Interactions Between Cells and the Extracellular Environment 161- What are the factors that influence the rate of diffusion across
a plasma membrane? What structural features are often seen
in epithelial membranes specialized for rapid diffusion? - Describe the cause-and-effect sequence whereby a genetic
defect results in improper cellular transport and the
symptoms of cystic fibrosis. - Using the principles of osmosis, explain why movement of
Na^1 through a plasma membrane is followed by movement
of water. Use this concept to explain the rationale on which
oral rehydration therapy is based. - Distinguish between primary active transport and
secondary active transport, and between cotransport and
countertransport. Give examples of each. - Describe the different types of regulatory molecules found in
the body. What are the target cells for each type of regulatory
molecule? - How do nonpolar and polar regulatory molecules differ in
terms of the location of their receptor proteins in the target
cells and the mechanism of their actions? - What are G-protein-coupled receptors? Explain their
function in regard to how particular regulatory molecules
influence different effector proteins in the membrane.
Test Your Analytical Ability
- Mannitol is a sugar that does not pass through the walls of
blood capillaries in the brain (does not cross the “blood-
brain barrier,” as described in chapter 7). It also does not
cross the walls of kidney tubules, the structures that transport
blood filtrate to become urine (see chapter 17). Explain why
mannitol can be described as osmotically active. How might
its clinical administration help to prevent swelling of the
brain in head trauma? Also, explain the effect it might have
on the water content of urine. - Discuss carrier-mediated transport. How could you
experimentally distinguish between the different types of
carrier-mediated transport? - Remembering the effect of cyanide (described in chapter 5),
explain how you might determine the extent to which the
Na^1 /K^1 pumps contribute to the resting membrane potential.
Using a measurement of the resting membrane potential as
your guide, how could you experimentally determine the
relative permeability of the plasma membrane to Na^1 and K^1?
31. Using only the information in this chapter, explain how
insulin (a polar polypeptide hormone) causes increased
transport of plasma glucose into muscle cells.
32. Using only the information in this chapter, explain how
antidiuretic hormone (ADH, also called vasopressin)—
a polar polypeptide hormone—can stimulate epithelial cells
in the kidneys to become more permeable to water.
33. Epinephrine increases the heart rate and causes the
bronchioles (airways) to dilate by using cyclic AMP as a
second messenger. Suppose a drug increased the cyclic AMP
in heart and bronchiolar smooth muscle cells; what effects
would the drug have? Could you give a person intravenous
cyclic AMP and duplicate the action of epinephrine?
Explain.
Test Your Quantitative Ability
Suppose a semipermeable membrane separates two solutions.
One solution has 0.72 g glucose to 1.0 L of water; the other
has 0.117 g NaCl to 1.0 L of water. Given that glucose has a
molecular weight of 180 and NaCl has a molecular weight of
58.5, perform the following calculations.
Calculate the molality and osmolality of each solution (see
fig. 6.10 ).
Given your answers, state whether osmosis will occur and
if so, in which direction (assuming that the membrane is
permeable to water but not to glucose or NaCl).
Use the Nernst equation and the ion concentration provided in
figure 6.26 to perform the following calculations.
Calculate the equilibrium potential for K^1 ( E (^) K ) if its
extracellular concentration rises from 5 mM to
10 mM. Comparing this to the normal E (^) K , is the change
a depolarization or hyperpolarization?
Using the chloride (Cl^2 ) concentrations provided, calculate
the equilibrium potential for Cl^2. Given your answer, should
Cl^2 enter or leave the cell if the plasma membrane suddenly
becomes permeable to it (given a membrane potential of
2 70 mV)?
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