PROTOZOA AS CELLS 19 



single organelle. Some intermediate enzymes are dissolved in the 

 mitochondrial matrix. In addition, many mitochondria contain the 

 requisite enzyme systems for fatty acid oxidation, and they appear 

 to participate in active and selective transport of water and certain 

 solutes. Absorption and active extrusion of water occur, depend- 

 ing on the oxidation-reduction state of components of the respira- 

 tory chain on the mitochondrial membranes. The abundance of 

 internal membranes may sometimes be correlated with the oxida- 

 tive rate of the mitochondrion; in tissues such as mammalian liver 

 where mitochondria serve many accessory functions, internal 

 membrane area is small and matrix volume large. These facts give 

 added interest to the association of mitochondria with the contrac- 

 tile vacuole (see below) in some protozoa, and suggest that 

 Pelomyxa, with its occasional patterned mitochondrial structure, 

 might be favorable material for further exploration. 



Nearly every other cell component has been implicated as the 

 source or site of development of mitochondria. In the protozoa 

 these include small, undifferentiated, membrane-limited, precursor 

 bodies arising de novo {Paramecium, Wohlfarth-Bottermann, 1958c) 

 and nuclei (Pe/omyxa, Brandt and Pappas, 1959; Paramecium, 

 Ehret and Powers, 1955). In Pelomyxa, Brandt and Pappas, 

 employing light microscopy, saw ordinary mitochondria adhering 

 to the nuclear surface during and just after nuclear division. In 

 electron micrographs the limiting membranes of the two organelles 

 seemed to be continuous, and a filamentous material occurred in 

 mitochondria that was similar to a substance layered under the 

 nuclear membrane. An interesting sidelight on the question is 

 provided by studies of the zooflagellates Trypanosoma (Steinert, 

 1960) and Bodo (Pitelka, 1961b). In the trypanosome, a mito- 

 chondrion is observed to be joined to the kinetoplast, an extra- 

 nuclear, deoxyribonucleic acid (DNA)-containing body of 

 unknown significance. In Bodo (Fig. 5, PL I), the cell's single, very 

 long mitochondrion is continuous at both ends with the kinetoplast. 

 In the absence of any developmental studies, it is not possible to 

 conclude that mitochondria can originate at the nucleus, but 

 temporary or permanent fusion, with opportunity for exchange of 

 substances, between mitochondria and DNA-carrying bodies is 

 conclusively demonstrated. 



