Life: Its Nature and Origin 33 



membrane into the cytoplasm. These nutrients may first be broken 

 down to simple compounds. Gradually they are built up into more 

 complex compounds and finally into duplicates of the structures of 

 the original young cell. When the cell has exactly doubled in weight, 

 it undergoes maturation and division again, as was described 

 above. 



Simple though these steps sound, each is highly complex. The 

 cell membrane is a series of meshes of proteinaceous and other 

 molecules oriented by inherent physical properties of attraction 

 and repulsion into a precise network. Conditions either outside or 

 inside the cell cause the network to change the "mesh" so that dif- 

 ferent compounds can pass through it at different times. Only 

 molecules in aqueous solution can go through the membrane, and 

 they do it according to the physical laws of diffusion. Diffusion 

 unaided could not supply the cell as we know it with the large 

 amount of nutrients needed for its growth. Several methods for 

 supplementing diffusion have evolved. In one method, the size 

 of the mesh of the membrane decreases, preventing the difi^usion 

 of certain substances out of the cell. This causes differences in 

 solution concentrations in the two sides of the membrane, effecting 

 an automatic flow of the solvent into the cell ( osmosis ) . In another 

 method (Weinstein, Robbins, and Perkins, 1954; Thomas, 1956) it 

 is thought that complex, organic bonding agents (chelating com- 

 pounds ) combine with needed molecules to form compounds which 

 will diffuse into the cytoplasm. Then they release the captive com- 

 pounds to other compounds in the cytoplasm. The freed molecules 

 of the chelating agents then diffuse back to the outer regions of the 

 membrane, and the process can start over again. This process is in 

 reality a miniature molecular machine, kept going by material ar- 

 riving from the external environment and being consumed within 

 the cell, geared by automatic chemical and physical conditions 

 inherent in the cell. 



Once inside the cell, these nutrients are consumed in two types 

 of reactions. In one, the enormously varied and complex com- 

 pounds of the cell are built up, that is, doubled, from the ingested 

 raw materials. In the other, the raw materials take part in reactions 

 which produce the energy to perform the synthetic reactions in- 

 volved in this replication. 



Of all the compounds in the cell, the proteins are the largest 

 and most complex. A tremendous number of proteins have been 

 observed in living systems. The basis of proteins are amino acids, 

 of which 22 kinds have been identified ( Fruton, 1950 ) . Individually 



