THE CELL AS A FUNDAMENTAL UNIT OF LIFE 



19 



(inside your mouth). This will loosen some of the cells of this region 

 which can then be mounted in a little water and placed under the 

 microscope. When viewing such cells you may get the impression that 

 a cell is a very simple thing. There will be a clearly defined outer 

 surface which is known as the cell membrane, inside there will be 

 a spherical nucleus, and in between there will be a clear material called 

 the cytoplasm. The cell has many other parts, however, which this 

 study does not reveal. To see these it is necessary to use some method 

 that will give us a greater contrast between the various components 

 of the cell. We can achieve such contrast in living cells by using a 



Golqi bodies 

 Centrosome 

 Nucleus 

 Chromatin 



Nucleolus 



Nuclear membrane 



Cytoplasm 



Vacuole 



Mitochondria 



Plasma membrane 

 Cell membrane 



Fig. 2.1. A typical animal cell. Diagram of the parts of a typical animal cell that 

 can be brought out by various microscopic techniques. 



phase contrast microscope which transforms slight differences in the 

 refractive index of the parts of the cell into visible differences in con- 

 trast. For a still more detailed study we can apply special stains to 

 the cells. Cell components differ in chemical structure and hence differ 

 in their reaction to various stains. Some parts absorb one stain readily, 

 but repel other stains. By using several stains it is possible to get a 

 varicolored image which brings out fine details of cell structure. We 

 will describe some of the parts of the cell which can be seen by such 

 techniques. 



The outer surface of the living part of the cell is known as the 

 plasma membrane. This very thin membrane, which lies just under 

 the cell membrane, serves to hold the contents of the cell and to 



