160 - The Cell 



Fig. 9-2. Chloroplast of a corn plant: electronmicrograph with a total magnification of about 24,000. Chloro- 

 phyll is restricted to the grana (one granum encircled). The form of a granum, in three dimensions, is suggestive 

 of a stack of coins. The several grana are suspended in the chloroplast by strands of material, called stroma. 

 (Courtesy of A. E. Vatter, University of Colorado, Medical Center.) 



of the granum (Fig. 9-3). Precise measure- 

 ments of the thickness of each lamella and of 

 the density of the components indicate that 

 the layers of chlorophyll are regularly flanked 



Fig. 9-3. Single granum at higher magnification 

 (about 160,000). This reveals the lamellar structure. 

 A thin layer of chlorophyll lies between the two mem- 

 branes in each lamella, and the lamellar membranes 

 appear to be made up of protein and lipid compo- 

 nents. (Courtesy of A. E. Vatter, University of Colo- 

 rado, Medical Center.) 



by layers of protein and of a speciali/ed lipid 

 material. In any event, the fine structural or- 

 ganization of the grana and of the other parts 

 of the chloroplast provides an amazingly 

 effective molecular mechanism for utilizing 

 light energy. This mechanism permits chloro- 

 phyll, working in partnership with various 

 other enzymes and coenzymes, to transform 

 and conserve light energy in the form of or- 

 ganic compounds newly synthesized from in- 

 organic forms of matter. 



Some Modern Developments. Since 1905, 

 starting with the work of a British botanist, 

 F. F. Blackman, it has been known that the 

 synthesis of carbohydrates and other organic 

 compounds by green plants must involve two 

 separable kinds of reactions: (1) light reac- 

 tions, an exceedingly rapid set of reactions 

 that are energized by light; and (2) dark re- 

 actions, a series of slower reactions that can 

 proceed in the absence of light. But truly im- 

 pressive progress in the field of photosyn- 



