306 Essays in Biochemistry 



plants and animals reveals certain interesting exceptions. Whereas 

 plants in general accumulate polysaccharides and are poor in lipids, 

 many important vegetable oils do occur. With few exceptions, how- 

 ever, major lipid storage in plants is restricted to seed parts. Again, 

 whereas it is the habit of animals to accumulate lipid rather than 

 polysaccharide, in the tissues of molluscs one finds large amounts of 

 glycogen and but little fat. These exceptions suggest the possibility 

 that the habit of preferential lipid storage is an adaptation to motility. 

 An obvious advantage of lipid storage over polysaccharide storage is 

 that, per calorie stored, lipid weighs far less than carbohydrate. This 

 is of little survival benefit to sessile forms of life such as the higher 

 plants, but it may be of real importance to the mouse, who must evade 

 his natural enemies, or the cat, who must capture the mouse. Of all 

 the tissues of higher plants, it is uniquely in the seed parts that light 

 weight may be considered to be of survival value. It is necessary both 

 for the survival of the individual and of the species that seeds be 

 disseminated, and, regardless of the vector, the less a seed weighs 

 the higher the probability that it will be carried away from its source. 

 Molluscs are undoubtedly motile, but of all animal species few are 

 better protected against natural enemies and few have less need for 

 motility in order to survive. What has been said of molluscs may also 

 be said of the mammalian fetus. 



From the foregoing it will be seen that in the starch-glycogen group 

 of polysaccharides nature has produced a system peculiarly and ele- 

 gantly adapted to the function of energy storage. Each glycogen mole- 

 cule acts as a minute and elastic glucose reservoir at a molecular level 

 and is continuously acquiring and losing glycosyl residues at its pe- 

 riphery. As an energy reservoir it suffers, however, in comparison 

 with fat in that, per calorie stored, it adds far more to organism weight. 

 Although this is unimportant to the survival of such forms as do not 

 depend upon motility, such as the tuber, the tree, the mollusc, or the 

 fetus, it may be of importance to the plant seed and the adult mammal. 

 In these latter forms the habit of lipid storage has largely supplanted 

 the habit of polysaccharide accumulation. 



Reverting to the question of whether mammalian-liver glycogen is a 

 biochemical vestige, we are still inclined to the opinion that it is not. 

 It is, however, regarded as entirely possible that a transfer from a 

 predominantly glycogenic habit to a predominantly lipogenic habit has 

 real survival value to many motile forms of life and that this change 

 has, in the long course of evolution, actually occurred. 



