402 ANIMAL BIOCHEMISTRY 



Microcrystals of inorganic material are distributed throughout tlie 

 bone, contributing rigidity. The preponderant compoimd appears to 

 be hychoxy apatite with the foraiula 



[Ca(OH) • Ca4(P04)3]2 



Since bone also contains magnesium, carbonate, citrate, and fluoride 

 ions, substitution in this formula is presumed in order to account for 

 the additional components. The sodium and potassium also present 

 do not fit conveniently in place of calcium, and their locations are 

 unknown. Since the chondroitin sulfate is a multivalent anion, all 

 four major cations of bone may also be associated with it. 



Practically every other cation of animals might be found in bone 

 since most heavy metals form insoluble salts. Lead and radioisotopes 

 have probably been studied the most in connection with their toxici- 

 ties. In these and other abnormalities, the calcium is displaced. 

 When the exchange is extensive enough, crystal structure changes, and 

 the bone becomes hard and brittle or soft and flexible. Both condi- 

 tions are potentially dangerous. 



Many lower animals have exoskeletons of other types. A variety of 

 these skeletons are predominantly organic matrices with calcium 

 carbonate embedded (corals and shellfish). Crabs and the like possess 

 skeletons composed primarily of chitin, a polymer of glucosamine 

 (chitosamine). Most insects also have external skeletons containing 

 25 to 50 per cent chitin with other organic materials, mostly protein, 

 making up the remainder. Obviously the chitinous skeletons are not 

 inorganic and thus do not depend so directly on a supply of particular 

 ions in the diet. 



Electrolyte Balance 



Cells cannot function properly in pure water or even in dilute 

 aqueous solutions of essential substrates. Indeed, many cells are 

 killed and even ruptured in such systems. If a suitable concentration 

 of electrolytes is provided, the cell survives. This need is commonly 

 expressed as a requirement for a particular osmotic concentration in 

 the environment. An excess of electrolyte is equally unsatisfactory, so 

 cells thrive only in characteristic ranges of osmotic concentration. In 

 mammals and birds this corresponds to about 0.I45M sodium chloride. 

 Frogs and other cold-blooded animals operate at internal osmotic 

 concentrations equivalent to about OAWM sodium chloride. 



Recall from Chapter 2 that osmotic pressure depends upon the 

 total number of particles present. Since sodium chloride is dissociated 



