Movement of Molecules Across Cell Membranes
117
V. The osmolarity of the extracellular fl uid is about 300 mOsm.
Because water comes to diffusion equilibrium across cell
membranes, the intracellular fl uid has an osmolarity equal to
that of the extracellular fl
uid.
a. Na
+
and Cl
ions are the major effectively nonpenetrating
solutes in the extracellular fl
uid; K
+
ions and various organic
solutes are the major effectively nonpenetrating solutes in
the intracellular fl
uid.
b. Table 4–3 lists the terms used to describe the osmolarity
and tonicity of solutions containing different compositions
of penetrating and nonpenetrating solutes.
Endocytosis and Exocytosis
I. During endocytosis, regions of the plasma membrane
invaginate and pinch off to form vesicles that enclose a small
volume of extracellular material.
a. The three classes of endocytosis are (1) fl uid endocytosis,
(2) phagocytosis, and (3) receptor-mediated endocytosis.
b. Most endocytotic vesicles fuse with endosomes, which in
turn transfer the vesicle contents to lysosomes for digestion
by lysosomal enzymes.
c. Potocytosis is a special type of receptor-mediated
endocytosis in which vesicles called caveolae deliver their
contents directly to the cytosol.
II. Exocytosis, which occurs when intracellular vesicles fuse with
the plasma membrane, provides a means of adding components
to the plasma membrane and a route by which membrane-
impermeable molecules, such as proteins the cell synthesizes,
can be released into the extracellular fl
uid.
Epithelial Transport
I. Molecules can cross an epithelial layer of cells by two pathways:
(1) through the extracellular spaces between the cells—the
paracellular pathway, and (2) through the cell, across both
the luminal and basolateral membranes as well as the cell’s
cytoplasm—the transcellular pathway.
II. In epithelial cells, the permeability and transport
characteristics of the luminal and basolateral plasma
membranes differ, resulting in the ability of cells to actively
transport a substance between the fl uid on one side of the cell
and the fl uid on the opposite side.
III. The active transport of sodium through an epithelium
increases the osmolarity on one side of the cell and decreases
it on the other, causing water to move by osmosis in the same
direction as the transported sodium.
KEY TERMS
active transport
104
aquaporin
108
basolateral membrane
114
caveolus
114
channel
100
channel gating
100
clathrin
113
clathrin-coated pit
113
cotransport
107
countertransport
107
diffusion
97
diffusion equilibrium
97
electrochemical gradient
100
endocytosis
112
exocytosis
112
facilitated diffusion
103
fl uid endocytosis
112
ux
97
hyperosmotic
111
hypertonic
111
hypoosmotic
111
hypotonic
111
isoosmotic
111
isotonic
111
ligand-gated channel
100
luminal membrane
114
mechanically-gated channel
100
mediated transport
102
membrane potential
100
net fl
ux
97
nonpenetrating solute
110
osmol
109
osmolarity
109
osmosis
108
osmotic pressure
110
paracellular pathway
114
permeability coeffi cient,
P
99
phagocytosis
112
phagosome
112
pinocytosis
112
potocytosis
114
primary active transport
104
receptor
112
receptor-mediated
endocytosis
112
secondary active transport
104
semipermeable membrane
110
transcellular pathway
114
transporter
102
voltage-gated channel
100
CLINICAL TERMS
diabetes mellitus
104
REVIEW QUESTIONS
1. What determines the direction in which net diffusion of a
nonpolar molecule will occur?
2. In what ways can the net solute fl ux between two compartments
separated by a permeable membrane be increased?
3. Why are membranes more permeable to nonpolar molecules
than to most polar and ionized molecules?
4. Ions diffuse across cell membranes by what pathway?
5. When considering the diffusion of ions across a membrane,
what driving force, in addition to the ion concentration
gradient, must be considered?
6. Describe the mechanism by which a transporter of a mediated-
transport system moves a solute from one side of a membrane
to the other.
7. What determines the magnitude of fl ux across a membrane in a
mediated-transport system?
8. What characteristics distinguish diffusion from facilitated
diffusion?
9. What characteristics distinguish facilitated diffusion from
active transport?
10. Describe the direction in which sodium ions and a solute
transported by secondary active transport move during
cotransport and countertransport.
11. How can the concentration of water in a solution be decreased?
12. If two solutions with different osmolarities are separated by
a water-permeable membrane, why will a change occur in
the volumes of the two compartments if the membrane is
impermeable to the solutes, but no change in volume will
occur if the membrane is permeable to solute?
13. Why do sodium and chloride ions in the extracellular fl uid and
potassium ions in the intracellular fl uid behave as though they
were nonpenetrating solutes?
14. What is the approximate osmolarity of the extracellular fl
uid?
Of the intracellular fl
uid?
15. What change in cell volume will occur when a cell is placed in a
hypotonic solution? In a hypertonic solution?
16. Under what conditions will a hyperosmotic solution be isotonic?
17. How do the mechanisms for actively transporting glucose and
sodium across an epithelium differ?
18. By what mechanism does the active transport of sodium lead
to the osmotic fl ow of water across an epithelium?
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