PLANT STRUCTURE AND COMPOSITION 193 



Lecithin is used in the food industry as an emulsifying agent and as 

 an antioxidant in the confectionary trade to preserve the "bloom" and 

 prevent graying of chocolate in hot weather. Lecithin is also used in 

 compounding cosmetics and pharmaceutical preparations. Addition of 

 small amounts to the surface of paints in cans prevents film formation 

 during storage. 



Proteins 



Although proteins are usually thought of as biological catalysts 

 (Chapters 5 and 6), in the seeds of many plants these important poly- 

 mers also serve as stored food reserves. Since most of the cereal 

 grains contain only about 10 per cent protein, these seeds depend 

 primarily on the carbohydrate reserves. Seeds of many species of 

 dicotyledonous plants, on the other hand, frequently accumidate larger 

 quantities of reserve proteins in the cotyledons (20 to 30 per cent). 

 Globulins (page 114), which constitute the largest part of dicotyle- 

 donous proteins, are extracted from the defatted seed meal with 

 5 to 10 per cent sodium chloride and may be precipitated by dialysis 

 against water, or by the addition of ammonium sulfate. Crystalline 

 globulins, prepared from hemp, squash, and melon seeds, have molecu- 

 lar weights of 200,000 to 430,000. 



Proteins from different vegetable sources, particularly from soybean 

 and cottonseed meals, have found wide industrial application. Plant 

 proteins from different sources are so similar in chemical composition 

 that they can be used interchangeably for many industrial purposes. 

 Ease of isolation and cost of the starting material are the determining 

 factors. The most important industrial uses of proteins are for the 

 production of plastics and adhesives, in coatings for paper products, 

 and in bonding plywood veneers. Artificial textile fibers can be pre- 

 pared from vegetable proteins but have not been used extensively be- 

 cause of competition from synthetic fibers. Similarly proteins in water- 

 based paints have been replaced largely by synthetic products. 



METABOLIC MACHINERY 



Proteins 



In the preceding section plant proteins were discussed as reserve 

 materials. Associated with the reserve proteins in the seed, and widely 

 distributed throughout the remainder of the plant, are the proteins 

 which serve to catalyze the multitude of reactions involved in the com- 

 plicated process termed metabolism (Chapter 9). Studies on the leaf 



