Yeast are single-celled fungi used for millennia in food production. They are classified as chemoorganotrophs, meaning they derive both their energy and carbon from organic compounds. Unlike plants, yeast are heterotrophs that must consume pre-formed organic matter, primarily sugars, from their environment to survive and grow. Obtaining energy involves breaking down these molecules to generate adenosine triphosphate (ATP), the universal energy currency of the cell. Yeast uses two distinct metabolic strategies to produce ATP, depending on the presence or absence of oxygen.
Feeding the Process: How Yeast Utilizes Sugars
The initial stage for yeast to harvest energy from sugars is glycolysis, a ten-step process occurring in the cytoplasm. It begins with the breakdown of a single glucose molecule into two molecules of pyruvate. The process requires an investment of two ATP molecules but ultimately generates four ATP molecules, resulting in a net gain of two ATP per glucose molecule.
Glycolysis also produces high-energy electron carriers in the form of two NADH molecules. The pyruvate and NADH become the starting materials for subsequent energy-production pathways. Because glycolysis does not require oxygen, it serves as the foundational metabolic route that all yeast species use to generate energy.
Maximizing Power: Energy Production Through Aerobic Respiration
When oxygen is available, yeast switches to aerobic respiration to maximize energy yield from the pyruvate produced during glycolysis. The two pyruvate molecules are transported into the mitochondria, where they are converted into acetyl-CoA. This acetyl-CoA then enters the tricarboxylic acid cycle, also known as the Krebs cycle, which takes place in the mitochondrial matrix.
The Krebs cycle completes the breakdown of glucose, releasing carbon dioxide and generating a small amount of ATP. Crucially, this cycle produces a large number of high-energy electron carriers: NADH and FADH2. These carriers feed their electrons into the electron transport chain, a series of protein complexes embedded in the inner mitochondrial membrane. This final stage, oxidative phosphorylation, uses the electron energy to pump protons, creating a gradient that drives the synthesis of a large quantity of ATP. Aerobic respiration yields a theoretical maximum of 30 to 32 ATP molecules per glucose molecule.
Survival Mode: Anaerobic Fermentation and Its Byproducts
If oxygen is absent, yeast shifts into alcoholic fermentation, a less efficient survival strategy. Pyruvate remains in the cytoplasm instead of entering the mitochondria. The purpose of fermentation is not to create more ATP—the yield remains only the two net ATP from glycolysis—but rather to recycle the electron carrier NAD+.
Without NAD+, glycolysis would halt. To regenerate NAD+, pyruvate is first converted into acetaldehyde by pyruvate decarboxylase, releasing carbon dioxide (CO2). Next, alcohol dehydrogenase converts acetaldehyde into ethanol, simultaneously oxidizing NADH back into NAD+. This recycling allows glycolysis to continue running, providing a minimal supply of two ATP per glucose molecule for cell maintenance. Both CO2 and ethanol are waste products for the yeast cell but are valuable to human industrial processes.
Real-World Results of Yeast Metabolism
The waste products of anaerobic fermentation—carbon dioxide and ethanol—have been harnessed by humans for thousands of years. In baking, carbon dioxide gas is the primary product of interest, becoming trapped within the dough matrix. The expansion of these gas pockets causes the dough to rise or leaven, giving bread its light texture. During baking, the heat causes the ethanol byproduct to evaporate completely.
In alcoholic beverage production, ethanol is the desired end product. Yeast strains are added to sugary liquids, and the lack of oxygen forces fermentation. Yeast converts sugar into ethanol and CO2 until the alcohol concentration reaches a level toxic to the yeast, typically around 15% alcohol by volume. This metabolic capability is also used in the large-scale production of ethanol for biofuel.