chapter 7



Membrane Structure and Function



  • The plasma membrane separates the living cell from its nonliving surroundings.
  • This thin barrier, 8 nm thick, controls traffic into and out of the cell.
  • Like all biological membranes, the plasma membrane is selectively permeable, allowing some substances to cross more easily than others.




Five dialysis bags, constructed from a semipermeable membrane that is impermeable to sucrose, were filled with various concentrations of sucrose and then placed in separate beakers containing an initial concentration of 0.6 M sucrose solution. At 10-minute intervals, the bags were massed (weighed) and the percent change in mass of each bag was graphed.


Which line in the graph represents the bag that contained a solution isotonic to the 0.6 M solution at the beginning of the experiment?


A) A

B) B

C) C

D) D

E) E


Which line in the graph represents the bag with the highest initial concentration of sucrose?


A) A

B) B

C) C

D) D

E) E


Which line or lines in the graph represent(s) bags that contain a solution that is hypertonic at 50 minutes?


A) A and B

B) B

C) C

D) D

E) D and E