All organisms require energy to fulfil their life functions. Like most living things, yeasts derive much of their energy by the breakdown of sugars in respiration. Respiration is the breakdown of sugars in cells to release carbon dioxide, water and energy.

Yeasts can also derive energy from the breakdown of sugars in fermentation. Fermentation can be defined as the breakdown of sugars in cells to release carbon dioxide, ethanol (or ethyl alcohol) and energy.

C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂ + Energy
Sugar → ethanol + carbon dioxide

Respiration generates about 20 times more energy than fermentation, and so is the energy generating pathway that yeasts prefer to use.
However, due to the high sugar concentrations in wine musts, yeasts can only break down sugars by fermentation. This is called the Pasteur Effect, and is why wine is alcoholic, whereas bread is not.