90 MACROMOLECULAR COMPLEXES 



pievioush obtained spectra for the in vivo Euglena chloroplast and 

 for digitonin extracts of their chloroplasts (Fig. 3). The concentra- 

 tion of chlorophyll calculated from the absorption data corroborates 

 the previous analysis that there are of the order of 10^ chlorophyll 

 molecules per chloroplast. By scanning across the chloroplast at 0.5/>t 

 intervals at the absorption maximum 675 mfj. and at the absorption 

 minimum 550 m/>i, it can be shown that the chloroph\'ll is evenly 

 distributed throughout the whole chloroplast (Fig. 4). Because 

 of the limitation in the resolving power of the instrument it is not 

 possible to locate chlorophyll precisely in the chloroplast lamellae. 

 Together with microspectrophotometry, the techniques of polariza- 

 tion, fluorescence, phase, and interference microscopy are the most 

 direct approach for exploring cellular structure. These methods per- 

 mit the study of the chloroplasts in their natural state. 



The electron microscope now permits us to see through the fixed 

 chloroplast in situ instead of seeing its shadow or replica. The in- 

 creased resolution of the electron microscope has greatly aided our 

 attempts to obtain a molecular picture of the chloroplast. In the 

 preparation for electron microscopy, the cells were fixed with 1 per 

 cent osmium tetroxide buffered with acetate-veronal pH 7 to 8. The 

 osmotic changes were partially controlled by adding sucrose 

 (0.15M) to the fixative. The fixed cells were embedded in a resin 

 ( /j-butyl methacrylate, methyl methacrylate, or mixtures of these ) . 

 Thin sections, less than 0.05/x in thickness, were cut with a glass 

 knife. 



Chloroplast Structure 



There is now good evidence from a great variety of plant species 

 that, although chloroplasts may differ in shape and size, they have 

 in common a basic internal organization. We have investigated the 

 structure of chloroplasts in several algae and higher plants with the 

 electron microscope. These studies reveal that the chloroplasts are 

 composed of lamellae. 



We have found the algal flagellate Euglena to be uniquely adapt- 

 able for experimental studies of the chloroplast structure, since 

 Euglena behaves as a photosynthetic plant cell in the light and as 

 an animal cell in the dark. Changes in the environmental conditions 

 ( light, darkness, temperature, drugs ) are reflected in the organisms' 

 chemistry and morphology. Using Euglena, we have been able to 



