ENERGY SOURCES 



253 



Energy sources 



What is the energy source for the synthesis of raw materials into proto- 

 plasm ? What substances are oxidized to carbon dioxide and water to provide 

 the energy for the synthesis of the large molecules in a growing cell ? In the 

 amphibian egg the amount of carbohydrate in the egg begins to decrease 

 sharply at about the time of gastrulation (Fig. 165). This carbohydrate 

 breakdown may furnish the energy for early development, just as carbohy- 

 drate breakdown in a muscle furnishes the energy for muscular contraction. 

 As a matter of fact, the same types of enzymes which play a part in the break- 

 down of carbohydrates in muscle are also present in the frog egg. 



Now the energy derived from the breakdown of carbohydrates may be 

 used for several purposes. It is needed, first of all, for the synthesis of the 

 protoplasm in the egg. Energy may also be necessary for the cell movements 

 which take place during gastrulation. It is probably also used in the process of 

 differentiation, because that process involves the formation of new com- 

 pounds, presumably proteins. And finally, energy is also necessary for the 

 maintenance of the embryo in a living condition. An embryo, like an adult, 

 is constantly expending energy to keep its cells in the living condition. 



Carbohydrate breakdown is not the only source of energy. At about the 

 time of hatching in the amphibian egg the lipids begin to be oxidized. If we 

 analyze for the amount of fat in the egg we find that it stays fairly constant 

 until just before hatching and then begins to be used up (Fig. 165) . 



CASTRULATIO 



TIME FROM FERTILIZATION 



TIME FROM FERTILIZATION 



Fig. 165. The utilization of carbohydrate and fat during early development 

 of the frog egg. Carbohydrate metabolism begins about the time when gastrula- 

 tion occurs. Fat metabolism starts later, at some time just before hatching, Stage 

 20, Figure 75. 



