Fermentation is an anaerobic process.  This process may occur if oxygen is not available for metabolizing glucose aerobically. Whether lactic acid or ethyl alcohol and carbon dioxide are produced in fermentation depends on the enzymes in the cells of the organism.  However, the reason for fermentation is the regeneration of NAD⁺ so that glycolysis can continue.  Remember that two molecules of ATP are produced in glycolysis.   However, glycolysis can't continue without a steady supply of NAD⁺.   Thus, fermentation continues to generate NAD⁺.   Organisms that rely on fermentation get two ATP molecules from each glucose molecule. Note that fermentation does NOT produce additional ATP - both molecules are coming from glycolysis.   Is fermentation adequate for sustaining the life of yeast cells? of humans?  What accounts for this difference?  See if you can write an equation for each type of fermentation and check your work with the equations on page 4-10 of the Study Guide.