12. Heat, Energy, and Chemical        Bonds   Previous PageNext Page
       Fuels, Combustion, and Energy

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?

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