26 I The Process of Evolution 



of convolutions forming lamellae or villae. The mitochondria are the 

 site of most of the reactions involved in cellular respiration, includ- 

 ing the formation of adenosine triphosphate (ATP). There is some 

 evidence also that mitochondria may play a role in cytoplasmic in- 

 heritance in the sense that they may be self -replicating. 



Closely related to the mitochondria structurally, but found only 

 in plant cells, are the plastids. Like the mitochondria, they are the 

 site of important reactions providing energy for the cell (indeed for 

 nearly all life). Plastids have a lamellar structure, and upon the 

 alternating layers of lipid and protein molecules are found layers of 

 special pigments such as the various types of chlorophyll, carot- 

 enoids, and others ( depending upon the plant group studied ) . Light 

 energy absorbed by the plastid is converted to chemical energy; in 

 a series of steps, energy from oxidation is utilized to phosphorylate 

 ADP to ATP. This photophosphorylation obviously is related to the 

 phosphorylation carried out by the mitochondria. The functional 

 units of chloroplastids in higher plants are minute particles called 

 grana. In the green bacteria and blue-green algae the grana are not 

 organized into plastids. 



The cytoplasm of most animal and many plant cells has a struc- 

 ture, known as the centrosome, adjacent to the nucleus. Within this 

 relatively clear area of cytoplasm there are one or two granules, the 

 centrioles. These organelles are important in the origin and function 

 of flagella and cilia and in nuclear and cell division. The nine 

 strands making up the outer portion of a flagellum or cilium are 

 continuous with nine tubelike or filamentlike structural components 

 of the centriole or basal granule. This remarkable similarity of 

 centrioles, basal granules, and cilia (which is preserved even in such 

 highly specialized cells as retinal photoreceptors ) is found through- 

 out the animal kingdom. The centriole and obviously related struc- 

 tures (such as basal granules) apparently have properties that lead 

 to the organization of fibrous protein molecules in special ways, for 

 example, in the formation of the spindle tubules (fibers) during 

 mitosis. 



The genetic information in the cell is located mostly in the chro- 

 mosomes within the nucleus. The precise state of the material is not 

 known. In the so-called resting or metabolic stage, chromosome 

 material in the nucleus usually is difficult to view. In some instances, 

 portions of chromosomes that have not undergone the usual trans- 

 formations accompanying mitosis may be seen. Often one or more of 

 these chromosome regions are associated with the nucleolus, a 

 usually conspicuous feature of the metabolically active nucleus. 

 Presumably during the metabolic stage, material is exchanged be- 



