Movement of Molecules Across Cell Membranes
119
7. What will happen to cell volume if a cell is placed in each of the
following solutions?
Concentration, mM
NaCl
Urea
Solution
(nonpenetrating)
(penetrating)
A
150
100
B
100
150
C
200
100
D
100
50
8. Characterize each of the solutions in question 7 as isotonic,
hypotonic, hypertonic, isoosmotic, hypoosmotic, or
hyperosmotic.
9. By what mechanism might an increase in intracellular sodium
concentration lead to an increase in exocytosis?
Chapter 4 Answers to Physiological Inquires
Figure 4.2
As shown in the accompanying graph, there would be
a net fl ux of glucose from compartment 1 to compartment 2,
with diffusion equilibrium occurring at 12.5 mmol/L.
Figure 4.19
Because it is a nonpenetrating solute, infusion
of isotonic NaCl restores blood volume without causing a
redistribution of water between body fl uid compartments due
to osmosis. An isoosmotic solution of a penetrating solute,
however, would only partially restore blood volume because
some water would enter the intracellular fl uid by osmosis as the
solute enters cells. This could also result in damage to cells as
their volume expands beyond normal.
Figure 4.22
Active transport of sodium across the basolateral
(blood side) membrane would decrease, resulting in an increased
intracellular concentration of Na
+
. This would reduce the rate
of Na
+
diffusion into the cell through the Na
+
channel on the
lumen side because the diffusion gradient would be smaller.
12.5 mmol/L
20
10
0
AB
C
Compartment 1
Compartment 2
Glucose concentration (mmol/l)
Time
12.5
15
Glucose added to
Compartment 1
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