to these persisting elements of cell structure 

 there is the mitotic apparatus (Fig. 3-3), 

 which appears transiently each time a cell 

 divides (Chap. 3). 



Granules of more or less inert materials 

 are found in virtually every kind of cell. 

 Many of these granules represent stored 

 materials, such as starch grains (in plant 

 cells), fat droplets or glycogen grains (in 

 animal cells), and yolk granules (in egg 

 cells). Others, particularly in gland cells, 

 are secretory granules. Secretory granules 

 are composed of special products, syn- 

 thesized and accumulated in the gland cell, 

 later to be extruded as components of some 

 secretion. Pigment granules give characteris- 

 tic colors to the cells and tissues containing 

 them; still other granules merely represent 

 waste products accumulated and precipitated 

 in the cytoplasm. The size of the granules 

 in the cytoplasm extends down to and beyond 

 the limits of microscopic and ultramicros- 

 copic resolvability to the level of relatively 

 small molecules, such as glucose and water. 



Intrinsic (Protoplasmic) Membranes. At 

 every surface, protoplasm displays an altered 

 structure, forming a subvisible specialized 

 layer, or membrane, which is denser and less 

 fluid than the rest of the protoplasm. The 

 plasma membrane, at the external surface of 

 the cytoplasm, is such a layer. In addition 

 there are a number of other intrinsic mem- 

 branes at internally placed surfaces in the 

 cell: at the boundary between the nucleus 

 and cytoplasm (the nuclear membrane); bor- 

 dering each vacuole (vacuolar membranes); 

 and at the contact surfaces between the cyto- 

 plasm and the visibly differentiated struc- 

 tures such as mitochondria, Golgi bodies, 

 chloroplasts, etc. All these living membranes 

 possess a number of unique and important 

 properties, but these properties have been 

 studied most extensively in the plasma mem- 

 brane. 



The Plasma Membrane. The plasma mem- 

 brane is so thin that it is not microscopically 

 visible as a separate structure. It appears 

 merely as the external boundary of the cyto- 



Protoplasm,i he Cell, and the Organism - 31 



plasm. Nevertheless the plasma membrane 

 has great functional importance. It plays a 

 dominant role in controlling the passage of 

 substances into and out of the cell. 



Because the plasma membrane is not re- 

 solvable with any type of light microscope, 

 many earlier biologists doubted its existence. 

 These workers compared the cell to a mass 

 of gelatin or other gelated material, in which 

 no differentiated boundary membrane is pres- 

 ent. To explain the fact that certain sub- 

 stances — particularly dyestuffs like phenol 

 red — fail to penetrate the cell from a sur- 

 rounding solution, it was said that the proto- 

 plasm had no "chemical affinity" for the dyes 

 in question. 



Proof for the existence of the plasma mem- 

 brane was finally obtained by means of 

 microinjection experiments (Fig. 2-17). If a 

 phenol red solution is microinjected into the 

 cytoplasm, it does color the protoplasm. In 

 fact the injected dye spreads throughout the 

 whole cytoplasm; but when it reaches the 

 boundary surface, it cannot pass out of the 

 cell. Therefore it is certain that the bound- 

 ary layer is different from the internal cyto- 

 plasm, and this differentiated surface layer is 

 called the plasma membrane. The plasma 

 membrane prevents the molecules of phenol 

 red from entering the cell from a surrounding 

 solution, and likewise it prevents the dye 

 from leaving the cell once it is placed inside. 

 Such an experiment, of course, must be per- 

 formed on a cell from which any extraneous 

 coating has been removed, since otherwise it 

 would not be possible to rule out the non- 

 protoplasmic membrane as a barrier to the 

 passage of the dye. 



The plasma membrane, like other parts of 

 the living protoplasm, displays a well-defined 

 capacity for self-repair. If the cell surface is 

 torn — as, for example, with a microneedle — 

 the gap seals itself spontaneously, provided 

 the tear is not too drastic. But if a very ex- 

 tensive rent is torn in the plasma membrane, 

 a visible wave of disintegration sweeps over 

 the surface of the cell, and within a few sec- 

 onds all parts of the protoplasm disintegrate. 



