Cells and Their Activities 269 



of various substances. Apparently they have some relationship to se- 

 cretion of materials inasmuch as they are more abundant in glandular 

 than in other cells. 



Another structure found in the cytoplasm is the Golgi apparatus. 

 Very often, this is located close to the nucleus ; its form varies consid- 

 erably from one cell to another and even within a single cell. Usually, 

 however, it is in the form of a dense network. Again, it is difficult to 

 assign any exact function to this Golgi apparatus. From indirect evi- 

 dence, it appears that it plays a part in the secretion of cells and may 

 act as a condensation surface for particular substances. 



Located near the nucleus is the small centrosome which is formed 

 of two parts : the deeply staining granule or granules, the centriole, 

 together with the surrounding denser mass, the centrosphere. The 

 centrosome is present in all animal cells where it plays an important 

 role in cell division. 



The outer limits of the protoplast are marked by an exceedingly 

 thin membrane, the plasma membrane. Usually this membrane is in- 

 visible, but its presence can be easily demonstrated. It is possibly 

 formed of a lipid layer to which are adhered a layer of protein mol- 

 ecules. The dynamic balance of the cell with its immediate environ- 

 ment is mediated by this membrane. By its ability to regulate the move- 

 ments of materials into and out of the cell, the plasma membrane plays 

 an absolutely essential role in the life of the cell. Thus the plasma 

 membrane is a true semipermeable membrane which permits certain 

 molecules to pass through while at the same time excluding others. 



Outside the plasma membrane, a heavier structure, the cell wall or 

 membrane, is often present. In plants this is a very conspicuous cellulose 

 structure, but in animals, it is often very thin and inconspicuous. Unlike 

 the plasma membrane, this wall is visible with an ordinary microscope. 



The movement of materials in and out of the cell as regulated by 

 the plasma membrane is subject to the laws governing osmosis and 

 diffusion. To a great extent this cell permeability is further influenced 

 by certain ions, such as those of calcium, sodium, and potassium. The 

 metabolism of these ions is in turn controlled by the endocrines — again 

 demonstrating the extreme complexity of the interrelationships of the 

 functioning organism. 



