PROTOPLASM 1 7 



it is a viscid, jellylike substance, slimy to the touch, which is colorless or 

 faintly yellow or pink. 



When seen under the light microscope, protoplasm may appear to 

 have granules or fibrils of denser material, droplets of fatty substances 

 or fluid-filled vacuoles, all suspended in the clear, continuous, semifluid 

 "ground substance." Protoplasm is a complex colloidal system (see p. 

 30), whose consistency varies from liquid (sol) to a firm jelly (gel). The 

 change from sol to gel is reversible and the consistency may vary from 

 moment to moment and from one part of the cell to another. Some of 

 the formed bodies within the protoplasm— mitochondria, microsomes 

 and Golgi apparatus— are specialized parts of the living substance; others 

 are nonliving accumulations of fat, protein, carbohydrate or pigments. 



Mitochondria. \V4ien animal cells are viewed through the electron 

 microscope (Fig. 2.2), the mitochondria are seen to be large, round, oval 

 or sausage-shaped structures with a double membrane separating the 

 mitochondrial substance from the surrounding ground substance. The 

 inner membrane is thrown into folds which extend deep into the center 

 of the mitochondrion. These membranes are about 50 Angstrom units 

 (A) thick, just about the thickness of a single layer of protein or of a 

 double layer of lipid. Mitochondria from all animals from protozoa 

 to man have the same basic structure. As we shall see in Chapter 4, there 

 is experimental evidence that the mitochondria are complicated enzyme 

 machines; it is probable that these folds within the mitochondria are the 

 sites of many of the enzymes which catalyze reactions by which the cell 

 obtains energy from foodstuff molecules. 



Microsomes. In addition to mitochondria, cells contain smaller 

 particles, not visible with the light microscope, known as microsomes. 

 The electron microscope reveals these to be thin membranes to which 

 are attached spherical particles (Fig. 2.2). There are many such particle- 

 covered membranes in each cell. When cells are cut in thin sections and 

 viewed in the electron microscope, these membranes, called endoplasmic 

 reticulum, appear as long thin strands, like strands of spaghetti. The 

 microsomes are, like mitochondria, organized masses of enzymes. The 

 enzymes of the microsomes are concerned with the synthesis of proteins 

 and of certain other complex molecules in the cell. 



Golgi Apparatus. The cytoplasm of most cells (mature sperm and 

 red blood cells are notable exceptions) contains another type of inclusion 

 known as the Golgi apparatus. These are visible in the light microscope 

 when the tissue section has been properly stained. They may appear as 

 granules, threads, rods or canals. Golgi bodies are stained by the dye 

 neutral red; mitochondria take up the dye Janus green. The Golgi 

 bodies appear to play a role in the production of cellular secretions. 



Much has been learned in recent years of the role each of these 

 particles plays in the economy of the cell. Cells are homogenized in spe- 

 cial glass grinding tubes to break the cell membrane and release the 

 intracellular structures. Then, by subjecting the homogenate to increas- 

 ing amounts of centrifugal force in an ultracentrifuge, first the nuclei, 

 then the mitochondria, and finally the microsomes can be sedimented 

 separately. W'hen these sedimented particles are examined in the electron 



