200 - The Cell 



of the vacuole is mainly water and is dis- 

 tinctly hypotonic to the protoplasm. As a 

 vacuole swells, therefore, water is forced to 

 flow "uphill" from a region of lesser to a 

 region of greater concentration. Accordingly, 

 the cell must expend energy in forcing water 

 to flow into the vacuole, and this energy must 

 be derived ultimately from metabolism. Thus 

 the contractile vacuole stops functioning 

 when a cell is deprived of oxygen, or when 

 its metabolism is depressed by narcotics; and 

 in such cases the cell swells gradually until 

 it reaches the bursting point. 



Responses of Multicellular Organisms. It 



should be emphasized that the complicated 

 behavior of higher organisms depends upon 

 responses occurring in the individual cells. 

 Receptivity, conductivity, and the capacity 

 to execute effective responses are general at- 

 tributes of all cells. However, considerable 

 specialization has occurred among the cells 

 of higher organisms, so that well-differen- 

 tiated tissues and organs serve as the re- 

 ceptors, conductors, and effectors in the 

 highly integrated responses of all higher 

 forms. 



TEST QUESTIONS 



9 



1. Define the term "stimulus." Differentiate be 

 tween internal and external stimuli, citing 

 an example of each. 



2. Differentiate between continuous and dis- 

 continuous responses, citing two examples in 10. 

 each case. 



3. Specify four criteria that distinguish the 

 state of excitation— that is, four differences 

 between an excited and an unexcited cell. 



4. Discuss the action potential in relation to: 11. 



(a) plant cells: (b) animal cells: (c) heart 

 muscle: (d) the polarity of the plasma mem- 

 brane. 



5. Define the terms: (a) resting potential and 12. 



(b) action potential. 



6. Explain how these potentials are related to 13. 

 (a) membrane permeability; (b) ion transport 

 mechanisms; and (c) ionic mobilities. 



7. Classify the various kinds of stimuli. 



8. Both the rate and the magnitude of a stimu- 14. 

 lating change are important in determining 



the effectiveness of a stimulus. Explain this 

 statement in terms of the human retina. 



Differentiate between receptors, conductors, 



and effectors, using (a) various unicellular 



organisms: and (b) man, to exemplify the 



discussion. 



Differentiate between: (a) cilia and flagella: 



(b) the effective and the recovery strokes; 



(c) ciliary fibrillae and myofibrillae; (d) the 

 plasmagel and the plasmasol; (e) amoeboid 

 movement and cyclosis. 



Both amoeboid movement and cleavage 

 probably depend upon the capacity of proto- 

 plasm to undergo gelation. Explain and dis- 

 cuss this statement. 



Secretion is an energy-expending process. 

 Explain and discuss this statement. 

 Receptivity, conductivity, and the capacity 

 to execute definite responses are general at- 

 tributes of living cells. Explain and discuss 

 this statement. 



What new evidence is available in regard to 

 the fine structure of cilia and flagella? Ex- 

 plain. 



FURTHER READINGS 



1. Unresting Cells, by Ralph W. Gerard; New 

 York, 1961. 



2. "How Cells Communicate." by Bernard Katz; 

 in Scientific American, September 1961. 



3. "How Cells Receive Stimuli," by W. H. Mil- 



ler, F. Ratliff, and H. K. Hartline; in Scien- 

 tific American, September 1961. 

 "Chemical Factors Controlling Nerve Activ- 

 ity," by David Nachmansohn; in Science, vol. 

 134, Dec. 15, 1961. 



