FIG. 1A. A mitochondrion and surrounding cytoplasm from the liver cell of a 

 bat (Myotis lucifigus) . The structure, as is characteristic of liver cell mitochondria, 

 shows few cristae (cr), which are paddle-shaped. It is evident that the total mem- 

 brane surface of the cristae contiguous with the matrix (mx) of the mitochondrion, 

 is relatively small. This matrix, which is homogeneous and shows no evidence of 

 patterned fine structure, makes up a large part of the mitochondrial content. The 

 double membrane across the center of the mitochondrion is clearly an extension 

 of the inner of the two membranes limiting the organelle, and may represent a 

 stage in division of the mitochondrion (see Fawcett, 1955, for similar forms). 

 The dense granules evident at various points in the structure are characteristic of 

 many kinds of mitochondria; their function is unknown. Among mitochondria, 

 these of liver cells display one of the simplest morphologies. Magnification: 

 36,000 X. 



FIG. IB. A group of mitochondria (sarcosomes) as found in cardiac muscle of the 

 bat. It is clear at a glance that these mitochondria differ from those of liver in 

 possessing many more cristae (cr) and that the individual crista is much more 

 extensive and more nearly partitions the interior of the organelle. The total mem- 

 brane surface thus provided is much greater than in the case of liver mitochondria, 

 but the volume of matrix materials is proportionately less. The structural features 

 of these sarcosomes are characteristic of mitochondria associated with systems hav- 

 ing large energy requirements. Presumably the production of ATP by such mito- 

 chondria is greater than in those of liver. The myofibrils are indicated at my, the 

 elements of the meager sarcoplasmic reticulum at SR, and Z and M bands of the 

 sarcomere by Z and M, respectively. The structure at the arrow is unidentified but 

 appears to be continuous with the margins of one or two adjacent mitochondria. 

 This cristae-rich variant on the morphology of the liver mitochondrion is one of 

 the commonest. Magnification: 36,000 X. 



FIG. 1C. A more unusual form of mitochondrion as found in Paramecium aurelia 

 The mitochondria here are limited by the typical double membrane, but are dis 

 tinctive in possessing tubular cristae. These microelements have a diameter ap 

 proximately equal to the thickness of cristae encountered in other mitochondria 

 The number of tubular cristae is obviously great and achieves a large contact sur 

 face between membrane and matrix. The dense granules in the cytoplasm surround 

 ing the mitochondria represent glycogen. (Courtesy of Giuseppe Millonig.) 

 Magnification: 36,000 X. 



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