ably it approximates 10.0 millimicrons in the 

 living cell. It appears to consist of two layers 

 of electron-dense material between which lies 

 a layer of less dense substance. Presumably, 

 the denser material is of protein composi- 

 tion, whereas the layer of lesser density is 

 lipid (p. 80). The structural and functional 

 characteristics of the plasma membrane and 

 of other intrinsic protoplasmic surfaces will 

 be considered more thoroughly in Chapter 6. 



Extraneous (Nonprotoplasmic) Membranes. 

 In most cases the cell is covered by some sort 

 of extraneous membrane, w,hich lies in con- 

 tact with the outer surface of the plasma 

 membrane. Such nonliving covers protect the 

 protoplasm from mechanical injury and help 

 to maintain the characteristic shape of the 

 particular cell. Extraneous membranes are 

 usually thick enough to be visible under the 

 microscope and are composed of inert sub- 

 stances that have been synthesized by the 

 protoplasm and deposited at the external 

 surface. 



The extraneous membranes of plant cells 

 are generally quite different from those of 

 animal cells, and consequently they are given 

 different names. In the case of plant cells it 

 is customary to speak of the cell wall; and in 

 the case of animal cells, one speaks of the 

 pellicle. In the tissues of higher animals, the 

 pellicular covering of the cells may be very 

 thin or even absent, although usually some 

 sort of extraneous intercellular material can 

 be demonstrated. 



The cell wall (Figs. 2-14 and 2-16) of plant 

 cells is generally stronger, more rigid, and 

 less elastic than the pellicle of animal cells — 

 as can be demonstrated by experiments. If 

 plant and animal cells are placed in distilled 

 water, which tends to make cells swell (Chap. 

 6), the results are quite different in each case. 

 Animal cells, owing to the greater elasticity 

 and weakness of the pellicle, continue to 

 swell. The cells become larger and larger, 

 until finally they burst. But plant cells swell 

 only slightly, and then stop. The strength of 

 the cell wall, like that of the casing of an auto- 

 mobile tire, prevents further swelling, despite 



Protoplasm, the Cell, and the Organism - 33 



the fact that the inflation pressure in the 

 plant cell may reach a value of several atmos- 

 pheres. 



The converse of this experiment demon- 

 strates the greater rigidity of the cell wall, as 

 compared to the pellicle. When plant and 

 animal cells are placed in strong salt solu- 

 tions, which induce the protoplasm to shrink, 

 marked differences are again observed. As the 

 animal cell shrinks, at first the pellicle 

 shrinks along with the protoplasm. But as 

 shrinkage continues, the flexible pellicle be- 

 comes wrinkled (Fig. 2-18) and distorted to 



n ® 



® 

 







o 



NORMAL 



SHRUNKEN 



Fig. 2-18. The shrinking of animal cells (human red 

 cells). Note the wrinkled pellicle in the shrunken cells. 



fit the reduced contour of the cell. In the 

 plant cell, on the other hand, the cell wall 

 does not shrink as the protoplasm shrinks; 

 nor does the cell wall become wrinkled. The 

 protoplasm merely pulls away from the rigid 

 wall and continues to shrink by itself until 

 it occupies only a small part of the original 

 tightly fitting compartment (Fig. 2-19). 



CELL WALL _ 

 CYTOPLASM _ 

 VACUOLE , 



Fig. 

 wal 

 sh 



2-19. The shrinking of a plant cell. The cell 

 I is rigid and does not shrink as the protoplasm 

 nks. 



