As observed in many unicellular organisms, the budding yeast Saccharomyces cerevisiae preferentially uses glucose over other carbon sources as it can directly enter the glycolytic pathway.
However, when glucose is unavailable, alternative carbon sources are used for the production of metabolic energy and cellular biomass.
Budding yeast is able to utilize a wide variety of different carbons; for example, other alternative sugars such as galactose, sucrose, maltose, and melbiose as well as nonsugar carbons such as ethanol, lactate, glycerol, acetate, or oleate may be used.
The enzymatic pathways required for the specific utilization of these carbon compounds are very well characterized. Quite often, enzymes needed for a specific pathway are produced only when required. This regulation is mainly (but not exclusively) exerted at the transcriptional level. A classical example is the galactose-induced expression of genes required for catabolism of this sugar by the transcriptional activator Gal4