What is the principle reason why organisms such as yeast produce alcohol? Please explain in terms...

The yeast species involved in alcohol production transport sugar into their cells where it can be used to extract energy through two processes depending on the presence, or absence, of oxygen. In both processes, electrons are transferred from a 6 carbon glucose to electron carrying molecules, NAD+, to form two 3 carbon molecules of pyruvate through a process called glycolysis. Energy is conserved through the transfer of phosphate groups from high energy intermediates of glycolysis to ADP, resulting in the production of ATP, or cellular energy. When oxygen is present, the pyruvate produced is shuttled through other metabolic pathways that result in the production of more ATP, however, without oxygen further energy extraction from pyruvate is not thermodynamically feasible.

The process of ethanol fermentation generates waste products, ethanol and carbon dioxide, from pyruvate. The biological purpose of this process is to remove the electrons transferred to electron carrying molecules, to regenerate them, thereby allowing their use in additional cycles of glycolysis.

This process generates two carbon dioxide and two ethanol molecules from one 6 carbon glucose.