PROTOPLASM 27 



shown that these may be converted in the animal body to carbohydrates 

 such as glucose. 



The nuclear membrane, the plasma membrane around the cell, and 

 the membrane around the mitochondria all contain fatty substances. The 

 myelin sheath which surrounds nerve fibers (p. 61) is exceptionally rich 

 in lipids. In some animals, such as mammals, there are large deposits 

 of fat just under the skin which serve as fuel reserves and as insulators 

 to decrease the loss of heat from the body. The lipid stores of animals 

 such as sharks and starfish are in the form of oils found in the liver. 



Related to the true fats are the phospholipids, waxes and cerebro- 

 sides, all of which contain fatty acids. The phospholipids, which contain 

 phosphorus and nitrogen in addition to glycerol and fatty acids, are 

 important structural and functional components of protoplasm and are 

 especially found in mitochondria and microsomes. Waxes, such as bees- 

 wax and lanolin, contain a fatty acid plus an alcohol other than gly- 

 cerol. Cerebrosides, as their name indicates, are fatty substances found 

 especially in nerve tissue. They contain galactose, long chain fatty acids, 

 and a long chain amino alcohol, sphingosine. The metabolic roles of 

 these special fats is not clear at present. 



Sferoids. Steroids are complex molecules containing carbon atoms 

 arranged in four interlocking rings, three of which contain six carbon 

 atoms each and the fourth of which contains five. Vitamin D, male and 

 female sex hormones, the adrenal cortical hormones, bile salts and 

 cholesterol are examples of steroids. Cholesterol (Fig. 2.5) is an important 

 structural component of nervous tissue and other tissues, and the steroid 

 hormones are of great importance in regulating certain aspects of me- 

 tabolism. 



Proteins. Proteins differ from carbohydrates and true fats in that 

 they contain nitrogen in addition to carbon, hydrogen and oxygen. Pro- 

 teins typically contain sulfur and phosphorus in addition. Proteins are 

 always present in protoplasm and are of prime importance as the basic 

 building materials of living matter. Protein molecules are among the 

 largest found in protoplasm and they share with nucleic acids the dis- 

 tinction of great complexity and variety. Hemoglobin, the red pigment 

 found in the blood of all vertebrates and many invertebrates, has the 

 formula C,o32H48i60872N78oS8Fe4 (Fe is the symbol for iron). Although 

 the hemoglobin molecule is enormous compared to a glucose or triolein 

 molecule, it is only a small-to-medium-sized protein. Many, indeed most, 

 of the proteins in protoplasm are enzymes, biological catalysts which 

 control the rates of the many chemical processes of the cell. 



CH3 CH2 



CH CHo CH3 



CH3 I I / 



X\\/\ CH2— CH 



CH, I ] \ 



HO 



CH 



Figure 2.5. Structural formula of a steroid, cholesterol. 



