380 THE FIRST ORGANISMS 



weight, is composed of different formed structures, particles 

 of various sizes, for the most part mitochondria and micro- 

 somes. The mitochondria are rod-shaped formations visible 

 under the microscope. Their internal structure has been 

 fairly well studied both as to its morphology and its physical 

 chemistry. They have an envelope consisting of two pro- 

 tein layers with a lipid layer between them. The internal 

 core has a complicated structure and is also made up of 

 proteins and lipids. ^^ The microsomes are submicroscopic 

 and can only be discerned with the electron microscope. 

 Their structure has still only been very poorly studied. 

 According to J. D. Bernal** the arrangement of the molecules 

 of protein and nucleic acid in them is reminiscent of the 

 structure of globular virus particles. 



Both the mitochondria and the microsomes are very rich 

 in lipids.*^ The mitochondria contain the iron-porphyrin 

 systems of the celP® while the bulk of the nucleic acid is 

 situated in the microsomes.*^ The mitochondria contain 

 large amounts of various enzymes. They seem to embody the 

 catalytic mechanism required by the cell for the processes 

 of oxidation and decomposition which lead to the liberation 

 of energy from the multifarious substrates entering the cell, 

 and also for the processes of transformation of this energy 

 into forms in which it can be used in synthetic processes and 

 for carrying out work in general. In particular, in the cells 

 of highly developed organisms capable of respiration, this 

 is carried out by means of the tricarboxylic cycle of Krebs 

 (a diagram of which is given in Fig. 40 on p. 466). This is 

 the most ^videspread system of oxidation of intermediate 

 products of the breakdown of various organic substrates.** 

 The Krebs cycle comprises a strictly ordered concatenation 

 of a large number of enzymic reactions, especially the hydra- 

 tion, dehydrogenation and decarboxylation of organic acids. 

 At particular points in the cycle there branch off side re- 

 actions leading to the formation of substances which can 

 serve as material for the synthetic processes of the cell. For 

 example, a-oxoglutaric acid is one of the links in the Krebs 

 cycle. It is formed from oxalosuccinic acid and later, in the 

 course of the transformations of the cycle, it is converted, by 

 oxidative decarboxylation, into succinic acid. This in its 



