Chap. 3 LIVING MATTER AND CELLS 31 



Fats resemble carbohydrates in being composed only of carbon, hydrogen, 

 and oxygen but differ in the proportions of each of these, the hydrogen atoms 

 being twice as numerous as. those of carbon and the amount of oxygen rela- 

 tively small. Fats are colloids, relatively insoluble in water. They liquefy at 

 various temperatures, oils at room temperature or lower, others near the 

 body temperature of the animals in which they occur. Those of snakes and 

 other cold-blooded animals liquefy at relatively low temperatures. 



The complex phosphorus-containing fats (phospholipids) include lecithin, 

 abundant in egg yolk, in nerve tissue, in bile, and blood. The steroids, an- 

 other group of fatty substances, include cholesterol, well known in the bile 

 and gallstones. The male and female sex hormones are also related to these 

 fats. Certain vitamins are associated with them; the growth vitamin A and 

 vitamin D, which prevents rickets, occur especially in butter and cod-liver oil 

 and in green vegetables; the fertility vitamin E is in butterfat and lettuce. 



Proteins. All protoplasm contains proteins. They are the keystones in its 

 organization and next to water its most abundant compound. Different pro- 

 teins occur in different kinds of cells. The proteins of every species of organ- 

 ism evidently differ from those of every other. The kinship of animals is 

 recorded in the proteins of their blood. Proteins in the blood of whales that 

 have lived in the sea for countless generations are more like those of their 

 relatives, the land mammals, than of their neighbor fishes. Proteins are 

 prominent in the nuclei of all cells. Chromatin, the chief physical basis of 

 heredity, is composed of nucleic acid and extraordinarily complex proteins. 

 The nuclei of the male and female sex cells together contain most of what 

 determines the inherited qualities of an offspring, maybe its chance to become 

 a codfish or a senator. 



Proteins are the most complicated and various of all substances. They are 

 composed not only of carbon, hydrogen, and oxygen, like the carbohydrates 

 and fats, but include nitrogen, sulfur, phosphorus as well. Their molecules 

 are very large, often containing thousands of atoms, and are complex, and 

 variable like living matter itself. This means variety of structure and enables 

 protein to interact with many other substances and to share continually in 

 the metabolism without which life ceases. 



Proteins are constructed of chains or groups of smaller molecules called 

 amino acids, the simplest of which is glycine (C1.H5O0N) which can be syn- 

 thesized in the body. Molecules of proteins are too large to enter cell mem- 

 branes, but those of amino acids go through them freely and form within 

 the cell the kind of proteins which are characteristic of it (Fig. 3.3). By 

 varied combinations of about thirty-odd amino acids, a variety of protein 

 molecules enormous beyond imagination is achieved. They not only differ 

 with every species but with every individual. This is shown in many ways, 

 such as the usual difficulty in skin grafting, even between nearly related 



