To summarize then, under anaerobic conditions, or in a medium containing high levels of substrate, yeast will metabolize through fermentative pathways, rather than through respiration. Fermentation, despite producing so much less energy than respiration, still achieves two important metabolic goals. It uses NADH to reduce the pyruvate (or one of its metabolites) generated by glycolysis, and consequently NAD+ is regenerated. This ion is used once more in glycolysis, so the cell can carry on metabolizing glucose. Also, fermentation allows the cell to produce a small amount of ATP through substrate level phosphorylation. Metabolic intermediates needed for cell growth will be supplied by glycolysis, the pentose phosphate pathway and the citric acid cycle.