278 UNITY AND DIVERSITY IN BIOCHEMISTRY 



longer tenable for it has been shown that they are the exclusive carriers 

 of certain enzymes like glucose-6-phosphatase and they are without the 

 typical enzyme of the mitochondria, cytochrome-oxidase. As already stated, 

 it is believed that protein synthesis takes place under the influence of 

 ribonucleoproteins. The microsomes, amongst the other cellular organites, 

 are the most abundantly provided with these substances, and this is why 

 Brachet considers that the microsome, rich in RNA and incorporating 

 amino acids very rapidly, is the probable site of protein synthesis. 



The enzymatic composition of microsomes is still obscure. Nevertheless 

 we know that they contain important amounts of cytochrome-reductases. 

 They must therefore presumably play some part somewhere in the stage 

 of electron transfer between DPNH or TPNH and cytochrome-c. 



In addition, according to Brachet, they contain a number of hydrolases : 

 phosphatases, amylases, cathepsin, and ribonuclease. Several experiments 

 also indicate the presence in microsomes of the enzyme system responsible 

 for the incorporation of alanine into proteins. 



B. Mitochondria 



We may say, and rightly, that the mitochondria is the engine which drives 

 the cell. For although the energy-rich bonds of ATP are formed in the 

 cytoplasm during glycolysis, the major production results from oxidative 

 phosphorylations which take place in the mitochondria. We may also liken 

 them to the central bank for the cell for it is from here that the currency 

 in which the cell deals, is issued. 



The mitochondria are cylindrical particles 1^ [jl in length and 0-3-0 -7 [x 

 thick. They are bounded by a double membrane of protein in the middle 

 of which there is a double layer of complex lipides. In mitochondria from 

 mammalian liver and kidney, each protein layer is 45 A thick and the 

 distance between the two protein layers is 70 A, so that the membrane 

 surrounding the mitochondria is 160 A thick. This membrane is prolonged 

 into the interior of the mitochondria by ridges penetrating into the sub- 

 stratum which appears to consist mainly of soluble proteins. 



The general features of mitochondria have been found in all the cells 

 of the different and varied organisms in which they have been studied : 

 mammals, birds, batrachians, molluscs, annelids, protozoa, yeasts, plants, etc. 



In mitochondria from rat liver the greater part (56%) of the organic 

 phosphorous is in the form of complex lipides and 20% is present in RNA, 

 the only nucleic acid they contain. A quarter of the total nitrogen of the 

 liver cell is in the mitochondria and about 15^o of the RNA of the cell is 

 found there. 



Cytochrome oxidase or cytochrome a^, appears to occur only in the 

 mitochondria. However, this is not the case for the cytochrome-reductases 

 (DPN and TPN specific) which, as stated above, are also found in the 



