PROTOPLASM AS A PHYSICAL SYSTEM 51 



plants and possibly in other motile organs. But in all 

 cases the work performed in the response represents 

 energy derived from metabolic breakdown, although this 

 energy may act in the intervals between stimulation by 

 developing a tension or turgor which is released only at the 

 moment of stimulation. A fundamental problem thus 

 arises with regard to the general nature of the conditions in 

 living matter which render its rate of chemical reaction so 

 readily alterable by physical changes in the system. 



The most significant general fact is that it is only 

 while the cell is living that its rate of metabolism is 

 readily and quickly changed by a stimulating condition. 

 In general, also, it is only during life that the energy- 

 yielding forms of metabolism have a high rate or intensity; 

 typically COa-production, heat-production, and con- 

 sumption of oxygen decrease greatly at death, although 

 they may not cease entirely. One of the most remarkable 

 peculiarities of living protoplasm, considered as a 

 chemical reaction-system, is that its chief energy- 

 yielding reactions, e.g., oxidation of sugar, proceed 

 rapidly at low temperatures, and in a medium which is 

 approximately neutral. To produce a corresponding 

 speed of reaction in vitro, high temperatures or strong 

 reagents are required. It is probable that the conditions 

 which determine the susceptibility to stimulation are 

 the same as those which are responsible for the high 

 velocity of the energy-yielding reactions. The nature 

 of these conditions is imperfectly understood at present; 

 but apparently they are especially favorable to certain 

 types of oxidation; e.g., of carbohydrates. 



The indications are that structural rather than purely 

 chemical factors are of chief importance, since the 



