7-3 THE YEAST CELL 



STAINABLE GLYCOGEN 



The granular carbohydrate reserve in yeasts stains dark red- 

 dish brown with Lugol's iodine -potassium iodide solution. In many 

 cells one can observe twenty or more small isolated granules 

 (fig. 7-1). Occasionally, these granules are linked by connecting 

 bands. Hundreds of small glycogen granules can be seen occasion- 

 ally in some cells in addition to fifteen or twenty larger ones. These 

 observations suggest that, in most of the cells containing a solid 

 mass of dark-staining glycogen, the distribution is similarly non- 

 homogeneous with the basic granular structure obscured by over- 

 staining. Under some circumstances the glycogen is deposited in 

 granules right under the plasma membrane; under other circum- 

 stances it forms a flocculent deposit throughout the cytoplasm. 



Unstained cells containing glycogen can be recognized by the 

 high refractive index of the cytoplasm. The nuclear vacuole in a 

 glycogen- containing cell is often concealed by the glycogen. The 

 visible vacuoles often appear to be multiple, but critical observa- 

 tion shows that the small vacuoles are all interconnected with each 

 other by fine canals and are merely separate compartments of one 

 major vacuole. This is consistent with the view that the vacuole is 

 the nucleus and that each yeast cell contains only a single vacuole; 

 if the vacuole is divided, all the separate compartments connect 

 with the centrosome. In many unstained glycogen -containing cells 

 no vacuole is visible, but staining with Lugol's solution always re- 

 veals the vacuole either compressed into the middle of the cell by 

 a surrounding sheath of glycogen, or at one pole of the cell. The 

 vacuole in a glycogen -containing cell is usually much reduced in 

 size. 



Budding is retarded or inhibited in cells containing much gly- 

 cogen, and occurs only after the glycogen has begun to disappear 

 from the cell. Growing cells during the lag phase contain enough 

 unidentified reserve to obscure or deform the vacuole. After the 

 cell has completed one or two divisions, the refractive index drops 

 and the vacuole reappears or loses its deformity. 



Deposition of granular glycogen is irregular at high (12 percent) 

 concentration of sugar and rarely fills the whole cell. The stained 

 granules are darker than those observed at lower concentrations of 

 sugar. Many granules may coalesce, often forming two large polar 

 deposits. Small granules of glycogen may be linked by arcs of gly- 

 cogen. At lower sugar concentrations (4 per cent), the deposition 

 of glycogen continues until it fills the entire cell with the exception 

 of a small region at the one end into which the vacuole is crowded, 

 or the vacuole may be concealed in the center of the cell inside the 

 spherical envelope of glycogen. After the deposit has reached a 

 maximum, the glycogen disappears on aeration, by peripheral dis- 



