EPILOGUE 1991 



corresponding to a 100% conversion of light energy into chemical energy, 

 and thus implying a complete absence of "friction losses" in the whole 

 reaction sequence of photosynthesis. There is, however, considerable 

 evidence that at low light intensities (or in short illumination periods) 

 intermediates of respiration can be used as substrates of photosynthesis, 

 with an apparent decrease in quantum requirement. 



One area in which great progress has been achieved since this monograph 

 was begun, is the morphology of the photosynthetic organs. We now 

 know, from electron-microscopic evidence, that a general feature of these 

 organs are lamellae, 70-200 A thick, either running through the whole 

 chloroplast, or forming cylindrical stacks ("grana") perhaps 2 n thick, 

 suspended in the "stroma." It seems likely that these lamellae are formed 

 by single or double protein layers, coated with monomolecular layers of 

 chlorophj'll ; but the precise arrangement of chlorophyll molecules remains 

 to be established, and practically nothing is known as yet about that of 

 the "accessory" pigments, the carotenoids, and the phycobilins. 



Since Chapter 37A was completed, the technique of sectioning, permit- 

 ting the preser\ation of much of the original structure of the chloroplasts, 

 has been further improved, and the relation between laminar and granular 

 chloroplasts has emerged with new clarity. As dimly perceived before, the 

 whole chloroplast has a laminar structure, with lamina converging and 

 merging at the two poles (c/. fig. 38.4). In a certain stage of the develop- 

 ment of most (but not all) chloroplasts of the higher plants, the lamina 

 become thicker and denser in certain regions, until, in many cells, these 

 denser regions accjuire the sharp outhnes of "grana." The latter remain, 

 however, connected by thinner lamina. The lamina probably contain more 

 lipoids than the interlaminar material (since they are preferentially fixed 

 by OSO4). This structure was demonstrated by Steinmann and Sjostrand* 

 in Aspidistra elatior; even more revealing in their clarity were the elec- 

 tron micrographs obtained by Vattert with maize (cf. figs. 38.1-38.3). 



The two-dimensional, lamellar structure may serve the purpose of easy 

 access to, and removal away from, the light-activated pigment molecules, 

 of chemical agents ("co-enzymes") or of reaction intermediates, including 

 organic acids or aldehydes (intermediates in the reduction of carbon di- 

 oxide), and peroxides (intermediates in the oxidation of water). In addi- 

 tion, however, this structure may also permit the migration of excitation 

 energy toward the reaction substrates or enzymes. The actual occurrence 

 and extent of such energy migration between the chlorophyll molecules 



* Steinmann, E., and Sjostrand, F. S., Experimental Cell Research, 8, 15 (1955). 

 t Vatter, A., Thesis, University of Illinois, 1955. See figures 88.1, 2, and 3. 



