Chapter 14 



THE CHLOROPLASTS AND CHROMOPLASTS 



A. Structure of the Chloroplasts * 



1. Number, Size, and Shape 



Chloroplasts are small green bodies enclosed in the cytoplasm of the 

 higher plants and green algae. Together with the corresponding organs 

 of red and brown algae, they are included in the more general term 

 chromoplasts. Blue-green algae do not contain any chromoplasts at all, 

 and the same is probably true of green and purple bacteria (c/. Metzner 

 1922). 



The importance of chromoplasts for photosynthesis is indicated by 

 the fact that all chlorophyll (as well as the other pigments related to 

 photosynthesis — the carotenoids and phycobilins) are concentrated in 

 them. (Only pigments of the blue-green algae are distributed more or 

 less uniforml}^ in the " chromatoplasm " of these primitive organisms.) 

 Sachs asserted, in 1862, that the formation of starch grains inside the 

 chloroplasts during photosynthesis proves that these bodies are the site 

 of the photosynthetic process. Reinke (1883) remarked that this proof 

 is not convincing, because chloroplasts can convert externally supplied 

 sugars into starch as well (c/. page 47). He agreed, however, that the 

 observation of Engelmann (1882, 1883) that oxygen-sensitive motile 

 bacteria are attracted by the chloroplasts gives an indisputable proof of 

 the production of oxygen in these bodies. 



It has been generally accepted since the time of Engelmann and 

 Reinke, that the reaction sequence of photosynthesis begins and ends in 

 the chloroplasts, despite the occasional, rather vague discussion of a 

 "protoplasmic factor" as a regulating influence in photosynthesis. 

 However, the inability of isolated — even if apparently intact — chloro- 

 plasts to carry out photosynthesis (c/. Chapter 4, page 62) is an indica- 

 tion that this process requires the cooperation of the cytoplasm. Experi- 

 ments of Frenkel, described on page 204, make it ajjpear probable that 

 the preliminary dark fixation of carbon dioxide may take place outside 

 the chloroplasts. Hill's obsei-vation (cf. page 63) that isolated chloro- 



* Bibliography, page 394. 



355 



