ISO PLANT BIOCHEMISTRY 



recently has been studied very extensively and is an excellent example 

 of the association ol biocatalysts and cofactors within discrete cellular 

 areas. This organization leads to efficient function at maximal local 

 substrate concentrations. Mitochondria contain relatively high 

 amounts of diphosphopyridine nucleotide, triphosphopyridine nucleo- 

 tide, coenzyme A, fiavinadeninedinucleotide, diphosphothiamine, 

 lipoic acid, pyridoxal phosphate, folic acid, and biotin. A multitude 

 of cellular reactions depend upon the presence of such cofactors. 



The cytoplasm also contains submicroscopic particles, visible under 

 the electron microscope, called microsomes. Microsomes are rich in 

 ribonucleic acid. The activity of this fraction has been associated with 

 nucleotide and protein synthesis as well as with fat metabolism. 



The separation of the above-named cytoplasmic components and 

 nucleus can be accomplished by grinding the cells in a suitable buffer 

 with the proper osmotic properties, that is, 0.25 to 0.90M sucrose, 

 followed by centrifugation at different velocities. Low-speed cen- 

 trifugation sediments the heavier fragments, such as cellular debris. 

 At higher speeds, that is, 5,000 to 21,000 x G* the mitochondria are 

 sedimented. Prolonged centrifugation in an ultracentrifuge at 60,000 

 to 80,000 X G permits the recovery of the microsomal fraction. The 

 supernatant remaining after removal of the microsomes contains the 

 soluble enzymes readily extracted from the cells by buffer solutions. 

 Most of the enzymes associated with the glycolysis pathway are asso- 

 ciated with this fraction. For example, 90 to 100 per cent of the phos- 

 phoglucomutase, hexosediphosphatase, and aldolase activities is recov- 

 ered in the supernatant. 



Starch granules, oil vacuoles, and chromoplasts of various types may 

 also be found in the cytoplasm in large quantities, particularly in cells 

 of organs with special functions, such as storage organs or flower 

 petals. 



Cell Wall 



The cell wall forms a more or less rigid encasement for the proto- 

 plasmic constituents of the cell. The rigidity of the cell wall depends 

 on the age of the plant part. In young tissues the cell wall consists 

 of a single, thin, non-rigid layer. In older tissues it contains two or 

 more thickened layers forming a very rigid structure. The cell wall 

 consists primarily of polysaccharides and polysaccharide derivatives. 

 In some species only a single polysaccharide is present, although in 



* C represents ihe centrifugal force exerted on the particle expressed in gravita- 

 tional force units. 



