Chapter 2 - Biochemical Horsepower

Abstract

Efficiency is a matter of context. What represents a good measure of efficiency will therefore depend on that context. We often assume yield, the metabolic output per unit of nutrient consumed, is the right measure of efficiency. When we said oxidative phosphorylation is more efficient than fermentation, we meant oxidative phosphorylation has a higher yield of ATP per molecule of glucose than fermentation. But yield, as a measure of efficiency, is only relevant in a context where glucose supply is limited. In an environment plenty of glucose, oxidative phosphorylation does not give any advantage over fermentation. In the absence of any further evidence, we would conclude that, in an environment with plenty of glucose, oxidative phosphorylation and fermentation should be utilized by cells indistinctly. What else besides nutrients could limit metabolism?