STAINING, PRACTICAL AND THEORETICAL 



acid or basic, applied to tissues without the use of a mordant, 

 remains in the tissues in sufficient strength to colour them, and 

 cannot be extracted by ordinary solvents, as already indicated, it 

 could be bound to the tissue elements through chemical union 

 or by physical factors, such as adsorption. 



Proteins which are found everywhere in tissues and which are 

 the most important biological substances, constitute an enormous 

 group composed of carbon, hydrogen, oxygen, nitrogen, and 

 usually sulphur. Many proteins contain phosphorus in addition 

 to the elements named above, and some contain small amounts of 

 iron, iodine, copper, zinc, etc. Proteins are intimately associated, 

 in biological cells, with nucleic acids, lipids, carbohydrates and 

 their derivatives; hormones, enzymes, vitamins, etc. Hormones 

 and enzymes themselves have been found to be protein in charac- 

 ter. The chemistry of proteins is exceedingly complex. There is a 

 greater degree of diversity in their molecular structure and chemi- 

 cal composition than in any other group of biological substances. 

 The natural proteins consist of large numbers of molecules, of 

 the same or different amino acids which are united through 

 condensation with the loss of the elements of water. Proteins are 

 characterized in biochemistry by their amino acid components. 

 Amino acids are organic acids in which one or more of the hydro- 

 gen atoms has been replaced by an amino group. Amino acids 

 are amphoteric because their molecules contain both acidic 

 (carboxyl) and basic (amino) groups. Proteins also are amphoteric 

 because they are composed of amino acids. Because proteins are 

 amphoteric the possibilities of building up and otherwise modify- 

 ing their molecules by processes of nature is enormous. In fact, 

 the number of types of protein molecules varying in structure and 

 chemical composition is almost infinite. Their molecular weights 

 are in many cases enormous. As mentioned above, proteins 

 found in tissues are intimately associated with nucleic acids, 

 lipids, carbohydrates, etc., to form highly complex substances. 

 It seems reasonable to imagine the molecules of such substances 

 as consisting of chains of protein-complexes from which the 

 tissue elements are woven by nature. Some tissue elements such 

 as the envelope of the red blood corpuscle and neurokeratin 

 are closely woven or "close-knit" (see page 77), while others 

 such as collagen appear to be loosely woven or " loose-knit ". 

 The outer surfaces of tissue elements might be imagined to 



