DISCUSSION QUESTIONS 131 



relatively low concentration in the medium in contact with tliat side of the 

 membrane. A seemingly more probable hypothesis is that the cargoes of ad- 

 sorbed, dissolved, or chemically bound molecules are not carried all the way 

 across the membrane by one micelle, but pass from particle to particle as im- 

 pacts occur between them. Each micelle would, according to this hypothesis, 

 carry a given molecule only for a very short portion of its journey across the 

 membrane. 



The Mechanism of Changes in Membrane Permeability. — The per- 

 meability of the cytoplasmic membranes fluctuates continuously as a normal 

 feature of cellular activity. Some changes in the permeability of these mem- 

 branes are induced by variations in factors of external origin as previously 

 discussed, but others are due to unrecognized factors in the "internal environ- 

 ment" of the membranes. Furthermore, the living membrane of a given cell 

 may at times be more permeable to some substances in certain regions than 

 in others, whereas a short time later the distribution of the relatively permeable 

 areas may be very different. 



Various suggestions regarding the mechanisms by which changes in the 

 permeability of the cytoplasmic membranes can take place have been proposed. 

 Among these are: (i) changes in the viscosity of the membrane, (2) reversal 

 of phase (on the assumption that the membrane has an emulsion-like struc- 

 ture), and (3) changes in the proportion of water in different phases of the 

 colloidal system which is supposed to constitute the membrane. Since all of 

 these hypotheses are highly speculative any detailed consideration of them in 

 an introductory discussion would be unjustified. 



Discussion Questions 



1. How would you demonstrate that it is the cytoplasm of a living palisade 



cell that acts as a differentially permeable membrane and not the cell wall? 



2. A decrease in temperature apparently decreases the permeability of the cyto- 



plasmic membranes to water. What are some possible explanations? 



3. If the tonoplast and plasmalemma act as differentially permeable membranes 



why does not the water in the vacuole move osmotically into the cytoplasm 

 and cause the cytoplasm to displace much of the vacuole? 



4. How can sudden changes be produced in the turgor of the root hairs of 



solution grown plants without injury to the cells? 



5. What effect would an increase in the permeability to water of the cytoplasmic 



membranes of an algal cell immersed in pure water have upon the turgidity 

 of the cell? 



6. When algal cells are immersed in a strong glucose solution and similar cells 



in an ethylene glycol solution of equal molarity they plasmolyze (Chap. XI) 

 rapidly, but recovery from plasmolysis occurs much more rapidlv in the 

 latter solution than in the former. What does this indicate regarding the 

 permeability of the cytoplasmic membranes to these two compounds? 



