384 LIFE : OUTLINES OF GENERAL BIOLOGY 



the ordinary weapons of chemistry, such as strong acids and caustic 

 alkaHes, the large molecule of starch breaks down into several very 

 much smaller molecules of comparatively simple sugars. Recently, 

 moreover, chemists have begun to suspect that "starch" is really 

 a mixture of two or more chemical compounds. Each grain of starch 

 possesses a tough outer envelope of "amylo-pectin", which will not 

 dissolve in water, and a less resistant core of "amylose" or "granu- 

 losa". But there is an additional and unsuspected complication. The 

 recent work of Ling and Nanji shows that the outer envelope of 

 amylo-pectin contains a considerable proportion of the element 

 phosphorus. Now phosphorus is an important constituent of all 

 living cells, and it is likely that this hitherto undetected store of 

 phosphorus in the starch grains is of great importance to the plant, 

 and perhaps also to the animals that reincarnate what the plant 

 has made. This instance illustrates what happens so often in the 

 advance of science: an apparently simple thing (the composition of 

 starch) turns out to be more complicated than was supposed, and 

 the unravelling throws light on something fundamental (in this 

 case the phosphorus cycle). 



Before passing from the carbohydrates, it should be noted that 

 the complex forms resemble proteins in forming colloidal solutions 

 in water, their large molecules cohering in particles, while the 

 simple sugars resemble the amino-acids in forming true solutions, in 

 which the molecules float free. 



The third group of cell materials includes primarily the fats, in 

 which glycerol (popularly called glycerine) combines with three 

 molecules of fatty acids. The glycerol molecule contains three —OH 

 or hydroxyl groups 



CH,.OH 



I 



CH .OH 



I 

 CH,.OH 



and each of these unites with one of three acids, stearic, palmitic, 

 or oleic, to form three common fats, which usually occur together in 

 ordinary animals. The fatty acids have long, straight chains of carbon 

 atoms; thus stearic and oleic have eighteen, palmitic sixteen. When 

 they combine with a mineral base, such as caustic soda, they form 

 soap. Perhaps this slightly technical paragraph, familiar enough to 

 the student of chemistry, may be more intelligible to others if the 

 familiar fact be kept in mind that ordinary soap consists of the 

 sodium salts of stearic and pa'mitic acids, that it is made by boiling 

 up fats with caustic soda, and that glycerine is always produced at 

 soapworks. In other words, fats consist of glycerol united to fatty 

 acids. 



