THE MECHANISM OF PHOTOSYNTHESIS 295 



of molecules are involved (Wassink, 1946a, 1954*). The action of the 

 light is that of a stimulus, in Pfeffer's sense, and in more recent times the 

 connection between the primary action and the final effect has been suit- 

 ably termed "amplification" (Jordan, 1932, 1938). Biochemically it can 

 be visualized that in these cases an enzyme or carrier molecule is hit in an 

 early stage of the reaction chain, 'thus influencing a lot of molecules mov- 

 ing along this path in much the same way as the grid in a radio tube 

 directs and modifies the energy stream (Wassink, 1946a, b). In these 

 cases the light energy plays hardly any role in the energy balance of the 

 process. In photosynthesis, on the other hand, the light energy is essen- 

 tial to complete the energy balance of the process and, other things being 

 equal, of the cell. This statement is not invalidated by the fact that 

 some types of photosynthesis exist (e.g., in colored sulfur bacteria) in 

 which the gross reaction mechanism requires hardly any supply of energy. 

 Even in these cases some intermediate step also probably requires a 

 stoichiometric intervention of light energy for its completion. 



One remark should be added. In the future, it may become desira- 

 ble to denote "photosynthesis" as "carbon dioxide photosynthesis" or 

 "carbohydrate photosynthesis." It was mentioned previously that the 

 reduction of carbon dioxide very probably takes place in the dark with 

 the aid of something prepared in the light. Other cellular processes such 

 as nitrate reduction or formation of energy-rich phosphate bonds might 

 turn out in the future to have essentially the same relation to light as 

 carbon dioxide reduction. 



2. GENERAL CHARACTERIZATION OF THE PHOTOSYNTHETIC PROCESS 



Under the influence of light energy the green plant performs the over- 

 all reaction 



CO2 + H2O ^ tCH20) + O2, (5-1) 



in which (CH2O) stands for carbohydrate in general. Actually the proc- 

 ess going on in the plant consists of a number of links or partial reactions, 

 to be discussed later in greater detail. Carbon dioxide assimilation, 

 moreover, may result in growth of the plant, which means that photo- 

 synthesis is linked with other processes such as protein formation. This 

 will be the more so in cells such as quickly multiplying algae, which, 

 while photosynthesizing, show an active over-all metabolism. But Smith 

 (1943) showed that, in adult leaves of certain plants, the photosynthetic 

 process very closely approaches the over-all reaction, Eq. (5-1). 



Early in the study of photosynthesis it was recognized that only the 

 green parts of plants are capable of carbon dioxide assimilation. Ingen- 

 Housz (1779), in particular, clearly demonstrated the effect of light and 



* See Addendum. 



