20 THE CELL AS A FUNDAMENTAL UNIT OF LIFE 



regulate the passage of materials into and out of the cell. In most 

 plant cells there is a cell wall outside the plasma membrane, but this 

 is commonly lacking in animal cells. This wall is secreted by the cell, 

 but is not considered to be a living part of the cell. It is always 

 quite porous in comparison with the plasma membrane and does not 

 interfere with the movement of materials into or out of the cell. The 

 cytoplasm which lies between the plasma membrane and the nucleus 

 is a viscid fluid somewhat like the white of an egg and contains numer- 

 ous bodies suspended in it. There will be vacuoles, which appear as 

 bubbles, that usually contain water together with dissolved salts and 





Photo by Winchester 



Fig. 2.2. Mitochondria. In this living grasshopper cell which has almost completed 

 division, the mitochondria can be seen as a bundle of rod-like bodies which is the last 



part of the cell to divide. 



food materials. Scattered through the cytoplasm there are also mito- 

 chondria, which usually appear as rod-like or thread-like bodies which 

 recent research has shown to be centers of enzymes that play such 

 an important part in the physiology of the cell. These enzymes stim- 

 ulate chemical changes which are a part of the normal functioning 

 of a cell. Each enzyme is specific in its reaction, so there must be as 

 many enzymes as there are needed reactions. 



In most animal cells and in a few plant cells there is a centrosome 

 which may be seen near the nucleus. This appears as a central body 

 (sometimes it is a double body) surrounded by an area of cytoplasm 

 which is different in appearance from the rest of the cytoplasm. In 

 animal cells only there will be Golgi bodies which are usually collected 

 in an irregular mass near the centrosome. The function of these is not 



