6 



Define a Cell. Write a short Account of its Structure and 

 Physiology. 



A cell may be defined as a unit mass of living matter or 

 protoplasm which has a more or less definite boundary 

 and which usually contains a nucleus. Both in size and in 

 shape cells vary greatly. 



Morphologically, the general substance of the cell is 

 called cytoplasm, and this term includes the living matter 

 or protoplasm and the non-living or metaplasm. Besides 

 the cytoplasm and the nucleus, there are other organised 

 structures in the cell. Such are the vacuoles, cavities of 



Chromatin 



Nucleolus 



Nucleus 



Cytoplasm 



Centrosome 



FIG. 1. Diagram of a Cell. After Camoy.) 



the protoplasm lined with a specialised portion of it, and 

 the minute centrosomes which play an important role in 

 cell- division. 



The centrosomes are usually situated close to the nucleus 

 and are often contained in a distinct, but transient part 

 of the cytoplasm known as the archoplasm. 



The nucleus is the governing centre ; it is the seat of 

 dynamic activity ; it controls the metabolism of the cell. 

 For example, it has been shown experimentally that nutri- 

 tion is impossible without it. " Life depends upon the 

 mutual reactions of nucleus and protoplasm." The nucleus 

 is essential also in cell-division (in the multiplication of 

 ordinary body cells), and in fertilisation (in reproduction 



