METABOLISM OF GREEN PLANTS 35 



1 . Food Making 



In short, Sphaerella takes materials from its surroundings 

 in the form of simple compounds, as carbon dioxide, 

 water, and mineral salts, which are relatively stable and there- 

 fore practically devoid of energy, and, through the radiant 

 energy of sunlight, shifts and recombines their elements in such 

 a way that products rich in potential energy result. Sphae- 

 rella thus exhibits the prime diagnostic characteristic of green 

 plants the power to construct its own foodstuffs. 



The key to this power of chemical synthesis by light 

 PHOTOSYNTHESIS resides in a complex chemical substance 

 called CHLOROPHYLL. 1 This pigment, which is segregated in 

 special cytoplasmic bodies known as CHLOROPLASTIDS, gives 

 to Sphaerella during its active phases and to the foliage 

 of plants in general their characteristic green color. The 

 chlorophyll arrests and transforms a small part of the energy 

 of the sunlight, which impinges upon it, in such a way that 

 the protoplasm can employ this energy for food synthesis. 



The first great step in the constructive process is a com- 

 bination of carbon with hydrogen and oxygen to form a 

 carbohydrate. Sphaerella gets these elements from carbon 

 dioxide and water by a process of molecular disruption. We 

 know that when charcoal, for instance, is burned, carbon and 

 oxygen unite to form carbon dioxide, and energy in the form 

 of light and heat is liberated. Obviously Sphaerella must 

 employ an equal amount of energy in separating the carbon 

 and oxygen of carbon dioxide; that is, in overcoming their 

 chemical affinity. And this kinetic energy which the plant 

 employs is then represented in the chemical potential which 

 exists between the oxidizable carbon and free oxygen it is 



1 A rough approximation of the formula of chlorophyll has been given as: (MgN- 

 CajHwOO) (COOCHs) (COOCzoHsO. A slight chemical modification of chlorophyll 

 results in hematochrome, which gives at certain times the reddish tinge to Sphaerella. 



