The Chemical and Physical Structure of Protoplasm - 81 



Q o 



ARROWROOT 



CORN 



POTATO 



WHEAT 



Fig. 4-9. Starch grains from different plants, all drawn to the same scale. 



soluble in water, but readily soluble in such 

 organic solvents as ether, chloroform, and hot 

 alcohol. Lipids that are liquid at room tem- 

 perature (about 20° C) are commonly called 

 oils (olive oil, cod-liver oil, etc.); but even 

 fats such as butter, lard, and tallow, which 

 are solid or semisolid at room temperature, 

 are generally liquid at the body temperature 

 of the warm-blooded animals in which they 

 occur. Generally the lipid components of 

 the protoplasm are relatively light (low spe- 

 cific gravity) and tend to be thrown to the 

 "light end" when the cell is centrifuged (Fig. 

 4-10). 



Because of their diverse chemical nature, 

 lipids are not treated as a single group, but 

 are subdivided into three groups: (1) the 

 true fats, (2) the phospholipids, and (3) the 

 steroids. 



The True Fats. The true fats include 

 many familiar substances, such as olive oil, 

 butter fat, and beef fat. Beef fat (C 57 H 110 O 6 ) 

 shows the typical chemical structure of a true 

 fat in that: (1) true fats are composed en- 

 tirely of C, H, and O; and (2) the natural fats 

 have large molecules, containing usually 

 about 50 and 100 atoms respectively of C and 

 H, but only 6 atoms of O. 



The fats in protoplasm serve mainly as 

 accessory fuels. However, fats do not oxidize 

 as readily as glucose, although the quantity 

 of energy per gram of oxidized fat is con- 

 siderably greater (9 Cal as compared to 4). 

 Much of the fat in complex animals such 



as man is localized in special cells, which col- 

 lectively make up the adipose tissue of the 

 body. 



A molecule of fat represents a combina- 

 tion of simpler units that can be liberated 

 by hydrolysis. In practice this hydrolysis is 

 accomplished by boiling the fat in a strongly 

 basic solution, although cells hydrolyze fats 

 at ordinary temperatures. When hydrolyzed, 

 each fat molecule liberates: (1) one molecule 

 of glycerol, which is commonly called glyc- 

 erin, and (2) three molecules that the chem- 

 ist identifies as fatty acid. 



lipids 



nucleus 



clear protoplasm 



lighter granules 



heavier granules 



Fig. 4-10. The fatty materials of protoplasm are rela- 

 tively light and rise to one end when a cell (such as 

 this Arbacia egg) is centrifuged strongly. (Courtesy of 

 Ethel Browne Harvey.) 



