THE CELL AS A FUNDAMENTAL UNIT OF LIFE 21^ 



definitely known, but it is thought that they are associated with se- 

 cretions of the cells. Most plant cells, but few animal cells, contain 

 plastids which are bodies concerned with constructing substances within 

 the cell. Some of these plastids (chloroplasts) contain the vital chloro- 

 phyll which manufactures food ; others are centers which convert simple 

 sugar into starch. 



The nucleus of a cell usually appears as a large spherical body 

 separated from the cytoplasm by a nuclear membrane. Within the 

 nucleus there is an irregular network of thread-like bodies which is 

 commonly called the chromatin network. The individual thread-like 

 bodies are known as chromosomes — these become very distinct as they 

 become shorter and thicker during various phases of cell reproduction. 

 The chromosomes contain many very tiny bodies, known as genes, 

 which are the units of heredity. Much remains to be learned about 

 the method of gene action, but we have discovered that at least some 

 of the genes produce their results through enzymes which they pro- 

 duce or which they cause to be produced within the cell. There also 

 may be one or more nucleoli within the nucleus. These appear as 

 spherical bodies associated with the chromosomes. 



All of the living parts of the cell including nucleus and cytoplasm 

 come under the general heading of protoplasm. 



Cell Metabolism 



Metabolism is a word that refers to the sum of the various chemical 

 activities which provide for the growth, repair, and energy release of 

 protoplasm. It is commonly divided into two categories — anabolism 

 which involves the building of protoplasm from the raw materials com- 

 ing; into the cell, and catabolism in which there is a breakdown of the 

 materials within the cell with a release of energy. In a reducing diet 

 an effort is made to reduce the intake of food to a point where the 

 forces of catabolism will exceed those of anabolism, and some of the 

 stored food in the body will be used to supply the extra energy. Un- 

 fortunately, the balance between anabolism and catabolism varies in 

 different persons, and it is much more difficult to maintain a proper 

 body weight when either of these forces tends to overbalance the 

 other. Many of you have probably had a basal metabolism test made 

 to determine your rate of catabolism when you are in a state of relaxa- 

 tion. This is done by measuring the amount of oxygen which you 

 consume during a given period of time. Since catabolism uses oxygen 

 in the release of energy, this gives an indication of the rate of the 



