34 - The Cell 



The different physical properties of the cell 

 wall and pellicle reflect a fundamental dif- 

 ference of chemical constitution. The cell 

 wall of the typical plant cell is composed 

 mainly of a carbohydrate substance, cellu- 

 lose, which is the special component of all 

 "woody" materials. But the pellicular cover- 

 ing of typical animal cells is composed mainly, 

 if not entirely, of mucoprotein substances 

 (Chap. 4). 



Extraneous material, found between the 

 cells of certain tissues, is designated generally 

 as intercellular matrix. In some tissues (for 

 example, bone and cartilage) the matrix is 

 more abundant than the protoplasm (Fig. 

 15-17). Like the cell wall or pellicle, the ma- 

 trix is secreted by the cell it surrounds, and 

 the matrix is not part of the protoplasm. 

 Even in tissues possessing no visible matrix, 

 small amounts of an analogous material, the 

 intercellular cement, are usually present. 

 This material binds the cells together — pelli- 

 cle to pellicle, or cell wall to cell wall, as the 

 case may be. 



The nonliving nature of extraneous mem- 

 branes and matrices generally is emphasized 

 by the fact that they are not essential to the 

 life of the cell. By microdissection ami other 

 techniques, the cell wall or pellicle can be re- 

 moved from a number of plant or animal 

 cells. Such naked cells are more vulnerable to 

 mechanical injury and may fail to maintain 

 their normal shape, but they do continue to 

 live. In time, the naked cells may replace 

 the missing protective material. 



Functional Responsibilities of the Nucleus 

 and Cytoplasm. By microdissection it is pos- 

 sible to remove the nucleus from certain cells 

 (Fig. 2-20) or to cut the cell into two surviv- 

 ing pieces of roughly equal size, one with a 

 nucleus and the other without. 



The common fresh-water amoeba, because 

 of its large size and ease of culture, provides 

 a favorable cell for the performance of such 

 microoperations. Virtually none of the cyto- 

 plasm is lost during the enucleation and the 

 cell membrane repairs itself instantaneously 

 when the nucleus is carefully pulled out 



.<*•••>..:- 



*•: 



Fig. 2-20. Using microneedles to remove the nucleus 

 from an amoeba. The bottom needle holds the 

 amoeba while the top one pulls out the nucleus 

 through the cell membrane. (Photograph retouched.) 

 (Courtesy of Robert Chambers.) 



through it (Fig. 2-20). Also, when the amoeba 

 is properly cut in hall, the cell membrane on 

 both sides of the cut becomes sealed imme- 

 diately. 



The enucleated amoeba continues most of 

 its vital activities for about two weeks. It can 

 move and ingest food; it digests and absorbs 

 this food; ami it even shows some slight 

 signs of growth. But it is not able to repro- 

 duce. Gradually growth and activity cease 

 and the cytoplasm begins to waste away. 

 Finally, usually within two to three weeks, 

 the cell remnant disintegrates. Observations 

 on the anucleate halves of cut amoebae yield 

 similar results, except that the survival time 

 is usually less. 



The nucleated half of a cut amoeba has a 

 different fate, however. Its activity and 

 growth continue vigorously. Within a day or 

 two it has more than regained its original 

 size, whereupon cell division occurs and re- 

 production is achieved. In fact the nucleated 

 half of a cell displays a lull potential for con- 

 tinuing all its vital activities, including re- 

 production. 



