STRUCTURE AND ACTIVITIES OF CELLS 



the plasma membrane (Fig. 2.6.4 and E). This is the semipermeable barrier 

 through which everything entering or leaving the cell must pass. Elucidation 

 of its structure has defied the highest magnification of the electron microscope. 

 If photographs of isolated cells in tissue culture are taken at frequent intervals 

 through a light microscope, a remarkable phenomenon called pinocytosis is 

 revealed. This involves the "gulping," as it were, of fluid outside the cell 

 through what are apparently small temporary breaks in the plasma membrane. 

 In electron micrographs of sections of cells, minute vesicles are seen to be 

 continuous through the plasma membrane with the intercellular space 

 (Fig. 2.6E). These may indicate that pinocytosis occurs on a very small 

 scale when cells are in their normal positions, but this is not an established 

 fact. If the plasma membrane is grossly torn or otherwise damaged during 

 manipulation, the contents of a cell escape and become disorganized; the cell 

 dies. If the break is very limited, the plasma membrane may be reformed 

 after minimal loss has taken place and the cell remains alive. 



In cells that occur singly, such as the unicellular animals and the egg cells 

 of some marine animals, there is a narrow zone of relatively firm or gel-like 

 cytoplasm immediately internal to the plasma membrane (p. 230). This 

 ectoplasm is relatively dense to electron penetration and contains many very 

 fine fibers as seen in electron micrographs (Fig. 2.6E). An ectoplasmic zone is 

 found at the exposed surfaces of cells lining many tubes, such as the digestive 

 tract, in many-celled animals. However, ectoplasm cannot be distinguished 

 at the periphery of cells that are in close association with one another. 

 Instead, the cytosome is crowded with many different structures distributed in 

 a relatively fluid ground substance or endoplasm, such as is found beneath 

 the ectoplasm of free cells. 



Under high magnification with the light microscope, the endoplasm appears 

 to contain delicate fibers which many have thought formed a net or reticulum. 

 In electron micrographs this endoplasmic reticulum is seen to be a system of canals, 

 sharply limited by a continuous membrane (Fig. 2.6B). These channels are 

 very narrow in places but widen in varying degree as they course throughout 

 the endoplasm and communicate with the nuclear envelope in a manner to be 

 described presently. The contents of the canals are of variable but low density 

 and are unidentified chemically. With the magnification and resolution of the 

 electron microscope, the endoplasm is found to contain great numbers of a fine 

 fibrillar component (Fig. 2.6B). These seem to be like the very fine fibers 

 present in the ectoplasm and are undoubtedly responsible for the viscosity of 

 the cytoplasm. 



Embedded in the endoplasm in the interstices of the reticulum are the rod- 

 shaped mitochondria, found in varying numbers in all types of cells (Fig. 2.6A). 

 In photographs of isolated cells in tissue cultures taken at frequent intervals 

 through a light microscope, the mitochondria are seen to be in a constant state 

 of flux — bending, fragmenting, uniting, and being carried from place to place 

 by streaming of the endoplasm (Fig. 2.1 B). The electron microscope reveals 

 that each mitochondrion is bounded by a continuous envelope consisting of two 



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