22 THE CELL AS A FUNDAMENTAL UNIT OF LIFE 



catabolic phase of metabolism. Certain hormones can be used to speed 

 the rate of metabolism when it is too low. 



The cell is the unit in which metabolism occurs. Whether plant 

 or animal, it is in the cell that food is altered chemically by the enzymes 

 of the cell with the resulting formation of living protoplasm. These 

 enzymes function as catalysts, which means that they stimulate a 

 reaction to take place without being used up in the reaction them- 

 selves. When we study the many chemical changes that take place 

 within a cell, we can but marvel at the many enzymes which must be 

 present to accomplish all these changes. All cells require a certain 

 amount of energy release in order to continue living, so catabolism must 

 take place to a certain degree at all times. In dormant seeds, or in 

 animals which are in a dormant state such as hibernation, the rate of 

 catabolism may be reduced to a very low level. If the spark of life 

 is to remain, however, it must take place. Again, it is the enzymes 

 within the cell which cause the breakdown of the substances that have 

 been built up by anabolism and the ultimate release of energy from 

 these substances through a combination with oxygen. 



Metabolism is the force of life itself. 



Two Cells from One 



Of all the fascinating properties of life there is none more re- 

 markable than the ability of living matter to generate additional living 

 matter like itself. To illustrate, let us consider an amoeba crawling 

 along in the debris at the bottom of a pond of water. The amoeba is 

 a microscopic water animal whose entire body consists of only one 

 cell. This one cell carries on all of the processes necessary for the 

 continuation of life. It takes in food and oxygen, throws off waste 

 matter, and responds to its environment, as is true of larger forms 

 of animal life with many cells in their bodies. Some of the food taken 

 into the body, however, is converted into additional protoplasm within 

 the cell. Thus, the amoeba grows in size — it becomes larger, but such 

 enlargement cannot continue indefinitely. Eventually it would become 

 so large that there could not be sufficient exchanges of material through 

 its plasma membrane to maintain life and it would die as a result. 

 Something happens, however, to prevent this tragedy. 



The amoeba ceases its normal movements and certain internal 

 changes become apparent. Small, rod-like bodies (chromosomes) ap- 

 pear in the nucleus — these line up in a row and separate into two 

 groups, each of which forms the center for a new nucleus. Then a 

 constriction develops which separates the cell into two equal parts each 



