Reserves 1 Stored Carbohydrate Metabolism

Fatty acids stored in adipose tissue, in the form of neutral TAG, serve as the body s major fuel storage reserve. TAGs provide concentrated stores of metabolic energy because they are highly reduced and largely anhydrous. The yield from complete oxidation of fatty acids to CO2 and H2O is nine kcal/g of fat (as compared to four kcal/g of protein or carbohydrate, see Figure 27.5, p. 357). 

Since starch is the most important and widespread reserve carbohydrate in seeds we will begin by surveying the metabolic reactions through which this polysaccharide is mobilized into a form available for the embryo. Details of the breakdown of other stored carbohydrates (hemicellulose, galactomannan, oligosaccharides) are discussed in the appropriate sections. 

In the normal human diet, about 25% to 50% of the caloric intake consists of fats and oils. These substances are the most concentrated form of food energy in our diet. When metabolized, fats produce about 9.5 kcal of energy per gram. Carbohydrates and proteins produce less than half this amount. For this reason, animals tend to build up fat deposits as a reserve source of energy. They do this, of course, only when their food intake exceeds their energy requirements. In times of starvation, the body metabolizes these stored fats. Even so, some fats are required by animals for bodily insulation and as a protective sheath aroimd some vital organs. 

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