IX 



A Community of Cells 



H 



By and large, all living cells have underlying mechanisms which are 

 basically similar; they are all variations of the same theme. Thus to 

 some extent the activities we have described so far are possessed by 

 every living cell but, of course, some features are emphasized in some 

 forms of life and repressed in others. The cells of unicellular organ- 

 isms, like many bacteria, carry out all the functions of living by them- 

 selves. But in organisms consisting of many cells, there is usually 

 specialization — some cells perform one function, some another. The 

 organism becomes a society or community of cells in which each 

 cell has a trade of its own. So we must now consider the life of the 

 cell as a member of a community. 



The first thing to note is that most of the organisms we are familar 

 with contain very large numbers of cells. Even a small fly contains 

 many millions of cells of many kinds. So however specialization 

 started, it has obviously gone very far and we are not likely to learn 

 much about how it came about from highly developed plants and 

 animals. 



The animal organism is particularly complex. There are cells set 

 aside for the manufacture of special enzymes. In a mammal, the pan- 

 creas produces the digestive enzymes required in the intestine. The 

 liver accumulates starch and also breaks it down to sugar. It also 

 manufactures the main proteins of the blood serum. The red blood 

 corpsucles are highly specialized structures for carrying oxygen and 

 for regulating the carbon dioxide content of the blood. 



So we have to ask how all these highly specialized cells come into 

 existence. It is obvious that different forms of life have very different 

 abilities to reproduce specialized structures. As a rule a plant is able 

 to reproduce the whole plant from a small part of it. If you break off 

 part of the stem of a growing plant and put it in the ground, it will 

 often develop into a complete plant with roots and the ability to 

 form flowers and seeds. The whole of the hereditary character of the 



