Resting Cells 3 



granular nature. In the living condition protoplasm is generally 

 considered to be an emulsion type of colloid consisting of a 

 watery background in which are many tiny globules of an im- 

 miscible substance, giving it the appearance of milk that has 

 been shaken up. In the watery part may be suspended many 

 extremely small particles or granules, which may be arranged 

 so as to form an interlacing network. In the liquid part also are 

 various dissolved substances such as salts and sugars. Although 

 protoplasm is generally fluid and has a specific gravity only 

 slightly higher than water, it may sometimes be firmer in con- 

 sistency than water and more like a jelly. 



Protoplasm in all typical living cells can be differentiated into 

 two parts, the cytoplasm and the frequently more jelly-like 

 nucleus. The outer region of the cytoplasm is firm and mem- 

 branous and forms the plasma membrane. This is of great im- 

 portance physiologically as it permits some substances to enter 

 and leave the cell and prevents others from doing so. In most 

 plant cells a cell wall surrounds the plasma membrane, but this 

 structure is not concerned with the passage of materials into 

 and out of the cell. In young plant cells this wall may be 

 very thin, but in most older cells a thicker secondary wall is 

 also present. In some types of specialized cell this secondary 

 wall becomes very thick. 



In the embryonic plant cell (Fig. 1), the space inside the cell 

 wall is occupied by protoplasm. When the cell is not dividing, 

 the nucleus, usually centrally located, is a round or ellipsoidal 

 mass of protoplasm separated from the cytoplasm by the nuclear 

 membrane, a barrier that may separate nuclear and cytoplasmic 

 material. In the mature, unspecializecl type of plant cell known 

 as a parenchyma cell, a large vacuole is present in the center of 

 the cell and the cytoplasm is mostly restricted to the periphery. 

 In the cytoplasm may be found living structures, such as the 

 plastids and chondriosomes or mitochondria, and many non- 

 living substances, including starch grains, protein granules, drop- 

 lets of fat or oil, and various crystals. 



In a typical animal cell there is no large central vacuole, and 

 in the cytoplasm are chondriosomes and secreted granules, but 

 no plastids. Lying in the cytoplasm to one side of the nucleus 

 is the centrosome, a structure intimately connected with cell 



