We mentioned previously that hydrogen gas was the most efficient
of all fuels in terms of heat emitted per gram of fuel burned. Gasoline
is less efficient by nearly a factor of three, as the table at the
right shows. Hydrogen gas releases 34 kcal per gram upon combustion
in air; gasoline yields less than 12 kcal per gram.
Fats, the main energy-storage system in animals, produce 9.5 kcal
per gram upon combustion, and are almost as efficient in energy
storage as gasoline.
Starch, the main energy-storage molecule in plants, is a long-chain
polymer of glucose. Glucose can store only 3.7 kcal per gram, so
on a weight basis, starch is an inefficient material for storing
energy. Then why is the entire photosynthetic and energy-storage
machinery of green plants based on starch?