LIVING CELLS (I) 



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(Readings: Weisz, pp. 55-67. S.P.T., pp. 39-58 and 488^98. Villee, pp. 35-^2. 

 J. Brachet, "The Living Cell," Sci. Am. 205, No. 3, pp. 50-62, Sept. 1961, 

 Reprint No. 90.) 



The cell is the minimum organization that 

 displays the properties and processes that we 

 refer to collectively as "life." We know life only 

 in the form of living cells. They are called 

 "cells" because each is enclosed in a continuous 

 boundary, the cell membrane, and sometimes 

 also a cell wall. 



Some living organisms are composed of a 

 single cell; others are multicellular. A multi- 

 cellular organism may be composed of many 

 different types of cell, each type playing a dif- 

 ferent role. A group of similar cells, specialized 

 for a single type of function, is called a tissue. 

 Between the unicellular and the multitissued 

 organisms, we find a few multicellular forms 

 that, because all the cells are of much the same 

 type, we speak of as "colonial." 



Most living organisms can be characterized 

 clearly as plant or animal, though one tends 

 now to recognize a third great kingdom, that 

 of the protists, which includes all unicellular 

 and colonial forms. Typical plant cells are 

 characterized by a rigid cell wall, made largely 

 of cellulose, and may contain organs of photo- 

 synthesis, the chloroplasts. Both plant and ani- 

 mal organisms may be uni- or multicellular, or 

 colonial. Among the unicellular or colonial 



forms, in addition to those that are clearly 

 plant (algae) and clearly animal (protozoa), there 

 is an interesting group that does not fit easily 

 into either category — the green flagellates. 



We shall devote the first two laboratories to 

 examining a variety of living cells and what 

 they do. You will see that though they look 

 very different from one another, they share 

 many properties in common. Toward the end 

 of the second laboratory, we will try to mimic 

 some of their activities with simple inorganic 

 models. The cells are made of molecules, much 

 the same types of molecules in all living cells. 

 Immediately after this work with living cells, we 

 shall go to work with the molecules. It is a 

 striking fact that the chemistry of living organ- 

 isms varies much less than their anatomy. 



CELLS OF MULTITISSUED ORGANISMS 

 Onion epidermis 



Remove a fresh inner scale from an onion. 

 With a scalpel and forceps strip off a layer of 

 the epidermis from the inner side of the scale. 

 Mount a piece in tap water on a slide, and with 



