The experiments by Anderson and his team grew out of increased understanding in recent years of the role of calcium in heart function. With each beat of the organ, molecules of calcium whiz in and out of tiny compartments called mitochondria that are powerhouses of heart and other cells. Inside the mitochondria, calcium is generally a good thing -- it helps generate energy that the cells use to stay alive. But for almost half a century, researchers have also known that too much mitochondrial calcium can overwhelm and cause cells to die. And after a heart attack or stroke, a sudden rush of calcium into the organelles sets off this cell death pathway, leading to long-term damage.