428 HAROLD F. 5LUM 



exactness for different structures. In some parts of cells, and in some 

 of the fluids outside the cell proper, there are substances dissolved in 

 simple solution, but this is probably the exception rather than the 

 rule. In such organized systems the quantum states of a given mole- 

 cule may be different from those of a molecule of the same kind that 

 finds itself in a simpler environment, say in an organic solvent. 

 Close chemical combination with another molecule may alter some of 

 the electronic energy levels, and even very loose association of mole- 

 cules may be expected to alter their rotation and vibration states; 

 changes in absorption spectra must result. Minor alterations, par- 

 ticularly of rotation-vibration states, would usually not be expected 

 to affect the absorption spectra of large molecules to a great extent; 

 and the general character of the absorption spectrum may often be 

 retained. Thus, one may hope in many instances to find close re- 

 semblance between the absorption spectra of compounds extracted 

 from living systems and the same compound inside the living cell, 

 but one must also be prepared for discrepancies of considerable mag- 

 nitude. Hydrogen ion concentration is particularly liable to affect 

 absorption spectra of substances in solution, as witness the case of 

 acid-base indicators, and hence, when the pH of the extracted solu- 

 tion differs appreciably from the in vivo condition, wide discrepancy 

 in absorption spectra may appear. 



Another factor that may lead to discrepancy is failure of Beer's 

 law, e.g., the absorption spectrum of some compounds in solution may 

 show considerable shift in absorption spectrum with dilution. The 

 causes for such changes are not clearly understood, and it is well to 

 guard against disagreement by making absorption measurements of 

 extracted substances in as nearly as feasible the same concentrations 

 as those at which they exist in the living system concerned. 



With the complex nature of the systems under consideration, it is 

 impossible to make any general rule regarding the nature of the dif- 

 ferences to be expected between the absorption spectrum of a com- 

 pound inside a living cell and the same compound when extracted 

 therefrom. This is a matter calling for the exercise of careful judg- 

 ment in interpretation. 



3. Effect of Light Scattering 



All molecules scatter light to a certain extent; in general, the 

 scattering increases with molecular size and with decrease in wave- 



