174 THE INDIVIDUAL ORGANISM 



as fast as it is made and the sugar concentration prevented from becom- 

 ing injuriously high. At night these starch granules in the chloroplasts 

 disappear, because of the reconversion of the starch into soluble sugars 

 that are carried to other parts of the plant. 



Cellulose is another polysaccharide formed from glucose. Although 

 related chemically to starch, it is very much more insoluble and is highly 

 resistant to chemical change. It is not affected by the ordinary digestive 

 enzymes; animals such as ruminant mammals and termites, which use 

 cellulose as food, are able to do so only through the presence in their 

 digestive tracts of microorganisms capable of breaking down this sub- 

 stance. Fats, oils, and waxes are synthesized from sugars without the 

 addition of any other elements; but the proportions of carbon, hydrogen, 

 and oxygen in the molecule are changed, and the latter element is so 

 greatly diminished in relative amount that the quantity of stored energy 

 is very considerably increased. Fats and oils are extensively used by 

 plants for food storage on account of this high energy content. In the 

 synthesis of amino acids and proteins, carbohydrates are combined with 

 nitrogen from the soil and sometimes also with sulfur, phosphorus, or 

 other elements. The proteins thus formed constitute, with water, the 

 bulk of the protoplasm of the cells; some of them are also stored as food 

 (especially in the seeds of legumes) or are used in the making of enzymes 

 and other cell products. 



Enzyme action in plants. The chemical transformations that occur 

 within the plant are nearly all brought about through the agency of 

 various enzymes. We have already seen something of the nature and 

 functioning of these organic catalysts in our study of the human body, and 

 it will be recalled that one of their characteristics is a high degree of 

 specificity in the substances upon which they act. Each enzyme affects 

 only a single chemical reaction. In view of the multiplicity of the chemical 

 reactions that occur within the plant cell, it would therefore seem that 

 each cell must either be equipped with a battery of enzymes or else be 

 able to produce any given enzyme at need. Some of the enzyme-con- 

 trolled reactions are reversible, the direction of the reaction being con- 

 trolled by the relative concentrations of the substances taking part; thus, 

 under experimental conditions, lipase, found in plants as well as in 

 animals, in the presence of excess fat splits the fat into glycerol and fatty 

 acids, but in the presence of excess glycerol and fatty acids causes these 

 to combine into fat. The plant cells are apparently able to control the 

 direction of this process by the expenditure of energy. Most enzyme- 

 controlled reactions, however, are in practice unidirectional; thus some 

 enzymes build up a complex substance from simpler ones with the ex- 

 penditure of energy (synthesis), whereas others break down a complex 

 substance into simpler ones with liberation of energy. 



