phosphorylate ADP to ATP and is called photophosphorylation. The 

 fixation or incorporation of carbon dioxide into ribulose 1,5-diphosphate 

 leads to the production of 3-phosphoglyceric acid. The latter is subse- 

 quently reduced to 3-phosphoglyceraldehyde by TPN and ATP gener- 

 ated in the photophosphorylation reaction. Reversible operation of the 

 Embden-Meyerhof cycle of glycolysis starting with 3-phosphoglyceralde- 

 hyde proceeds through a series of enzymatic steps, all of which are 

 exergonic, to produce glucose (Figure 3-17). 



Recent biochemical studies have demonstrated that both intact and 

 fragmented chloroplasts and grana are all capable of carrying out photo- 

 phosphorylation. These observations, particularly those pertaining to the 

 grana, recall the previous comments concerning the respiratory enzyme 

 assemblies isolated from mitochondria which were found to exhibit oxi- 

 dative phosphorylation. 



As in the case of mitochondria, it is not known whether plastids are 

 produced de novo in the cell or by some form of replication. Indirect 

 evidence favors the latter view. For example, plastids which for one 

 reason or another have lost their capacity to produce chlorophyll appar- 

 ently give rise to similarly deficient plastids. It is interesting to note that 

 chloroplasts and mitochondria somewhat resemble each other in that 

 both: (1) increase in number in the cell, (2) segregate at cell division, 



(3) contain respiratory pigments, (4) consist largely of lipoprotein, and 

 (5) possess a double-layered limiting membrane and an internal lamel- 

 lar organization. In spite of the apparent similarities in structure and 

 behavior of these two organelles, there is no real evidence to indicate that 

 one can be converted into the other. 



The development of chloroplasts in green plants has been studied with 

 the aid of the electron microscope. According to von Wettstein (1959), 

 the steps involved (Figure 3-18) are the following: (1) synthesis of 

 vesicles within the plastid primordia, (2) aggregation of these vesicles 

 to form single chains of interconnected vesicles, (3) rearrangement and 

 fusion of the chains to form parallel double membranes or lamellae, 



(4) multiplication of the lamellae, and (5) the growth and differentiation 

 of the lamellae into grana and stroma lamellae. This developmental 

 sequence is obviously under the control of many genes, and mutations 

 would be expected to block development at numerous points. In most 

 cases the phenotypic endpoint would be the same, namely, a functionless 

 chloroplast. Comparison of the development of chloroplasts in known 

 mutants may often serve to show the location of the block, at least with 

 respect to the structural deficiency. For example, in the white or albina- 

 20 mutant of barley, electron microscopy studies have shown that the 



44 / CHAPTER 3 



