20 : 2/ Photosynthesis 



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is shown in Figure 1 b. In euglena, the entire chloroplast is one granum. 

 In most other organisms, there are 10 to 100 grana per plastid. All of 

 the chlorophyll, and presumably all of the light absorbing pigments of 

 the plastid, are contained in the grana. 



The grana can be isolated by breaking the plastids and then centri- 

 fuging. They contain large amounts of lipid material. Each granum 

 is a highly oriented system of anisotropically arranged molecules. 



(a) 



(b) 



Figure 2. Green grana of corn leaves, (a) A sketch of an 

 electron micrograph of a section of granum within an intact 

 chloroplast of Zea mais. There are about 50 such grana per 

 chloroplast. Each granum has about 15 parallel lamellae 

 which are about 400 A thick and about 4,000 A in diameter. 

 (b) A sketch of an electron micrograph of a granum which is 

 believed to be dissociated into separate disks. After E. I. 

 Rabinowitch, Photosynthesis, II, 2 (New York: Interscience 

 Publishers, Inc., 1956), from Vatter, unpublished, modified. 



Figure 2 shows the lamellar structure found within grana by electron 

 microscopy. The granum appears to be made up of piles or stacks of 

 plates. The individual plates are about 100 A (10 m/x) thick; there are 

 about 40 plates per stack. The structure is very similar to that of the 

 rods of the vertebrate retinas. As shown in Figure lb, the grana are 

 regions where the density of lamellae is greater than elsewhere. Many 

 of the lamellae are continuous with those outside the granum. 



Although apparently endless variations exist on the structures out- 

 lined in the preceding paragraphs, the general characteristics are 

 common to all photosynthetic organisms, except bacteria; namely, the 

 absorbing pigments are oriented on a molecular level in small plates. 



