Organisms can be classified based upon the effect oxygen has on growth and metabolism. Organisms that require oxygen for growth and as the final electron acceptor for aerobic respiration. Microorganisms that only grow in the absence of oxygen and are, in fact, poisoned by it. Yeasts, many bacteria and mammalian muscle cells are facultative anaerobes. Some facultative anaerobes make ATP only by fermentation in the absence of oxygen, and make ATP by respiration in presence of oxygen.
Glycolysis is common to fermentation and respiration.
In some facultative anaerobes, the fate of pyruvic acid will be either fermentation or respiration depending upon the presence of absence of oxygen. Glycolysis is the most widespread metabolic pathway, so it probably evolved early. Glycolysis occurs in the cytoplasm and does not require membrane-bound organelles. Eukaryotic cells with organelles probably evolved about two billion years after prokaryotic cells. Organisms obtain most calories from fats, proteins, disaccharides and polysaccharides.