Properties and Activities of Living Protoplasm 91 



plants) so must be secured in sufficient amounts in the diet in order to 

 ensure that the various metabolic processes are performed normally. 

 Each vitamin which has been successfully analyzed so far has a different 

 chemical formula, and each has somewhat specific functions. Today we 

 know the chemical structure of most of the vitamins so far discovered, 

 and many of them have been prepared synthetically. Vitamins were 

 discovered about 1912, and much scientific progress has been made since 

 that time through chemical analyses, animal experimentation, etc. 



From recent experiments dealing with the exact functions of a vita- 

 min it has been observed to act as a catalyst for some fundamental 

 reaction common to all protoplasm. When there is a vitamin deficiency 

 below a certain level, certain behaviors and metabolic functions are 

 impaired, the particular effects being determined by the type of vitamin 

 involved. Plants probably have their specific vitamin requirements just 

 as animals, although this has not been studied so extensively as in the 

 animals. Plants synthesize most of the vitamins so they must play im- 

 portant roles in their metabolisms. Some of the vitamins are compo- 

 nents of enzymes which control many physiologic processes in cells or 

 contribute to the actual formation of certain enzymes. It is probable 

 that many vitamins in plants act as the components of enzymes or as the 

 progenitors of enzymes. In green plants, vitamin B is essential for nor- 

 mal root development. Vitamin K regulates the oxidation-reduction 

 processes in living cells. The various roles of vitamins in the numerous 

 processes of living organisms are considered in greater detail in a later 

 chapter. 



G. Enzymes 



An enzyme (Gr. en, in; zyme, yeast, or leaven) is a nonliving, complex 

 organic (protein) catalyst (kat' a list) (Qr. katalysis, dissolving) produced 

 only by living protoplasm, and controls the speed of a chemical reaction 

 without taking part in the reaction itself or without being consumed as 

 the result of it. Because one of the first enzymes was isolated (by the 

 German scientist BUchner) from crushed yeast cells, the unknown sub- 

 stance was called an enzyme. Every animal and plant cell contains 

 many different enzymes, each with a specific function. A pure enzyme is 

 specific because it controls one type of chemical action and acts on a 

 specific kind of substance known as the substrate. Many enzymes per- 

 form their function where they are formed, but some operate outside the 

 cell which produces them. Digestive enzymes illustrate the latter. Most 



