378 PHOTOSYNTHESIS 



colloidal state. While this has been widely accepted, the work of Stern 

 has cast some doubt upon the validity of this conception. It, moreovei 

 assumes that the colloidal state is due to the union of secondary valencies 

 and accepts the possibility of a chlorophyll-carbonic acid compound. 

 Nevertheless, the theory is not without interest and is also suggestive of 

 possible mechanisms of the action of other colloidal substances which be- 

 have like catalysts, as for example, some of the enzymes. The theory 

 suggests that the condensing or synthesizing action of enzymes may be due 

 to the fact that these are colloidal substances or aggregates of molecules 

 which absorb or adsorb molecules of simple structure, A. The adsorp- 

 tion compound undergoes change, e.g., loss of water, and subsequently 

 the enzyme and a complex compound, the product of the condensation 

 of several molecules of A, are formed. 



2. The Chloroplasts 



The extreme difficulties which are associated with any endeavor to 

 interpret the reactions of a living organism in terms of the concepts of 

 physics and chemistry become evident when the finer anatomical structure 

 of the organisms is considered. Our knowledge of chemistry has been 

 obtained largely from intensive study of the properties of matter under 

 known and well defined conditions. One of the purposes of laboratory 

 investigation has been, as it were, to isolate and confine single reactions and 

 to study these under control of all imaginable conditions. It has thus 

 been possible to determine what conditions influence the course and rate 

 of the reactions. This has been a comparatively simple task, though it 

 has led to a prodigious mass of information regarding the properties of 

 matter and the conditions under which this changes form. There are 

 many chemical reactions, of inorganic and carbon compounds, taking 

 place in a homogeneous system, which are now fairly well understood, 

 and while in heterogeneous systems more complexities are introduced, 

 nevertheless in most of these reactions the properties of the substances 

 involved are either already well known or can eventually be determined. 



The chemical reactions in living organisms, on the other hand, occur 

 in minute compartments, the cells. In no case are the chemical changes 

 taking place in a cell confined to one reaction or a group of similar re- 

 actions. In the leaf of a plant, at least, each cell is a whole chemical 

 laboratory in itself. The cells are microscopic units of complex physical 

 structure, which can be discerned only under the microscope. Many 

 of the reactions in cells are apparently catalytic, i.e., enzymatic or sur- 

 face reactions. The agents inducing such reactions are usually present 

 in very small amounts, are exceedingly complex substances, easily dis- 

 integrated and therefore not amenable to study by the same means 

 applicable to other chemical reactions. There is an enormous difiference 

 between a clean glass vessel into which substances can be introduced at 



