146 CLELAND 



the protoplasm. This discovery has added a new importance to the chon- 

 driosomes and has focused renewed attention upon them. Cases are now 

 known where chondriosomes appear to have mutated — to have changed their 

 fundamental character by the loss of certain enzymes which they normally 

 possess. For instance, Ephrussi (1951) found in yeast a case of mutation 

 as a result of which cells previously capable of complete aerobic respiration 

 were now capable only of the less complete anaerobic type of energy release 

 because of the loss of the enzyme cytochrome oxidase. He found by the use 

 of Nadi reagent (a test for the presence of cytochrome oxidase, which helps 

 to make possible the use of oxygen in energy release) that, whereas unmutated 

 cells had particles in the cytoplasm which would stain blue with this reagent, 

 thus showing the presence of cytochrome oxidase, the cells no longer capable 

 of aerobic respiration no longer showed stainable bodies. Thus, aerobic respira- 

 tion was related to stainable bodies or chondriosomes, and the loss of an en- 

 zyme essential to the carrying on of aerobic respiration was shown to have 

 occurred in these bodies, confirming the conclusion that they are centers of 

 respiratory activity. 



The renewed interest in the cytoplasm has resulted in an increased use of 

 the electron microscope in order to learn more about the structure of the 

 bodies found in this region. Since these bodies are so small, their structure 

 is beyond the capabilities of the ordinary microscope to elucidate, and electron 

 microscopy seems the only feasible way now available to get at the details of 

 their architecture. The results of recent studies by such workers as Frey- 

 Wyssling (1953), Steinmann (1952), and Sjostrand (1956) have revealed 

 an amazing complexity of organization in bodies which had previously been 

 considered to be essentially structureless. Chloroplasts, which can often be 

 seen under the ordinary microscope to be filled with granules, or ''grana," are 

 now found to present a finely layered appearance, the grana as well as the 

 surrounding material having such a structure. The grana, which contain the 

 chlorophyll, seem to be composed of alternate layers of protein and fatty 

 material, and since the tadpole-shaped chlorophyll molecule is attracted at 

 its head end by water and at the tail end by oil, it takes up a position with 

 its head in the protein layer and its tail in the lipid layer. Thus the chlorophyll 

 becomes oriented in a very orderly and precise manner. 



Chondriosomes also appear to have a very intricate and precise structure. 

 The work of Sjostrand and others shows that they have a double membrane 

 with cross membranes extending part or all the way across the body. Like 

 the chloroplasts, therefore, they also have a form of lamellate structure. 

 Sjostrand has also shown that, in some kinds of cells at least, the cytoplasm 

 itself has a very elaborate structure, composed of closely folded, parallel 

 membranes which separate the protoplasm proper from a watery material 

 which fills part of the cell. 



