Chap. 3 LIVING MATTER AND CELLS 27 



conditions around it such as the degree of temperature, its chemical environ- 

 ment, and its age, or phase of life. The streams of protoplasm which pour 

 like water into the forming pseudopodia .of an ameba are in the sol state; 

 their borders are changeful, now sol, now gel. If the cell membrane is broken 

 slightly, a little of the sol will flow out and "set," thus healing the wound. 



Chemical Characteristics. Protoplasm has substantially the same chemical 

 content in all plants and in the great procession of animals whether jellyfish, 

 redbird, or man. The four elements, oxygen, carbon, hydrogen, and nitrogen 

 make up 96 per cent of living matter. No element occurs in protoplasm 

 which is not also present in nonliving substance. It cannot be recalled too 

 often that it is not the content of protoplasm but the way it is put together 

 that is unique. 



Water. The most abundant compound in active protoplasm is water, in 

 general terms of weight at least 75 per cent of it. A jellyfish may be 96 per 

 cent water, a paramecium 80 per cent. The gray matter often called the 

 "thinking part" of the adult human brain is at least 80 per cent water; in early 

 youth the percentage of water is still greater. The water content of a cell is 

 controlled by the living membrane which encloses it. Protoplasm has a water- 

 regulatory power which resembles that of gelatin in that it takes in water 

 and swells to a limited amount and no more. Water heats slowly and holds 

 its heat. Thus the temperature of an animal with its high water content 

 rises slowly and tends to hold its level. Water works toward a temperate 

 climate for protoplasm, whether it is in the body cells of a fish or surrounding 

 the fish in a stream. Certain very important changes in the water content 

 of their protoplasm make animals of low metabolism relatively cold-hardy, 

 such as the numberless cold-blooded ones, insects and others that withstand 

 temperatures of zero (F.) and far below. As winter approaches their proto- 

 plasm loses water, but this is only part of the cold hardening. The water 

 which remains is not all in the same state; it may be free or bound, more 

 of one than the other. Free water is water that contains truly dissolved 

 materials and acts as a dispersion medium for them. In both plants and ani- 

 mals it transports digested foods and waste products and forms a liquid 

 base for secretions. Bound water is held in a loose chemical combination 

 with other molecules. Ordinarily bound water does not freeze. Free water 

 freezes readily forming ice crystals, which because of their size and pressure 

 kill the protoplasm. Studies on the bound and free water in gelatin and egg- 

 white show that part of the water freezes when the temperature reaches 

 — 6°C. (21.2°F.) while what remained did not freeze even at — 50°C. Thus, 

 for the beetle that must endure a northern winter there are striking advantages 

 in having a content of bound water. 



Chemical Activity. Water is the closest approach that we have to some- 

 thing which dissolves everything. This is the basis of its prominence in diverse 



