PROTOPLASM AND THE CELL 57 



to combine with more oxygen in oxidation and therefore release 

 more energy. Fat is extremely well adapted as a form of material 

 for storage, since Aveight by weight it contains more potential energy 

 than any of the organic group. Such common substances as lard, 

 butter, tallow, whale blubber, and cottonseed oil are good examples. 

 Fats serve a double function in protoplasm : constitution of a part 

 of the structure of the cell and, secondly, the storage of food. 



Proteins constitute the bulk of the foundation or framework of the 

 cellular structure and are the most abundant organic constituents. 

 They are composed of carbon, hydrogen, oxygen, and nitrogen, with 

 the frequent addition of traces of sulphur, phosphorus, magnesium, 

 and iron. All of the proteins have large molecules, each being com- 

 posed of thousands of atoms ; as an illustration, take hemoglobin of 

 the red blood corpuscles with its formula C7i2Hii3oN2i40245FeS2. Pro- 

 teins have a slow rate of diffusion, high resistance to electric cur- 

 rent, and usually coagulate upon heating or upon addition of acids, 

 alcohol, or salts to form a clot. Egg albumen, gelatin, and lean 

 meat are common examples of proteins. They are split into numer- 

 ous amino acids which serve as the building stones of the stable 

 portions of protoplasm. 



Enzymes are substances whose exact chemical nature is not yet 

 known, but whose importance to protoplasm is probably unequaled. 

 Chemically and physically they seem to be more like proteins than 

 anything else. These substances are found not only in the cells, 

 but they are also secreted by cells into the digestive tract and into 

 the blood stream, where they act as organic catalysts. The general 

 function of the catalyzer or catalytic agent is that of facilitating 

 and speeding up certain chemical exchanges without the agent itself 

 entering into the reaction. The well-known example of catalysis 

 is the effect of a small amount of platinum in increasing the rate 

 at which hydrogen and oxygen combine to form water. A particu- 

 lar enzyme is usually specific for one kind of reaction, but not for 

 the species of animal in which it will function. Enzymes taken 

 from one species will usually facilitate the same kind of specific 

 reaction in other species. The digestive enzymes may be thought 

 of as an example. Of these, pepsin will bring about the same gen- 

 eral reaction, whether it is in the stomach of a frog or of a man, 

 under favorable conditions. Since many enzymes influence only one 

 specific type of chemical reaction and since there are numerous 



