbon dioxide> helps dough rise; the <ethanol> is useful in alcoholic drinks. The <metabolism> also supports production of <ATP>, a usable cellular <energy> carrier. This is one metabolic ability of one <species>, not the definition of every <yeast>. See https://openstax.org/books/biology-2e/pages/7-5-metabolism-without-oxygen[Clark, Douglas and Choi, Biology 2e, 7.5: alcohol fermentation] and the Burgess, Powers and Mell account above.
For <glucose>, the ideal net <stoichiometry> is
$$
\mathrm{C}_6\mathrm{H}_{12}\mathrm{O}_6\longrightarrow 2\mathrm{C}_2\mathrm{H}_5\mathrm{OH}+2\mathrm{CO}_2.
$$
One <molecule> of <glucose> supplies six <carbon> <chemical atoms>: four end up in the two <ethanol> <molecules> and two in the two <carbon dioxide> <molecules>. The twelve <hydrogen> and six <oxygen> <chemical atoms> also balance. This is a net material balance across several <enzyme>-catalyzed steps, not a single <chemical reaction> step or the complete bookkeeping for cellular <energy> carriers. The standard pathway accounting yields a net two <ATP> per <glucose>: the initial breakdown produces the <ATP>, while the later conversion to <ethanol> regenerates a <coenzyme> needed to keep the breakdown going. See https://openstax.org/books/biology-2e/pages/7-2-glycolysis[Clark, Douglas and Choi, Biology 2e, 7.2: net ATP accounting] and the section 7.5 cited above.
\Image[/saccharomyces-fermentation-carbon-balance.png]
{title=Ideal <alcoholic fermentation>: <glucose> <carbon> divides between <ethanol> and <carbon dioxide>}
{description=Original schematic and theoretical toy calculation, not measured culture data. The dots count <carbon> <chemical atoms>; they do not depict molecular bonds or traced individual atoms. Starting with $10\,\mathrm{mmol}$ of <glucose>, conversion of $x\,\mathrm{mmol}$ leaves $10-x$ and forms $2x$ each of <ethanol> and <carbon dioxide>. The coincident product lines count <molecules>, not equal masses.}
At complete conversion, this toy gives $20\,\mathrm{mmol}$ of <ethanol> and $20\,\mathrm{mmol}$ of <carbon dioxide>. The <carbon> balance is $6\times10=2\times20+1\times20=60\,\mathrm{mmol}$ of <carbon> <chemical atoms>. A real culture also builds cellular material, can use other metabolic routes, and may retain dissolved <carbon dioxide>; the toy is a reference balance rather than a prediction of collected gas or beverage yield.
Although <alcoholic fermentation> does not require <molecular oxygen> as a reactant, the presence of <molecular oxygen> does not force <Saccharomyces cerevisiae> to stop fermenting. De Deken's 1966 experiments reported predominantly fermentative <glucose> use during exponential growth in air. Consequently, bubbles do not establish either absence of <molecular oxygen> or an exclusively fermentative <metabolism>. See https://www.microbiologyresearch.org/content/journal/micro/10.1099/00221287-44-2-149[De Deken, The Crabtree Effect: A Regulatory System in Yeast (1966): primary abstract].
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