90 Introductory Biology 



vulsionSj or even death. Normal contractions of heart muscles can 

 occur only if there is a proper balance of sodium, potassium, and cal- 

 cium ions. Mineral salts maintain the normal osmotic balance between 

 the living protoplasm and the various environmental factors. All in all, 

 it may be that deficiencies in mineral salts are more important than 

 temporary deprivations of the organic foods. About fifteen elements are 

 known to be essential in mineral salts in human diet, although some are 

 needed only in traces. About 30 grams of these mineral salts are lost 

 from the human body daily through feces, urine, and sweat, and they 

 must be replaced. Rich food sources for minerals include vegetables, 

 milk, cheese, meat, eggs, etc. Iron is necessary to build hemoglobin, and 

 iodine is necessary to produce thyroxin, the hormone of the thyroid 

 gland. 



E. Water 



A rather large part of all protoplasm is water, the percentage varying 

 with the type of protoplasm, the conditions under which it has lived 

 previous to the analysis, etc. More water is present in aquatic organ- 

 isms than in those living in dry environments. Water itself is not alive, 

 but it forms an arena in which the various nonliving substances which 

 make up the living protoplasm may perform (Fig. 27). Many of the 

 movements associated with living protoplasm are influenced in part, at 

 least, by the water content. Water acts as the dispersing medium of 

 the colloidal systems of living protoplasm. Water is essential in passing 

 foods into a living organism and in eliminating wastes from it. Water 

 also assists in equalizing temperatures throughout an organism as well 

 as in diluting certain detrimental substances which may have entered. 

 Water also tends to reduce friction and prevent structures from abnor- 

 mally adhering to each other. A certain amount of water seems to be 

 necessary for the reception of certain stimuli, especially those of the 

 senses of taste and smell. 



F. Vitamins 



i 



Vitamins (L. vita, life; amine, formed from ammonia) are rather 

 simple organic compounds which even in small quantities are essential 

 to life. The various vitamins are quite different chemically and origi- 

 nally were thought to be amines formed from ammonia; hence the name 

 is now incorrect from a chemical composition standpoint. In general, 

 they cannot be manufactured by the animal body (but are produced by 



