30 THE FOUNDATION Part I 



fructose and the loss of a molecule of water — glucose (CiHu-Oo) + fructose 

 (C,iH,:.0(i) — HjO = sucrose (Ci^-H^i-On )• When it is hydrolyzed sucrose 

 gives one molecule of glucose and one of fructose. 



Other compound sugars are starch, glycogen, and cellulose. These contain 

 units of simple sugars combined into large molecules. Starch is the common 

 storage form of carbohydrate in plant cells and glycogen or animal starch in 

 animal cells. The molecules of both are too large to go through the cell mem- 

 branes, but protoplasm can hydrolyze both and obtain glucose with its smaller 

 molecules. 



Fats. Fatty substances take part in the composition of cell membranes 

 and therefore in their selective permeability (Fig. 3.3). In animals they 

 constitute the principal supply of food. They produce more energy per gram 

 than carbohydrates but oxidize more slowly and are less quickly accessible. 

 Fat persons get hungry just as soon as lean ones. Fats are the backlogs of 

 the fire of which carbohydrates are the kindling. Fats are abundant in animals 

 and by no means absent in plants. They may be in the cells, as in bacon, 

 or in the secretions that cells produce, as in cream, or in the wax of honey- 

 comb. 



Fig. 3.3. Diagram of a cell membrane where there is continuous activity, con- 

 stant separation of what shall and shall not pass in and out of the cell. These 

 processes are discovered by chemical analysis. Here, the cell membrane is shown 

 cut so that its inner surface is at the left and its outer edge at the right. Lipoid 

 (fatty) particles are shaded, protein particles are white. Water channels (arrows) 

 permit water and other smaller molecules to pass. Larger molecules are blocked by 

 the small pores but those that are soluble in fats may enter the lipoid (shaded) 

 particles of the membrane, mix with their molecules and thus pass in or out of the 

 cell. (Courtesy, Gerard: Unresting Cells. New York, Harper & Bros., 1940.) 



