The episode presents metabolism as the combined activity of trillions of specialized cells. It traces mitochondria’s bacterial origins, maternal inheritance, and cell-specific functions, then follows glucose and pyruvate as resources that can be used for ATP production or biomass. The discussion connects these choices to fasting, exercise, heart failure, aging, cancer, treatment resistance, and emerging ways to visualize metabolism. A central theme is that health and disease can be understood through how cells allocate limited resources.