PROTOPLASM 51 



in fat. Variable amounts of free fat occur in cells, but much of the fatty 

 material exists in some form of combination with the proteins. 



The compound fats are combinations of fatty acids, nitrogen-contain- 

 ing bases, and either phosphorus or carbohydrates. Such fat -like com- 

 pounds containing nitrogen \\\t\\ or without phosphorus are called lipides. 

 The phospholipides are of special interest, for they appear to perform a 

 major role in the formation and activit}^ of protoplasmic surface mem- 

 branes, thus sharing largely in determining permeability and water- 

 immiscibility. Lecithin, a prominent member of this class, is abundant 

 in all cells. It occurs in a finely divided state in the cytoplasm, and by 

 virtue of its possession of hydrophile and lipophile groups it probably 

 functions in maintaining the colloidal state. 



The proteins, which with lipides and water represent the main con- 

 stituents of protoplasm, are elaborate compounds containing carbon, 

 hydrogen, oxygen, nitrogen, often sulphur, and sometimes phosphorus. 

 They are built up from amino acids with XHo substituted for H in the 

 group attached to the COOH group. The protein casein has about 20 

 amino acids in its molecule. Every kind of organism evidentl}^ differs 

 in some measure from every other in its proteins, a fact that is of impor- 

 tance with respect to such matters as immunity, allergy, and the differ- 

 entiation of species. 



The simple natural proteins, which yield only amino acids when 

 hydrolyzed by enzymes or acids, are present in great variet}^ in protoplasts. 

 Albumins and globulins are important constituents of cytoplasm and are 

 often present in large quantities in eggs and seeds. The enzyme urease 

 is a globulin, and other enzymes also are proteins. Other simple proteins 

 characterize the cereals {e.g., glutenin, oryzein, zein, gliadin, hordein) 

 and animal tissues (e.g., keratin, elastin, gelatin, collagen). The histones 

 are relatively simple, while the simplest and most basic of all natural 

 proteins are the protamines. The best known protamine (salmin) from 

 fish sperm has only four amino acids, and its formula is C81H115N45O18. 



The conjugated proteins in nature are simple proteins in combination 

 with other organic groups; they yield amino acids and nonproteins when 

 hydrolyzed. As examples may be mentioned the glycoproteins in mucus 

 and "tissue cements," the chromoproteins in certain plant and animal 

 pigments, the lecithoproteins probably present in all cytoplasm and its 

 membranes, and the nucleoproteins found in various parts of the proto- 

 plast and of special importance in nuclei. 



Special attention should be given to the proteins of nuclei. As pointed 

 out previously (page 50), the material composing the chromonemata, 

 which are significant constituents of the chromosomes, is mainly a nucleo- 

 protein made up of proteins and nucleic acid. This highly stainable 

 material has been called nuclein or, more commonly, chromatin. The 



