Ghloroplast Structure and Its Relation to Photo- 

 synthesis 



S. GRANICK, Rockefeller Institute for Medical Research, New York Citij, 



New York 



Considerable progress has been made in recent years in the study 

 of the structures of the chloroplast. I should like to summarize very 

 briefly what is kno\\Ti of these structures and discuss what they might 

 represent. 



On the basis of the electron microscope studies of various workers 

 (1) the following interpretation is presented of the structure of the 

 generalized chloroplast of higher plants. A chloroplast is a saucer- 

 shaped structure 4 to G m in diameter and 0.5 to 1 m in thickness. The 

 chloroplast is surrounded by a semipermeable membrane 50 to 100 A 

 thick, that is, equivalent to the diameter of 1 or 2 protein molecules. 

 Each chloroplast contains about 50 dense "grana" which lie embedded 

 in a colorless stroma or protein matrix. 



A dense granum has the shape of a column or cylinder 4000 to 

 6000 A in diameter and 5000 to 8000 A in height. Each granum con- 

 sists of a stack of 15 or more dense parallel membranes or lamellae, 

 piled one on top of another like a stack of poker chips. The thickness 

 of a lamella is about 35 to 50 A. In one interpretation a pair of ad- 

 jacent lamellae is considered to represent the upper and lower mem- 

 brane of a "disc." The disc would be as wide as a granum (4000 to 

 6000 A) and have an overall thickness of 130 A; the disc would con- 

 sist of a dense upper and lower membrane (each 35 A thick) enclosing 

 a space of about 65 A between the membranes. Attached to each 

 upper and lower membrane of a disc is a delicate membrane, 30 A 

 thick, which extends out into the stroma and appears to connect one 

 granum with another. In addition there is a space of about 65 A which 

 separates these fine membranes from each other. Thus, proceeding 

 downward through a granum we meet in succession a disc meml)rane 

 35 A thick, an intradisc space of 65 A, a disc membrane of 35 A, an 

 interlamellar membrane of 30 A, a space of 65 A, an interlamellar mem- 

 brane of 30 A, and again a disc membrane of 35 A, etc. (2). 



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