Animals, which must carry their energy reserves with them, universally have developed the involved chemistry of fats and fatty acids, for the sake of the weight reduction, in using a nearly gasoline like fuel. Stationary plants, for which portability and a high energy-to-mass ratio are of no advantage, have opted instead for the simplicity of carbohydrate chemistry. However, animals do make one use of carbohydrates as an energy reservoir.

They synthesize glycogen (animal starch), which is a more highly branched version of amylopectin, with branching every 8-10 glucose units. Glycogen in the liver and muscle tissues serves as a special rapid access energy store - a buffer between the immediate needs of the animal and the long-term energy supply in fats.

Thus animals have the best of both worlds, and a good analogy exists between this energy-storage strategy and currency.

Paper money in Europe was developed in the 1600's to combat the danger and inconvenience of carrying large amounts of gold and silver coin from place to place.

The Italians developed the Girobanks ("circulation banks") to assist the transfer of credit from one city to another, and these Girobank notes gradually became accepted as substitutes for the money they represented.

Paper currency was easier to carry than metal, but sometimes was difficult to use. Persuasion and discounts would be needed to get currency accepted as payment in out-of-the-way places. A sedentary businessman could keep all of his wealth in gold and silver.

The traveler would transfer his bulk accounts in bank notes (starch) for the sake of efficiency, but would be careful to keep small amounts in coin for immediate use. Glycogen is the metal coinage of animal energy needs.