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Photosynthesis /20 : 2 



responsible for the green color. A wide variety of experiments indicate 

 that photosynthesis in green plants and euglena occurs only in the 

 chloroplasts. 



The size and shape of the chloroplasts vary quite widely. The most 

 studied organisms are two genuses of one-celled green algae called 

 Chlorella and Scenedesmus. A diagram of a cross section through a chlor- 

 ella cell is shown in Figure la. Chlorella has only one cup-shaped 

 chloroplast per cell. It differs in this respect from many other algae 



Pyrenoid 



Cell Wall 



Cup shaped 

 Chloroplast 



Nucleus 



(b) 



Figure I. (a) Three Chlorella cells. This diagram emphasizes 

 that the single cup-shaped chloroplast occupies most of the cell. 

 The pyrenoid is associated with starch and/or protein synthesis 

 and/or storage, (b) A corn chloroplast. Sketch of an 

 electron micrograph of a chloroplast from Zea mais. The 

 dark regions are the grana. They are cylinders about 4,000 

 to 6,000 A in diameter and 5,000 to 8,000 A in height. After 

 E. I. Rabinowitch, Photosynthesis, II, 2 (New York: Interscience 

 Publishers, Inc., 1956), from Vatter, unpublished, modified. 



and most higher plants, all of which have many chloroplasts per cell. 

 The chloroplasts of higher plants are shaped like a saucer with a diameter 

 of 4 to 6 fx and a thickness of 0.5 to 1.0 (jl. In nongreen plants, the 

 pigmented organelles responsible for photosynthesis are called by other 

 names such as chromoplasts. A more general term used for both chloro- 

 plasts and chromoplasts is plastid. 



The algal cells have one to 50 plastids per cell. When these plastids 

 first form they are homogeneous, but as they develop, structure appears. 

 They are filled with smaller dark bodies called grana, which contain all 

 the photosynthetic pigment. A simplified cross section of a corn plastid 



