Recently, a fascinating, well-executed, molecular study regarding the direct influence of mitochondrial reactive oxygen species (ROS) formation by the electron transport chain (ETC) on mitochondrial morphology in baker's yeast appeared in PNAS. The findings highlight some very interesting connections between the choice of metabolic substrates, points of entry into the ETC, ROS formation, efficiency of ATP generation, and mitochondrial structures. These reflect both ancient eukaryotic constraints and later specific adaptations of Saccharomyces cerevisiae. However, by not addressing these adaptations, the important wider implications of the article's findings run the risk of being overlooked. There are illuminating connections to the FADH/NADH ratio concept and new studies replacing ETC components with yeast counterparts in diverse metazoan cells, which will also be discussed.