Chap. 3 LIVING MATTER AND CELLS 33 



A cell is enclosed by thin protoplasmic layers forming a semipermeable 

 membrane. This membrane is the lifeguard of the cell. It is permeable to 

 certain dissolved substances but impermeable to others, a constant control 

 over what may enter or leave the cell. The plant cell produces on its outer 

 surface a definite wall that is not living, an important difference between it 

 and the animal cell. 



Cells may live independently of others and if so each behaves like a com- 

 plete organism, as an ameba does. In multicellular animals each cell is con- 

 tinually affected by its relations with others, and by the behavior of the whole 

 cellular community comprising the animal of which it is a part. A cat consists 

 of billions of cells, yet when it springs on a mouse it moves as a single organ- 

 ism. 



Origin and Importance of Cells. Every cell originates from a preexisting 

 one and in no other way. This is a complex process during which the new 

 cells receive equal amounts of this essential substance of a parent cell. Every 



1P1 za,s-m.o^o-co.e 

 Ciar ora&t l-rx- 





Cy to-piasin 



PlasUci- 





P'a.t ^loioTjules 



Ceil wall 

 ■Ceil 



Vea.c-cLoie 

 hA.itoc'hio-rxS-tPi.m 



Fig. 3.4. Diagram of body cell. Some of the parts are visible only after special 

 preparation and very high magnification. Plasmosome is another term for nuclet)]us. 

 The karyosome is a body of nuclear substance. Organoids such as the centrosome, 

 chondriosomes (mitochondria), Golgi bodies, and fibrillae are parts of the cell that 

 have particular functions. Plastids are characteristic of plant cells. There may be 

 many nonliving inclusions, e.g.. droplets of water; and granules of yolk: the yolk 

 of a hen's egg is loaded with these. The inner cell membrane, an extremely thin 

 layer of protoplasm, is ordinarily invisible. It is in close contact with the porous 

 outer cell membrane (or "wall"); m animal cells the inner and outer membranes 

 are together and commonly called the cell membrane. (Courtesy, Stiles; Individual 

 and Community Health. New York, The Blakiston Company, 1953.) 



