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.
These complex molecules must be enzymatically hydrolyzed into simpler molecules or monomers that can enter an intermediate reaction of glycolysis or the Krebs cycle. Carbohydrates Glycolysis can accept a wide range of carbohydrates for catabolism. Starch is hydrolyzed to glucose in the digestive tract of animals. In between meals, the liver hydrolyzes glycogen to glucose. Enzymes in the small intestine break down disaccharides to glucose or other monosaccharides.