I96 PHYSIOLOGY OF NUTRITION 



amino acids enter into the simple proteins of plants (and animals also). The simplest 

 of these acids is glycocoll (alpha-amino-acetic acid, CH2NH2COOH). As examples of 

 the most complicated amino acids may be mentioned cystin (alpha-diamino-beta- 

 dithio-dilactic acid) and triptophan (beta-indol-alpha-amino-propionic acid). The 

 constituent amino acid molecules are joined into groups called polypeptides, and 

 polypeptide groups are united to form the simple protein molecule. The simple 

 proteins, of which there are a large number, act as foods and are not considered here 

 as really constituents of the protoplasm, though they occur in the cytoplasm 

 of cells. They mainly occur as protein grains or dissolved in the cell sap 

 (which is the more liquid material lying within the cytoplasm of ordinary 

 plant cells). 



The nucleo-proteins furnish examples of the complex proteins. They are com- 

 binations of simple proteins with nucleins, and nucleins are combinations of the more 

 complex simple proteins with nucleic acids. The latter are rich in phosphorus. The 

 simplest nucleic acid (derived from yeast) has the formula,, C40H59N14O22 — 2P2O5. 

 These acids give phosphoric acid, on being decomposed. 



3. Enzymes. — Enzymes are catalyzers that occur in plant cells. They are organic 

 in their nature and act, like other catalyzers, to accelerate (or retard) chemical changes 

 or to alter the equilibrium point at which a chemical change ceases. In their presence 

 many chemical reactions go on that would not go on appreciably without their aid, or 

 would soon come to equilibrium and cease. The rate of change depends on the amount 

 of enzyme present, as well as upon the temperature and the other substances in the 

 medium. The enzyme is not used up in the process that it promotes. The chemical 

 nature of enzymes is not yet known; they are known by their effects. 



Diastase is a widely distributed plant enzyme. It causes the transformation of 

 starch into glucose, with the consumption of water. It may be obtained from germin- 

 ating seeds, such as those of barley (malt), from leaves, etc. Diastase, as here defined, 

 consists of a mixture of two enzymes, amylase and maltase. Amylase transforms 

 starch to maltose, and maltase forms dextro-glucose (dextrose) out of maltose. — Sac- 

 charose (or invertase) is another common enzyme in plants. It converts saccharose 

 (cane sugar) into dextro-glucose (dextrose) and dextro-fructose (levulose), with the 

 consumption of water. — Proteins are transformed and rendered soluble in water by the 

 action of proteolytic enzymes, which decompose proteins, with consumption of water, 

 into simpler compounds. — Fats are decomposed into glycerine and fatty acids, with 

 consumption of water, by lipases. 



Besides these, and other hydrolytic enzymes (operating with consumption of 

 water), oxidases are present in plant cells. These cause the oxidation of organic 

 substances, with molecular oxygen. — Another non-hydrolytic enzyme is zymase, 

 which promotes the formation of alcohol and carbon dioxide from glucose. It occurs 

 plentifully in active yeast cells. 



Living protoplasm is supposed to contain specific enzymes that promote the various 

 chemical changes of vital activity, not only decompositions but syntheses. Hill 

 showed that maltase acts in either direction, to form glucose from maltose (with addi- 

 tion of water) or to form maltose from glucose (with production of water), according to 

 the concentrations of maltose and glucose in the mixture. 



Tissues may be killed without destroying all of their enzymes, as follows from the 

 fact that many enzymes may be extracted from tissues that have been killed in the 

 process of extraction. Enzymes may remain active in dead tissues for a considerable 

 time, but enzyme activity in dead cells is not automatically regulated and coordinated 



