Chapter 7 

 STORAGE OF RESERVES IN THE CELL 



When a yeast cell is suspended in a medium containing an ex- 

 cess of available carbohydrate, glycogen is deposited in the cyto- 

 plasm. The ability to store reserves is a complicating factor in all 

 attempts to analyze the growth and fermentative ability of yeast 

 cells. The following more or less general rule seems to hold: Most 

 available nutrients and vitamins when present in excess of rriain- 

 tenance requirements are stored in the cell as a reserve and the 

 accumulated reserves generally interfere with growth, fermenta- 

 tion and respiration. When the proper reserves accumulate in suf- 

 ficient variety, the cell becomes dormant. 



One would expect that a small excess of each metabolite must 

 be available in the cell, otherwise the mechanism would stop fre- 

 quently because of the absence of an essential metabolite. This 

 complicates the problem of studying growth, or indeed any meta- 

 bolic process, tor the tendency to store metabolites that are not im- 

 mediately used may be a general phenonenon. 



Many free -living, single -celled organisms can suspend anima- 

 tion when one nutrient is deficient; under similar conditions a meta- 

 zoan organism or even a more complex fungus would succumb. The 

 ability of the free-living yeast cell to store nutrients against a fut- 

 ure need may be a characteristic difference between free -living 

 cells and many -celled organisms. In higher organisms the func- 

 tion of storage is usually assigned to specialized cells and the func- 

 tionally active organs may be composed of cells containing minimal 

 amounts of reserve material. Cells whose cytoplasm is not "clut- 

 tered up" with stored reserves can operate at a maximal rate with- 

 out the interference of reserve materials with cytoplasmic reactions. 

 Therefore differentiation in metazoans involves keeping those cells, 

 which must maintain maximal activity, without stored reserves, but 

 nutrients must be kept available for them in some easily assimilable 

 form as in the blood stream. By contrast each yeast cell must per- 

 form both metabolism and storage and since these functions are 

 mutually exclusive, periods of dormancy or of low metabolic activ- 

 ity occur frequently depending on the environmental conditions. 



We are now familiar with at least three types of reserve sub- 

 stances which are stored in the cytoplasm of the yeast cell. They 

 are glycogen, fat and ribose nucleoprotein. Metaphosphate accumu- 

 lates on the chromosomes and this may also be considered a re- 

 serve substance, although its presence in the cytoplasm is a sign 



7-1 



