accumulating large amounts of starch (amyloplasts), oil (elaioplasts), 

 or protein (aleurone-plasts) within their interior. The most important 

 of the chromoplasts, so far as present knowledge is concerned, appears 

 to be the chloroplast, which is concerned with the elaboration of chloro- 

 phyll and carbohydrate photosynthesis. The photosynthetically active 

 chromoplasts (i.e., chloroplasts) contain both chlorophyll and carote- 

 noid pigments, while chromoplasts exhibiting no photosynthetic activity 

 generally contain carotenoids but are lacking in chlorophyll. 



Chloroplasts are present in all green plants and are particularly abun- 

 dant in cells actively engaged in photosynthesis (Figure 3-13). Not all 

 organisms capable of photosynthesizing carbohydrate contain organized 

 chloroplasts, for example, blue-green algae and certain bacteria which 

 photosynthesize in the infrared. In these cases the photosensitive pig- 

 ments appear as minute granules or vacuoles, called chromatophores, 

 dispersed in the cytoplasm (Figure 3-14). The typical chloroplast is a 

 more or less ovoid structure bounded by a double-layered membrane and 

 containing a relatively homogeneous matrix called the stroma in which 

 are embedded granules or lamellae referred to as the grana. As viewed 

 with the electron microscope, the grana appear to have a layered struc- 

 ture consisting of a series of membranes stacked one on top of the other 

 (Figure 3-15 (a) and (b)). The grana, which are more or less centrally 

 placed in the body of the chloroplast, are connected with each other by 

 paired membranes which also extend out into the stroma {stroma lam- 

 ellae). Spherical, highly electron-dense bodies called globiili are often 

 found freely dispersed in the more internal parts of the organelle. In higher 

 plants, the dimensions of the grana appear to be in the order of 0.5 mi- 

 crons, while the number of membranes or larnellae making up the individ- 

 ual grana vary considerably. The membranes of the grana measure about 

 40 to 60 A in thickness and appear to be double-layered structures. The 

 stroma lamellae are apparently also paired membranes and are somewhat 

 thinner, measuring about 20 to 30 A (Figure 3-16 (a) and (b)). The 

 lamellae of the chloroplast are considered to be protein structures and 

 to carry one or more layers of chlorophyll intermixed with other com- 

 ponents of the photosynthetic system such as the carotenoids and pyri- 



Figure 3-15. (a) Electron Micrograph of Chloroplast from Tobacco 

 Leaf showing the grana, starch grains (large, clear spherical areas), and 

 double-layered enveloping membrane. Approximately 60,000 X. (b) Highly 

 Magnified Section of a Barley Leaf Chloroplast showing the lamellar ar- 

 rangement of the grana. Approximately 1 90,000 X. (Courtesy of Dr. D. von 

 Wettstein, Forest Research Institute, Stockholm, Sweden.) 



STRUCTURE AND FUNCTION OF CYTOPLASMIC ORGANELLES / 41 



