422 PROTOPLASM 



instead, a new molecule results (page 176). However intimate 

 or loose the association may be in hydration, we can at least 

 assume that the water molecule is actually held, and more firmly 

 in some cases than in others. Water of hydration may often be 

 driven off by a mild application of heat, while at other times it is 

 separated only with great difficulty. Water held osmotically, in 

 the strict sense, is free water (though obviously it is not free to 

 move against an osmotic gradient). Water held by imbibition 

 forces is usually bound water. To squeeze the water out of some 

 gels requires terrific forces, and not always then can it be done. 

 No known force will keep gelatin or starch from taking up water. 



Bound water may also be defined as water that will not freeze ; 

 this serves as a method for determining the amount of bound 

 water present. The problem of a bound ^ free water equi- 

 librium in biological systems has centered primarily around 

 the question of freezing — the so-called frost resistance or winter 

 hardiness of plants and animals. When reduced to its simplest 

 form, it is a problem of the cold resistance of protoplasm. The 

 survival of a cell at low temperature appears quite definitely 

 to rest upon the relative amount of free and bound water. 

 Bound water does not freeze; free water freezes readily, forms 

 large ice crystals, and these, simply because of size and mechan- 

 ical pressure, kill the cell. Death is, therefore, not due to a 

 freezing of the protoplasm itself, i.e., freezing of the water held 

 by adsorption to organic matter in the cell. In their studies on 

 the proportion of bound and free water in gelatin and egg white, 

 Jones and Gortner found that all the water that would freeze 

 was frozen when the temperature reached — 6°C. and that no 

 additional water froze even when the temperature reached 

 -50°C. 



The problem of bound vs. free water has a practical application 

 in the study of winter hardiness in plants. Free water freezes 

 readily. Bound (adsorbed) water does not freeze. The relative 

 proportion of free and bound water determines drought and frost 

 resistance (winter hardiness) of plants and animals. Increase 

 in the concentration of dissolved substances (sugar), which 

 lowers the freezing point of aqueous solutions, would be one 

 effective way for plants to survive great cold, but still better is 

 it to bind the water to proteins and other colloidal substances 

 by adsorption. 



