THE CHLOROPLAST 99 



been shown to be photoconductive (Nelson, 1957). Monolayers of 

 chlorophyll, chlorophyll plus ^-carotene, or yS-carotene alone, spread 

 on various surfaces, have been demonstrated to be photoconductive 

 (Arnold and Maclay, 1958). The role of the carotenoids in the 

 chloroplast is more controversial. Lynch and French (1957) found 

 that when the chloroplasts were extracted with petroleum ether, the 

 chloroplasts were inactivated, whereas the addition of ^-carotene 

 restored their activity; hence, /3-carotene must be an active com- 

 ponent of the photochemical system. This study neglected the role 

 of the lipids in the complex. Fujimori and Livingston (1957), from 

 flash-photoh'sis studies, also indicated that carotene plavs a direct 

 part in the primary act of photosynthesis. On the other hand, Stanier 

 (1958), working with mutants of the purple bacterium Rhodospiril- 

 lum, and Sager (1958), with mutants of the alga Chlamijdomonas, 

 indicate that the carotenoids may act only in catalvtic amounts and 

 protect the chlorophyll molecules against high light intensities, tem- 

 peratures, and oxygen concentrations. Piatt (1959) suggests that 

 if the carotenoids are part of the primary sequence, there is a 

 "photosynthetic unit" that can transfer their absorbed energy to a 

 single reaction site. 



Chlorophyll and carotenoid in their natural state within the 

 chloroplast do not exist as free pigments but are bound to proteins 

 or lipoproteins. There is an obvious analogy to hemoglobin, where 

 heme is bound to globin, i.e., a chloroglobin. In order to build some 

 models for the chloroplast structure, we have undertaken several 

 experimental approaches. 



Chlorophyll-Protein Complex. Investigations of chlorophvll 

 complexed with proteins indicated that some of the complexes have 

 properties similar to those of active chloroplasts (Rodrigo, 1955). 

 We have tried to learn something of the nature of these chlorophyll- 

 protein complexes formed with a variety of proteins. The chloro- 

 phyll was extracted from the chloroplasts in 85 per cent acetone 

 (including the carotenoids and other extractables ) and was added 

 to various protein solutions (20 mg/ml in 0.9 per cent NaCl), until 

 no further precipitation occurred. The heavy green chlorophyll- 

 protein precipitate was then washed and resuspended in salt solu- 

 tions of various concentrations. The stability of the complex was 

 deteitnined from absorption spectra, electrophoresis, and analytical 

 ultracentrifugation. It was found that the extracted chlorophyll 

 complexed more readily with native globulins containing lipid and 



