PROTOPLASM 49 



tendons) and in certain ]irodiicts of protoplasmic activity (hair, siliv). 

 The molecules in silk, which is inelastic, are fully extended, whereas those 

 in wool, which is elastic, are wavy or folded. In other proteins the chain 

 may be very closely folded, forming a "globular molecule." With a 

 molecular weight of 36,000 such a molecule would have a diameter of 

 about 0.005m. Such molecules constitute the so-called mobile proteins, 

 while the fibrous molecules form the structure proteins. It has been possi- 

 ble to separate the two kinds in cells: about an eighth of the proteins in an 

 echinoderm egg and about two-thirds of those in kidney cells are of the 

 structural type. There is controversy over the question of the degree of 

 distinctness and the relative importance of the two types in determining 

 the physical characters of protoplasm, but the extremely suggestive nature 

 of the above findings is evident. 



Chemical Nature of Protoplasm. — The immense difficulty of ascertain- 

 ing the chemical composition of protoplasm with any degree of accuracy 

 scarcely needs to be pointed out. With protoplasm are always associated 

 some of its products ; relative amounts of the constituents vary in different 

 tissues and at different phases of activity; the high sensitivity to reagents 

 and the alterations occurring at death greatly complicate the problem of 

 analysis. It is nevertheless possible to form a general idea of its composi- 

 tion, and with further improvements in method our knowledge of it will 

 doubtless gain in definiteness. In general it is found that protoplasm in 

 the active state consists of more than 75 per cent water and less than 25 

 per cent materials representing the dry weight. The dry matter is 

 roughly 90 per cent organic (proteins, fats, carbohydrates) and 10 per cent 

 inorganic. 



Water, one of the commonest substances in nature, is of the utmost 

 importance to organisms. Without water or something like it — and there 

 is nothing like it — life as we know it is inconceivable. Water acts as a 

 solvent and conveyor of reacting materials, is a medium of reaction, and 

 participates in reactions through hydrolysis and dehydration. Because 

 of its unique properties it very largely determines the character of the 

 colloidal system of which it is a part as well as the types of reaction that 

 occur. In inactive protoplasm, such as that in dry spores, the percentage 

 of water may fall to a very low value. The protoplast contains free 

 water and water bound at the surface of the colloidal particles. The 

 bound water is difficult to remove by heat, and it also resists the effects 

 of very low temperatures, remaining unfrozen after the free water has 

 crystallized. It is probably this crystallization of free water that kills 

 protoplasm at low temperatures: spores that have been deprived of their 

 free water may survive the temperature of liquid air. It has recently 

 been found that undehydrated cells may not be killed by intense cold if the 

 temperature is lowered very rapidly through the freezing range: the water 



