Ultimately, respiration and fermentation are simply different ways to conclude sugar catabolism. Following the energy-yielding reactions, respiring cells pass electrons to oxygen (or other external inorganic oxidant), while fermenting cells dump electrons back onto a partially oxidized organic molecule (such as pyruvate), which then exits the cell as a waste product. Either way, the goal is to do which of the following?

A. fully oxidize NADH into CO2 and H2O
B. recycle NADH back to NAD+
C. generate as much NADH as possible (then synthesize more NAD+ if necessary)
D. break down NADH into biosynthetic precursors
E. stimulate de novo production of NAD+
recycle NADH back to NAD+

Both respiration and fermentation share a common pathway, that is glycolysis. The redox reactions of glycolysis use NAD+ as an electron acceptor and NADH is formed. To continue the process of respiration and fermentation, oxidation of NADH into NAD+ is required so that NAD+ can serve as an electron acceptor for glycolytic reactions.

Aerobic respiration oxidizes NADH into NAD+ by transferring its electrons to the terminal electron acceptor, the molecular oxygen. On the other hand, fermentation oxidizes NADH through the formation of lactic acid or ethanol from pyruvate.