10 



WATER 



which is not an integral part of the plant or animal tissue with which 

 it is associated. The major part of bomid water is held probably by 

 proteins, but other classes of compounds are known to retain relatively 

 large amounts of water. Thus adipose tissue contains considerable 

 water, certain of the compound lipides, such as lecithin, emulsify readily in 

 water, and the polysaccharides of plant tissues are decidedly hydrophilic. 



It is not certain just how bound water is held by colloidal material. 

 One explanation applied to proteins is that sharing of electrons between 

 the protein molecule and the water molecule sets up a binding force that 

 holds the water to the protein. Such a force is called a hydrogen bond 

 or bridge and consists of an electropositive hydrogen atom standing 

 between two electronegative atoms, e.g., N and 0, thus — N : H : — . 

 The hydrogen shares its electron with both the N and 0. 



Proteins contain many groups such as — NH2, — COOH that can form 

 a hydrogen bond with water. A protein molecule may contain several 

 thousand binding groups. For example, gelatin, a rather small protein 

 having a molecular weight of about 35,000, is calculated to have 960 

 molecules of water bound to each molecule of gelatin when a gel is 

 formed. There is much difference of opinion as to the quantity of water 

 held by proteins in solution, but 0.3 g. of water per gram of protein is 

 a commonly suggested figure. 



Bound water, especially that bound by the protoplasm of the cell, 

 appears to be one of the several important factors involved in frost and 

 drought resistance. Plants that are exposed to low temperatures in 

 winter increase the proportion of bound water and the concentration of 

 water-soluble protein in the cell sap, thus developing what is called 

 winter hardiness. Plants, such as cactus, that live under arid or semi- 

 arid conditions hold their water largely in the bound state. Insects also 

 increase the percentage of bound water under conditions of cold or 

 drought. 



Yeast cells furnish another example of the intimate association of 

 residual water and life processes. A commercial product known as active 

 dry yeast contains only about 8 per cent moisture, but the cells are 

 still alive and will survive for many months. If soaked in warm water 

 for a few minutes, the yeast promptly starts producing carbon dioxide 

 and can be used in place of baker's press yeast for bread-making. How- 

 ever, if the cells are dried to around 5 per cent moisture, they die, and 

 will not revive when placed in water. 



On the other hand, cultures of most microorganisms if lyophilized 

 (dried from the frozen state) can be kept in this condition for years and 

 still grow when placed in a suitable medium. 



