The methods of manufacturing and compressing hydrogen gas require great amounts of energy. To overcome these challenges, scientists have been tinkering with the biological powers of everything from common yeast to mysterious bacteria living on the ocean floor. At the University of California at Berkeley, mechanical engineering professor Liwei Lin is busy developing a microbial fuel cell that runs off the digestive activity of baker's yeast. The yeast feed on glucose, a simple sugar, and digest it in a process called aerobic metabolism. "We extract electrons from the yeast cells where the aerobic metabolism process happens," Lin explains. Controlling the movement of electrons to harness a renewable source of fuel remains the target for scientists designing fuel cells, which extract power from electrochemical reactions. The advantage of Lin's mechanism is that it runs on glucose, a naturally abundant resource produced by plants.