450 NITROGEN METABOLISM 



deadly nightshade {Atropa belladonna). Caffeine from coffee, tea, etc. and 

 thcobro/nine from the cocoa bean are often also classed with the alkaloids, 

 although actually they are purine derivatives. The formula for nicotine, a 

 representative alkaloid, gives some idea of the chemical nature of these 

 substances : 



HC 



HC 



CH H2C CH2 



\/ CH3 



N 



Most of the alkaloids are white solids, but nicotine is a liquid at ordinary 

 temperatures. They are all basic in reaction as the name indicates, and only 

 slightly soluble in water. The alkaloids are usually found in plant tissues in 

 the form of salts of organic acids. 



The physiological role of the alkaloids in plants, if any, is unknown. 

 Their restricted distribution suggests that they are not involved in any 

 processes of general importance. They are probably by-products of the nitro- 

 gen metabolism of the species in which they are found. IVIost of them have 

 pronounced physiological effects on the human body, and a number are of 

 therapeutic importance. 



Nitrogen Metabolism in Relation to Carbohydrate Metabolism. — Both 

 carbohydrate and proteinaceous foods are necessary for the development of 

 any plant. Deficiency of either soon results in the development of character- 

 istic and recognizable growth abnormalities. The supply of carbohydrate and 

 proteinaceous foods in a plant is influenced by many factors. Among these 

 are reciprocal relationships between available carbohydrates and available 

 organic nitrogenous compounds. As already described, synthesis of amino 

 acids, etc. occurs only at the expense of carbohydrates or their derivatives 

 which serve both as building material (together with nitrates) and as a source 

 of energy. Rapid amino acid synthesis therefore usually results in a diminu- 

 tion in the proportion of available carbohydrates in a plant, while plants in 

 which amino acid synthesis occurs slowly will usually be proportionately rich 

 in carbohydrates. 



Kraus and Kra5'bill (191 8) and subsequently many other investigators 

 have sought to give expression to the metabolic roles of carbohydrates and 

 organic nitrogenous compounds in terms of the relative proportions of the 

 two in the plant (often called the "carbohydrate-nitrogen ratio"). Working 

 with tomato plants they recognized four different metabolic conditions in 

 terms of the proportionate amounts of these two types of substances present. 



