The Chemical and Physical Structure of Protoplasm - 83 



H 2 C- 



OH 



O 



II 

 HO-C- 



HC- OH 



H 2 C— OH 



glycerol 

 1 mol 



+ 3HOH 



synthesis 



(dehydration) 



H O 



HO- 



O 



II 



-C- 



o 



II 



-c- 



fatty acids 

 3 mol 



-3HOH 

 hydrolysis 



H,C— O-C-R' 



HC— O- 



O 



II 



-c- 



H,C- 



O 



II 

 -O— c- 



fat 

 1 mol 



Fig. 4-1 1. Build-up and breakdown of fat as it occurs 

 in cells generally. 



like compound obtainable in good quantity 

 from egg yolk. Lecithin is probably a struc- 

 tural component in all cells. The molecular 

 structure of lecithin resembles that of a fat, 

 except that one of the three fatty acids is re- 

 placed by phosphoric acid in combination 

 with choline (an organic base). 



The most familiar steroid substance is 

 cholesterol (C 27 H 4 ,-OH), a normal compo- 

 nent of bile; several hormones, including 

 the sex hormones (p. 000), likewise belong 

 to the steroid group. Cholesterol and the 

 other steroids are constituted entirely of C, 

 H, and O; but the "carbon skeleton" of a 

 steroid is a complex of several interlinked 

 rings, as is shown in Figure 4-12. Steroids 

 and phospholipids are probably represented 

 in all protoplasm, as essential components 

 of the intrinsic membrane systems. 



Proteins. Proteins are all-important struc- 

 tural components in every cell. Each of the 

 genes represents a uniquely distinctive DNA- 

 protein complex, without which the cell loses 

 virtually all powers of growth and replica- 

 tion (Chap. 27). Nor can growth occur in 

 the absence of the RNA-protein components 

 of the protoplasm (p. 134). Likewise enzymes, 

 by means of which the cell carries on its 

 other metabolic activities (Chap. 5), always 

 possess an essential protein component; and 

 proteins are always represented in the struc- 

 ture of the plasma membrane and other in- 

 trinsic membranes of the cell. In short, pro- 



H, 



H.C- 



CH. 



H 



H, 



CH 3 





XH. 



CH 3 

 ~C — CH2 — CH2 - 



■CH, 



-CH, 



/CH 3 

 -CH 

 X CH, 



HO- 



H' 



H 2 



H 



XH, 



Fig. 4-12. Molecular structure of cholesterol, a steroid compound. Cholesterol is 

 present in most, if not all, cells. Also a number of hormones (Chap. 22)— such as the 

 male and female "sex hormones" (testosterone and "estrogen"), the "pregnancy hormone" 

 (progesterone), and the adrenocortical hormones (cortisone, etc.)— are steroids, which 

 have a similar, though not identical structure. 



