Abstract
This chapter discusses modern aerobic organisms that transduce the chemical bond energy of food molecules into the bond energy of adenosine triphosphate (ATP). It examines how aerobic organisms perform this feat where oxygen is used as the terminal acceptor of the electrons extracted from food molecules. The capacity to use oxygen to oxidise nutrients, such as glucose and fatty acids, yields a substantially greater amount of energy than does fermentation. The chapter recounts the accumulation of atmospheric O2 on Earth about 2 billion years ago, when existing organisms were confronted with a serious problem: molecular oxygen forms toxic oxygen ions and peroxides called reactive oxygen species (ROS). ROS react with and damage or destroy biomolecules. Consequently, exposure to O2 acted as a severe selection pressure.