Movement of Molecules Across Cell Membranes
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
ions are the major effectively nonpenetrating
solutes in the extracellular ﬂ
ions and various organic
solutes are the major effectively nonpenetrating solutes in
the intracellular ﬂ
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
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.
ﬂ uid endocytosis
permeability coefﬁ cient,
primary active transport
secondary active transport
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
8. What characteristics distinguish diffusion from facilitated
9. What characteristics distinguish facilitated diffusion from
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
Of the intracellular ﬂ
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?