136 TEXTBOOK OF BOTANY 



If this process were to continue, a polysaccharide consisting of a long 

 chain of many (CeHioOo) units would be formed. X-ray studies indicate 

 that the molecules of polysaccharides in plant cells do consist of long 

 chains of these smaller sugar units. In some substances, such as starch 

 and cellulose, the units are all (CcHioO.-,) from glucose. In other sub- 

 stances these units may be formed from another kind of sugar, or the 

 chain may be composed of units of different kinds of sugars, as in 

 hemicelluloses. The chain ma)' also consist of units of sugar and certain 

 acids derived from it, as in pectic compounds. 



One other important biological fact about condensation should be 

 appreciated, namely, that the chemically bound energy in the sugar is 

 still in the units of sugar in the new products formed. It is not liberated 

 in the process of condensation. 



Digestion, or hydrolysis, is the converse of condensation. As molecules 

 of water replace those lost during the condensation of simpler molecules 

 to more complex ones, the complex molecules are subdivided into the 

 simpler ones from which they were formed. 



Summary. Starch is formed in plastids ( leucoplasts and chloroplasts ) 

 in plant cells from glucose by chemical condensation — a process that is 

 just the converse of all ordinary processes of digestion. By condensation, 

 many molecules of sugar become joined together by a loss of one mole- 

 cule of water for every molecule of sugar added. Chemically bound 

 energy is not liberated b\' this process and the starch therefore contains 

 the potential energv that was in the sugar from which it was made. 

 In the same way, many other complex carbohydrates ( polysaccharides ) 

 are fomied in plants. 



Starch may be synthesized from sugar in both green and non-green 

 tissues, and in both light and darkness. It is especially dependent upon 

 the presence of plastids, certain enzymes, the concentration of sugar, 

 and the acidity of the cells. It is a reversible process. External factors, 

 particularly light and temperature, affect these internal conditions and 

 thereby indirectly affect the accumulation of starch in plants. These 

 internal conditions are not uniform throughout the plant, nor in differ- 

 ent kinds of plants. As a consequence, starch accumulation is more 

 abundant in some tissues, organs, and plants than in others. In some 

 plants it may occur onlv under exceptional conditions, or not at all. 



Since starch is insoluble in water, it does not pass from cell to cell. 

 It is formed in the cells in which it is found, and becomes valuable 

 as a food only where it can be digested. Enzymes that digest starch are 



