CYTOPLASMIC MATRIX 



The cytoplasmic matrix is the ground substance in which the organ- 

 elles of the cell (nucleus, mitochondria, plastids, endoplasmic reticulum, 

 etc.) are embedded. In the biochemical sense, this component of the 

 cell roughly corresponds to the subcellular fraction isolated from ho- 

 mogenates after removal of all cellular particulates, and contains 

 enzymes in the soluble form which are involved in a number of important 

 metabolic pathways (Lehninger, 1959). 



Among the important enzymes found in the soluble fraction are those 

 associated with degradation of carbohydrate to pyruvate (Embden- 

 Meyerhof cycle of glycolysis). Also, this is apparently the primary 

 location of the enzymes concerned in the hexose monophosphate shunt 

 which leads to the production of pentose units for nucleotide and nucleic 

 acid (RNA) synthesis. In addition, this fraction is also considered to be 

 the major site of fatty acid synthesis in the cell. 



What has been said concerning the mitochondria, endoplasmic reticu- 

 lum, microsomes, and matrix provides an excellent illustration of the 

 subtle interrelationships that may exist between various cellular com- 

 ponents with respect to metabolic function (Figure 3-28). It is quite 

 clear that we cannot really consider any one organelle as functionally 

 independent of the others. One should therefore consider in vitro experi- 

 ments, particularly those involving subcellular fractions, as providing 

 clues to potential, rather than as reflecting in vivo, operations. For 

 example, if we consider the problem of protein synthesis it becomes 

 apparent that a variety of cytoplasmic components are involved in this 

 activity (Figure 3-28) : the soluble fraction in activation of amino acids, 

 the microsomal fraction in their polymerization, the matrix as a source 

 of specific RNA, and the mitochondria as an energy supply in the form 

 of ATP. Likewise, as noted, the mitochondria are capable of oxidizing 

 fatty acids (Figure 3-11) but their synthesis is dependent on the cyto- 

 plasmic matrix. Therefore, fatty acid metabolism in general is deter- 

 mined by interaction between these two cell components. 



GOLGI COMPLEX 



In certain animal cells, particularly those which have a secretory 

 function (epididymus, liver, pancreas) a system of fibrous or globular 

 bodies which stain with osmic acid is readily apparent (Figure 3-29). 

 This system or organelle was first described by Golgi in 1898, and has 



60 / CHAPTER 3 



