CYTOPLASM 501 



gelatine/water solutions by changes of temperature or acidity, 

 and it is easy to imagine these having similar effects in the cell. 

 Cellular death induces — or perhaps is caused by — irreversible 

 changes, such as that which occurs when an egg is boiled. If 

 there were not some sort of wall or membrane at the cell surface 

 there would be nothing to prevent the aqueous cytoplasm mixing 

 with any water with which it came into contact. It does not do 

 so, and all cells appear to have a surface in which the protoplasm 

 is differentiated so as not to mix with water. This wall produces 

 a slight tension, about one two-hundredth of that at an oil/water 

 interface, which is usually called the surface tension of the cell. 

 Some substances penetrate the wall easily, others more slowly 

 or not at all, and since some of those which go through are water- 

 soluble and others are fat-soluble, it has been suggested that the 

 cell membrane is a mosaic of lipoids and proteins (Fig. 391). 

 In addition to the protoplasmic wall there may also be a gelated 

 layer, or a dead secreted layer such as the keratin of the skin. 



The normal hydrogen ion concentration of the cytoplasm 

 appears to be slightly on the acid side. 



NUCLEOPLASM 



Nucleoplasm is physically similar to cytoplasm. It is aqueous, 

 with a viscosity rather greater than that of water, it gelates when 

 damaged, and it is separated from the cytoplasm by a membrane 

 comparable to the cell membrane. Chemically it is rather simpler, 

 with a high proportion of protein, and with much phosphorus. 

 The chief substance which it contains is nuclein, which on 

 hydrolysis gives proteins and a compound called nucleic acid. 

 This is relatively simple, and is made up of carbohydrates, 

 amino-acids, and phosphoric acid. Nucleic acid from all animal 

 sources appears to be very similar, but that from plants differs in 

 the carbohydrate part of the molecule. Nucleoplasm lacks many 

 elements which are present in cytoplasm. Numerous experiments 

 have shown that although cells can hve without their nuclei, 

 their activities are much restricted ; roughly speaking, the cell 

 can carry out catabolic processes but not anabolic. The special 

 importance of the nucleus in transmitting the hereditary 

 characters is discussed in Chapter 29. 



