28 - The Cell 



Cytoplasmic Organelles. The organelles 

 are structurally specialized parts of the cyto- 

 plasm in which certain of the activities of the 

 cell are focalized. The mitochondria and 

 lysosomes are stable structures, probably pres- 

 ent in all cells; whereas spindles, asters, and, 

 perhaps, centrioles appear only transiently, 

 during cell division. Vacuoles, plastids, and 

 myofibrils, on the other hand, are character- 

 istic features found only in particular kinds 

 of cells. All of these structures, except the 

 lysosomes, can be seen under the light micro- 

 scope, but it is necessary to use (lie electron 

 microscope to resolve the details of their 

 morphology. 



Mitochondria. Usually several hundred of 

 these sausage-shaped structures are in the cyto- 

 plasm of animal and plant cells generally. 

 Prior to the development of electron micros- 

 copy, the mitochondria were differentiated 

 from other small cytoplasmic bodies by virtue 

 of their susceptibility to staining with a par- 

 ticular dye, janus green. Now, however, they 

 are recognized on the basis of their uniquely 

 characteristic ultrastructure (Fig. 2-12). 



As may be seen in the electronmicrograph 

 of Figure 2-12, or perhaps more clearly in 

 the diagram of Figure 2-13, each mitochon- 

 drion is a vesicle bounded by a double mem- 

 brane. The inner membrane displays numer- 

 ous complexly arranged infoldings called 

 cristae. As will be explained more fully later 

 (Chap. 8), the mitochondria are very impor- 

 tant organelles, especially with reference to 

 the development of useful energy within the 

 cell. The extensive, complexly organized 

 membrane system of each mitochondrion 

 provides for the patterned positioning of a 

 whole group of oxidizing enzymes. These 

 enzymes, working together as a team, maneu- 

 ver a stepwise series of energy-yielding trans- 

 actions (pp. 147 and 148). 



Lysosomes. Definitive recognition of these 

 somewhat smaller, usually round cytoplasmic 

 vesicles, which are found in the cytoplasm of 

 many, if not all, cells, came after the develop- 

 ment of the electron microscope. Each ly- 

 sosome (Fig. 2-13), unlike a mitochondrion, is 



bounded by a single membrane, and no 

 cristae are present. At present, the evidence 

 indicates that lysosomes are centers in which 

 certain enzymes, especially hydrolytic en- 

 zymes (p. 1 02), are localized. Such enzymes may 

 be useful to the cell, especially if reconstruc- 

 tion becomes necessary. The hydrolytic en- 

 zymes exert a disruptive action (p. 84) upon 

 proteins and other large organic molecules. 

 Presumably, therefore, they must be segre- 

 gated and kept out of contact with other 

 structures in the cytoplasm, except on spe- 

 cial occasion. 



Plastids. Typically, plastids are pigmented 

 bodies of fixed and definite form. The most 

 important and widely distributed kinds are 

 the chloroplasts (Fig. 2-14), found in the cells 

 of the green parts of all typical plants. The 

 chloroplasts, with their content of the green 

 pigment chlorophyll, are of tremendous im- 

 portance in the economy of life. They enable 

 plants to carry on photosynthesis (Chap. 9). 

 By this process, light energy is trapped and 

 utilized (indirectly) for the synthesis of glu- 

 cose, and other organic compounds, from 

 carbon dioxide and water (p. 161). This pri- 

 mary type of organic synthesis stores enor- 

 mous amounts of energy. Eventually this 

 energy is utilized, not only by the plants, but 

 also by all other forms of life. Moreover, 

 photosynthesis regenerates free oxvgen, and 

 thus it prevents a depletion of this important 

 component of our atmosphere (Chap. 9). 



Photosvnthesis is not a simple process. 

 Rather it is a complex series of interlinked 

 reactions about which more and more is 

 being discovered. Accordingly, it is not sur- 

 prising to find that chloroplasts possess a 

 complex internal ultrastructure, as may be 

 seen in Figure 9.2. Inside each chloroplast 

 are many smaller bodies, the grana (Fig. 

 9-3), each granum consisting of a svstem of 

 complexly folded membranes (Fig. 9-3). Just 

 how this highly organized system functions 

 in coordinating and achieving the processes 

 of photosynthesis is not entirely known, but 

 some aspects of the problem will be consid- 

 ered in Chapter 9. 



