With an ample supply of free oxygen in the atmosphere, the last great energy-managing invention appeared: oxygen-using respiration. The citric acid cycle developed to produce. NADH (whether originally for energy or for reducing power), and the respiratory chain evolved to use these molecules to make ATP. This led to the modern system of photosynthesis of glucose and oxygen from carbon dioxide and water, seen in green plants, and the complementary combustion of glucose and oxygen back to carbon dioxide and water, found in both plants and animals. One can think of the planet as a giant chemical machine, with cogs and gears made of glucose, oxygen, carbon dioxide, and water, absorbing energy from the sun and storing it in molecules of ATP to provide a continuing fuel source for that most unusual set of chemical reactions: Life.