32 GENERAL PHYSIOLOGY 



To begin with, the substances consumed by them, possess no potential 

 energy, but light, in connection with their content in chlorophyl, gives 

 rise to a splitting of the molecules of the carbon dioxid and water so 

 that the resulting atoms of carbon, hydrogen and oxygen are at liberty 

 to enter other chemical combinations. In this way, a number of com- 

 plex substances are produced, representing a large store of potential 

 energy, which is made use of later on by the animal cell. It is true, 

 however, that this assimilation and synthesis is associated with dis- 

 similation, in the course of which the plant gives rise to waste products 

 and generates certain forms of energy, such as motion, heat, light, and 

 osmotic power. It is quite apparent, however, that in the case of the 

 plants the kinetic energy is rather subordinate to the potential — a rela- 

 tionship which is reversed in the animal. 



The. energetics of a cell present themselves in various forms which, 

 as we have just seen, may be grouped as resting or potential energy 

 and as moving or kinetic energy. Among the former we have chemical, 

 osmotic, cohesion and gravitation forces, and among the latter mechan- 

 ical power, heat, light and electricity. But naturally, this classi- 

 fication is not fixed, because some of these energies may present them- 

 selves in either form. The chemical energy, for example, remains 

 potential only as long as the atoms retain their position toward one 

 another and becomes kinetic as soon as they rearrange themselves in 

 accordance with their specific affinities. Thus, the animal receives 

 potential chemical energy in the shape of complex organic substances 

 and as oxygen. The regrouping of the former under the influence 

 of oxygen eventually gives rise to carbon dioxid, water and simpler 

 nitrogenous bodies as well as to a large amount of actual energy. 

 Metabolism, therefore, is intended to keep the organism in energy- 

 equilibrium. The chemical intake and outgo are balanced in such 

 a way that the cells can continue to furnish the energy required of 

 them. The metabolic equilibrium and the dynamical equilibrium 

 must in the long run pursue a parallel course. 



Living substance presents itself in many characteristic forms, the 

 study of which has always been apportioned to morphology. It is 

 true, however, that a hard and fast line between the structural and 

 functional aspect of living matter cannot be drawn, because the former 

 changes constantly under the influence of different physiological 

 conditions. An organism is always in activity and conditions within 

 it are never at a standstill, although in many cases these processes 

 may be either very slow or too minute to be immediately apparent. 

 Thus, the metabolic changes are balanced in such a way that the 

 losses suffered in consequence of dissimilation are always made up, 

 allowing the cell to increase its substance and to grow. Growth is 

 the simplest manifestation of organic progress. In the second place, 

 living substance in any form is capable of reproducing its like so that 

 its continuance is assured as long as conditions favorable for its exist- 

 ence prevail. If the environment changes, living substance possesses 



