21. Lipids and Carbohydrates   Previous PageNext Page
       Polysaccharides: Cellulose and Starch

Although we do not have the enzyme cellulase, we do have amylase in our saliva and pancreatic juices. If you hold a piece of bread in your mouth for a minute or two, it will begin to taste sweet. Amylase breaks down the starch from the bread into a mixture of maltose and glucose, which can be sensed by the taste buds on your tongue. The job of cleavage is finished by hydrochloric acid in the stomach, and glucose is absorbed into the bloodstream to be carried off for eventual oxidation. The attractive feature of starch as an energy storehouse is the utter simplicity of the input and output mechanisms.

Only one chemical step is required to combine glucose molecules together to make starch, and with the breaking of only one kind of bond, starch is reconverted to glucose for instant use. In contrast, the synthesis of fats requires half a dozen metabolic steps, and the breakdown of fats into usable small molecules again is equally complicated. The great disadvantage of carbohydrates (starch) as energy-storage molecules is their low energy-to-weight ratio.

The table in the margin is a summary of an earlier table from Chapter 11, giving the heat per gram for burning several fuels. Hydrogen gas is the best fuel of all, but the hydrocarbons are second-best. Stearic acid is nearly as efficient, and this combustion value can be taken as representative of fats generally. Alanine, which represents protein fuels, is less than half as good, and glucose and its polymers are worse yet.

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