30 - The Cell 



Cell Sap Vacuoles. Cell sap vacuoles are 

 characteristic of large, mature plant cells 

 (Figs. 2-9 and 14). Each is filled with cell sap, 

 an aqueous solution of mineral sails. Fre- 

 quently the large cell sap vacuole occupies 

 all of the central part of the plant cell, so 

 that the cytoplasm is limited to a relatively 

 thin layer, between the vacuole and the cell 

 wall (Fig. 2-14). Usually the nucleus lies in 

 this thin, superficial layer of cytoplasm, al- 

 though sometimes it is found at the center 

 of the vacuole, surrounded by a small amount 

 of cytoplasm and suspended by strands of 

 cytoplasm that stretch across the vacuolar 

 space (Fig. 2-16). Cell sap vacuoles play an 

 important role in controlling the water bal- 

 ance in plant cells and tissues generally 

 (Chap. 6). 



CELL WALL 



. STARCH 



'sax /<#-? 



Fig. 2-16. One cell of Spirogyra, a colonial green 

 alga. 



Phagocytic and Pinocytic Vacuoles. Many cells 

 are capable of engulfing materials from the 

 fluids that surround them. Such active engulf- 

 ment always involves an inpocketing of the 

 surface membrane, followed by a pinching 

 off into the surrounding cytoplasm of one or 

 more vacuoles, of varying size, containing 

 the engulfed material. If the engulfed mate- 

 rial includes, in addition to fluid from the 

 outside medium, microscopically visible solid 

 material, such as bacteria or bits of organic 

 matter, the process is called phagocytosis 

 (literally "eating by the cell"). Such vacuoles 

 are referred to as phagocytic vacuoles, or 

 more simply as food vacuoles (Fig. 2-15). If 



the entrapped fluid does not include any 

 visible solid material, one speaks of pino- 

 cytosis (literally, "drinking by the cell") and 

 the vacuoles are referred to as pinocytic 

 vacuoles (Fig. 6-9). 



Phagocytosis has been recognized as an 

 important biological phenomenon since the 

 latter part of the nineteenth century, when 

 Metchnikoff, working at the Pasteur Institute 

 in Paris, first observed the engulfment ol 

 bacteria by white blood-cells (Fig. 17-4). The 

 formation of food vacuoles, indeed, repre- 

 sents the standard method by which amoebae 

 (Figs. 7-1 and 7-2) and other one-celled ani- 

 mals ingest their food. In the food vacuoles, 

 organic food substances are gradually di- 

 gested, in preparation for absorption, from 

 the vacuole cavity into the surrounding cyto- 

 plasm. 



Pinocytosis, on the other hand, was not 

 precisely described and named until 1931, 

 when Warren H. Lewis of Johns Hopkins 

 University recorded the process in tissue cul- 

 ture cells. It is difficult to observe pinocytosis 

 because the tubular inpocketings (Fig. 6-9) 

 are very delicate and the multiple pinocytic 

 vacuoles, which pinch oil internally, are very 

 small. In fact, some cells develop channels 

 and vacuoles that are of ultramicroscopic 

 dimensions and cannot be demonstrated 

 without the electron microscope. Probably 

 pinocytosis represents an important mechan- 

 ism by which cells take in substances of great 

 molecular size, as will be discussed later 

 (Chap. 6). 



Fibrillar Structures. There are a variety of 

 cytoplasmic structures that exhibit a pattern 

 of precisely oriented threads, or fibrils, either 

 microscopic or ultramicroscopic in dimen- 

 sion. These include myofibrils (p. 286), con- 

 tractile elements demonstrable in all types 

 of muscle tissue; neurofibrils (p. 195), conduc- 

 tive elements found in Paramecium and cer- 

 tain other unicellular animals; and ciliary 

 fibrils (p. 197), a precisely arranged group of 

 ultrafine, presumably contractile, threads 

 demonstrable by electron microscopy in all 

 cilia (p. 195) and flagella (p. 195). In addition 



