THE CELL 51 



1. Metabolism. — This term is used to designate those cellular 

 activities which have to do with the nutrition of the cell. A cell is 

 able (i) to take up from without substances suitable for its nutri- 

 tion and to transform these into its own peculiar structure, and (2) 

 to dispose of the waste products of intracellular activities. The 

 former is known as constructive metabolism or anabolism, tlie latter as 

 destructive metabolism or katabolism. 



It is possible, for example, to watch an amoeba send out projections 

 (pseudopodia) (see p. 52) around an adjacent bit of food material. 

 These projections coalescing finally completely enclose the food 

 material within the body of the amoeba where it is acted upon by 

 the protoplasm in such a way (digested) as to completely lose its 

 identity and to fmally become an integral part of the cytoplasm. 



There is normally maintained within the cell a state of equi- 

 librium between this constructive and destructive metabolism, be- 

 tween the intake of food on the one hand and the outflow of material 

 products or of energy on the other. Stated as an equation, intake 

 = outgo. Any marked -|- or — on one side of the equation 

 without corresponding H- or — on the other side must disturb this 

 equihbrium and must result in physical changes within the cell. 

 Thus any marked -f in intake without corresponding + in outgo 

 must result in growth of the cell, while any + in outgo without corre- 

 sponding + in intake must tend toward diminution in bulk of cell 

 and final destruction. The life-length of cells varies. Some cells, 

 for example, some of those of the central nervous system, probably 

 live throughout the life of the individual. At least there is good 

 clinical evidence that once destroyed they are never replaced. Other 

 cells, such for example as blood cells and many epithelial cells, are 

 constantly wearing out and being replaced by new cells. 



2. Special Function. — This is the special work which it is the part 

 of the cell to perform. It varies greatly for different cells. Some 

 cells, as, e.g., the surface cells of the skin, appear to act mainly as pro- 

 tection for more delicate underlying structures. Other cells — gland 

 cells — in addition to maintaining their own nutrition, produce 

 specific substances (secretions), which are of great importance to the 

 body as a whole. Still other cells, e.g., nerve cells and muscle cells, 

 have the power to store up their food substances in such a way as to 

 make them available in the form of energy. This appears to be 

 accomplished by the building up within the cell of highly complex 

 and, consequently, unstable molecular combinations. By reduction 



