Exercise 



LIVING CELLS (2) 9 



Most amebas are free-living, but one notori- 

 ous human parasite in this group is Endameba 

 histolytica, which causes amebic dysentery. It 

 is easy to think of such Khizopods as the ameba 

 as primitive and as ancestral to other types of 

 protozoa; but an interesting argument considers 

 the flagellates to be the most primitive protists, 

 and derives the ameba from them (cf. Weisz, 

 pp. 664-667). 



Reactions of Paramecium to its environment 



To another drop of Paramecium culture on a 

 fresh slide, add a small drop of a suspension of 

 powdered carmine before adding the methyl 

 cellulose. Observe how the granules of carmine 

 accumulate in a food vacuole at the end of the 

 gullet, which is pinched off and then pursues a 

 definite course around the cell. Follow one 

 such vacuole through its circuit. Unlike the 

 ameba, Paramecium has a special area which 

 serves for the egestion of solid wastes. This 

 anal spot is on the surface, about level with the 

 posterior end of the gullet. It can be detected 

 only during the act of egestion. You may be 

 able to see the elimination of carmine particles 

 through the anal spot. {Note: If the methyl 

 cellulose does not slow the organism sufficiently 

 for these observations, use in addition or instead 

 a bit of lens paper laid in your preparation.) 



Paramecia propel barbed, harpoonlike tricho- 

 cysts when disturbed. Place a large drop of 

 Paramecium culture on a slide, and put a very 

 small drop of ink next to it but not touching it. 

 Now bring the two drops into contact, put a 

 cover slip over them, and quickly examine 

 under low power. Note what happens when a 

 Paramecium swims into a blue zone. 



In a fresh preparation made up with lens 

 paper and not containing methyl cellulose, note 

 how Paramecium reacts to obstacles. Do you 

 see what is meant by its "trial-and-error" be- 

 havior? Can a Paramecium back up? How do 

 you suppose it does so? How does it synchronize 

 and integrate the beating of its cilia? Has it a 

 nervous system? Of what could a subcellular 

 nervous system be composed ? 



Other protozoa and algae 



Your instructor will also have available some 

 pond water containing protozoa and algae other 

 than those already studied. We are not inter- 

 ested in identifying them except in the roughest 

 way; but they are interesting to find and watch, 

 and to assign to the major groups. How many 

 different kinds do you see? Are organisms pres- 

 ent that are not unicellular? Of what types? 

 (Your instructor and reference books will help 

 you answer these questions.) 



You have now seen a wide range of living 

 cells, from the comparatively simple ones to the 

 exceedingly complex. With simplicity and com- 

 plexity we often associate such terms as "primi- 

 tive" and "advanced," or "lower" and "higher." 

 Would you say that Paramecium is a "lower" 

 organism? Does it seem "primitive" to you? 

 Does it seem more "primitive" perhaps than an 

 onion or a man, of which you have seen epider- 

 mal cells? Which is "higher" or more "ad- 

 vanced," Euglena or the ameba which may be 

 derived from similar flagellates? 



MODELS OF LIFE 



A discouraging thing about defining life is 

 that once one has made a definition, it is easy 

 to construct a model that satisfies the definition, 

 yet clearly is not alive. Such models are them- 

 selves instructive, because they sometimes pre- 

 sent much simpler systems that display proper- 

 ties exhibited by living organisms in ways that 

 permit closer analysis, and suggest physical and 

 chemical bases for these phenomena in the living 

 organisms themselves. Such an application of 

 models can be misleading; one needs to judge 

 carefully how far to pursue a model, and when 

 to leave it. 



In any case, the model we are about to 

 examine should be thought of in two ways : as 

 a demonstration that some phenomena of living 

 organisms are easily reproduced in inorganic 



