24 



ESSENTIALS OF BIOLOGY 



the formulae represent individual molecules and it is not such 

 that exist in the rocks and minerals. Thus rock-salt (sodium 

 chloride), as a molecule, is simply Na — CI, but in rock-salt crystals 

 we have really a solid '' lattice " : ' 



etc 



etc. 



etc. 



etc. 



And in crystals (out of which most mineral substances are con- 

 stituted) this lattice-work in three dimensions may be exceedingly 

 complex. Nevertheless, the molecular weights (that is the weights 

 relatively to the molecule of hydrogen = 2), when it is possible 

 to infer these, have the relatively low values of tens to hundreds 

 of units in the cases of inorganic substances. 



8^. Organic Chemical Categories. Here the molecular 

 weights have the values of thousands to hundreds of thousands 

 of units. 



Characteristic organic chemical substances are proteins, carbo- 

 hydrates and fats : each of these is really a category, so 

 that there are very great (though finite) numbers of difi"erent 

 proteins, etc. 



Protems. These are chemical substances, of which the mole- 

 cules may be regarded as being built up as follows : — 



The *' building-stones " are a,mino- acids ; 



Amino-acids are built up into polypeptides ; 



Polypeptides are finally built up into proteins. 

 An amino-acid may be arginine, which is 



NH, 

 NH = C - NH - CH2 - CHo - CH2 - CH(NH2)C00H. 



