Living Cells and 

 What They Are Made Of 



H 



Since all living things are either single cells or colonies of cells, the cell 

 is the real unit of life and we shall have gone far in the understanding 

 of life if we can find out what a single cell is made of and how it works. 

 However, this is a colossal undertaking. Although it is tiny — the 

 average cell is not more than 1 / 100 of a millimetre in diameter — we 

 shall see that it contains a very large number of chemical substances, 

 all connected by beautiful and complex mechanisms into a function- 

 ing whole. I shall try to explain something about these substances and 

 about these mechanisms — in other words, about the life of the living 

 cell. 



We do not usually need to specify, at least for a bird's-eye view, 

 what organism our cell belongs to. The remarkable fact has been 

 established — and this is one of the most significant findings of the 

 study of the chemistry of life — that all living organisms are very simi- 

 larly constructed and operate in very similar ways. There are naturally 

 quite large differences, e.g. between the cells of a woody plant, which 

 contain much cellulose, and those of, say, a jellyfish in which the cells 

 are entirely made of a jelly-like substance called protoplasm; but it 

 is found that the basic organization of all cells has much in common. 

 They are constructed to a considerable extent out of similar materials 

 and the chemical operations which occur in them are basically the 

 same. In other words all living things, from amoeba to man, are one 

 family. The smallest body which exhibits all the properties of living 

 things is the cell and it is only by studying life at the level of cells and 

 their contents that we can hope to understand the secrets of living 

 processes. 



First of all, then, we must know something of what cells are made 

 of. This study only really began in this century because it was impos- 

 sible to describe adequately the characteristic substances present in 

 cells until chemistry had advanced sufficiently to cope with them. In 



