(DPN and TPN, respectively). These electrons are then passed to mo- 

 lecular oxygen via a series of specialized enzymes known as the cyto- 

 chromes, which are also associated with the mitochondria. The energy 

 released by this electron transfer is conserved, in part at least, by cou- 

 pling oxidation with the phosphorylation of adenosine diphosphate 

 (ADP) to form adenosine triphosphate (ATP). This process is called 

 oxidative phosphorylation (Figure 3-10) and the energy thus produced 

 can be utilized by the cell through cleavage of the terminal phosphate 

 of ATP to give ADP plus energy. In addition to their role in the oxida- 

 tion of carbohydrate, isolated mitochondria have also been shown to be 

 capable of oxidizing long-chain fatty acids to carbon dioxide and water, 

 but there is little or no evidence that they can synthesize fatty acids 

 (Figure 3-11). In vitro experiments have indicated that the mitochondria 

 can account for practically all of the oxygen uptake of the intact cell and 

 the generation of as much as 90 per cent of the ATP formed (Lehninger, 

 1959). 



The mitochondria need not be intact in order to carry out oxidation 

 reactions, since isolated fragments have been shown to contain respira- 

 tory enzyme assemblies which exhibit oxidative activity and contain the 

 enzymes necessary for the coupling of phosphorylation with electron 

 transport (Watson and Siekevitz, 1956; Green, 1959). It has been 

 found that these assemblies (electron transport particles) of respiratory 

 chain enzymes are distributed throughout the membranes of the mito- 

 chondrion, particularly the cristae. On this basis, the mitochondrion may 

 be visualized as a polymer made up of a series of repeating units of 

 these respiratory assemblies (Green, 1959). Presumably the number of 

 assemblies in a single mitochondrion may vary and each may possibly 

 correspond to the unit of mitochondrial growth. One important aspect 

 of mitochondrial structure so far as function is concerned is the retention 

 of a double membrane. This is borne out by the fact that loss of the 

 double membrane structure results in inability of the respiratory unit 

 or electron transport particle to carry out oxidative phosphorylation. 

 Likewise, the capacity of mitochondria to carry out the reactions of the 

 Krebs citric acid cycle also appears to depend on the existence of a 

 double membrane structure. Most of the enzymes concerned with the 

 substrate reactions of the citric acid cycle and fatty acid oxidation are 

 easily extracted in soluble form following disruption of the mitochon- 

 drion and hence are considered to be located in the inner soluble matrix 

 of this organelle (Lehninger, 1959). 



The problem of change in number of mitochondria in a cell, particu- 



STRUCTURE AND FUNCTION OF CYTOPLASMIC ORGANELLES / 35 



