This lesson explores the third and fourth steps of the citric acid cycle and explains how the rate of ATP production is regulated according to the cell’s need for ATP. Together with lesson four, it explains how cells regulate their ATP production according to their needs and abilities. In the course of exploring this theme, we look at the role of AMP kinase (AMPK) in promoting energy uptake during exercise. We consider how AMPK and reactive oxygen species, so often at odds with one another, work together to produce fitness in response to exercise.