Chap. 3 LIVING MATTER AND CELLS 29 



Carbohydrates. All protoplasm is believed to contain carbohydrates. Those 

 of one group (pentoses) are one of the main components of the chromatin 

 in the nuclei of all cells. Qther than that important role, carbohydrates are 

 not actually a part of protoplasm but are only contained in it. Their great 

 function is the immediate supply of energy, of which they are the chief 

 source for all living organisms. 



The familiar carbohydrates are sugars and starches, the cellulose in the 

 walls of plant cells, pectin, and glycogen or animal starch stored in animal 

 cells (Fig. 3.2). Cellulose gives stiflfness to plant stems and forms most of 

 the fiber of cotton. Pectin, a carbohydrate of fruit, insures the stiffening of 

 jelly. Starch in plants and glycogen in animals are the reserve food supply 

 of the cells. They occur in the watery solution of protoplasm and the mole- 

 cules come and go through cell membranes (Fig. 3.3). 



All carbohydrates contain only carbon, hydrogen, and oxygen. In forming 

 them, untold numbers of green plants capture the energy of the sun, the 

 source of energy for all living matter, and use this energy to combine carbon 

 dioxide with water, thus creating the energy-packed food, glucose, and the 

 by-product oxygen. 



The simplest of the carbohydrates are sugars, all of them more or less 

 sweet. They include the simple sugars, pentoses with five and hexoses with 

 six carbon atoms (CoHjoOe), the latter including glucose (also called dex- 

 trose). This is an almost universal protoplasmic fuel. It is the form of sugar 

 present in human blood in which the essential blood-sugar content is about 

 0.1 per cent. One of the compound sugars (polysaccharides) is table sugar 

 (sucrose, Ci:.HooOii) from sugar cane and sugar beets. It is the commonest 

 sugar in the nectaries of flowers, easily tasted in violets and columbines. 

 Sucrose is produced by the union of a molecule of glucose with one of 



. .,^vV- 







Fig. 3.2. Glycogen (black) or animal starch in human liver cells. It is stored in 

 many kinds of cells but is most abundant in the liver and muscles. Soluble in water 

 and therefore in protoplasm it is a quickly available food. (Courtesy, Bremer and 

 Weatherford: Textbook of Histology, 6th ed. Philadelphia, The Blakiston Com- 

 pany, 1944.) 



