112
Chapter 4
As you will learn in Chapter 14, one of the major func-
tions of the kidneys is to regulate the excretion of water in the
urine so that the osmolarity of the extracellular fl uid remains
nearly constant in spite of variations in salt and water intake
and loss, thereby preventing damage to cells from excessive
swelling or shrinkage.
Table 4–3
provides a comparison of the various terms
used to describe the osmolarity and tonicity of solutions.
Endocytosis and Exocytosis
In addition to diffusion and mediated transport, there is
another pathway by which substances can enter or leave cells,
one that does not require the molecules to pass through the
structural matrix of the plasma membrane. When cells are
observed under a microscope, regions of the plasma mem-
brane can be seen to fold into the cell, forming small pock-
ets that pinch off to produce intracellular, membrane-bound
vesicles that enclose a small volume of extracellular fl
uid. This
process is known as
endocytosis
(
Figure 4–20
). A similar
process in the reverse direction,
exocytosis,
occurs when
membrane-bound ves
icles in the cytop
lasm fuse with the
plasma membrane and release their contents to the outside of
the cell (see Figure 4–20).
Endocytosis
There are three general types of endocytosis that may occur in
a cell. These are fl uid endocytosis, also known as
pinocytosis
(“cell drinking”), phagocytosis (“cell eating”), and receptor-
mediated endocytosis (
Figure 4–21
).
In
uid endocytosis,
an endocytotic vesicle encloses a
small volume of extracellular fl uid. This process is nonspecifi c
because the vesicle simply engulfs the water in the extracellu-
lar fl uid along with whatever solutes are present. These solutes
may include ions, nutrients, or any other small extracellular
molecule. Large macromolecules, other cells, and cell debris
do not normally enter a cell via this process.
In
phagocytosis,
cells engulf bacteria or large particles
such as cell debris from damaged tissues. In this form of
endocytosis, extensions of the plasma membrane called pseu-
dopodia fold around the surface of the particle, engulfi ng it
entirely. The pseudopodia, with their engulfed contents, then
fuse into large vesicles called
phagosomes
that are inter-
nalized into the cell. Phagosomes migrate to and fuse with
lysosomes in the cytoplasm, and the contents of the phago-
some are then destroyed by lysosomal enzymes and other
molecules. While most cells undergo pinocytosis, only a few
special types of cells, such as those of the immune system
(Chapter 18), carry out phagocytosis.
In contrast to fl uid endocytosis and phagocytosis, most
cells have the capacity to
specifi
cally
take up molecules that
are important for cellular function or structure. In
receptor-
mediated endocytosis,
certain molecules in the extracellu-
lar fl uid bind to specifi c proteins on the outer surface of the
plasma membrane. These proteins are called
receptors,
and
each one recognizes one ligand with high affi nity (see Chapter
3 for a discussion of ligand-protein interactions). In one form
of receptor-mediated endocytosis, the receptor undergoes
Figure 4–20
Endocytosis and exocytosis.
Table 4–3
Terms Referring to the Osmolarity and
Tonicity of Solutions*
Isotonic
A solution that does not cause a change in cell
volume; one that contains 300 mOsmol/L
of nonpenetrating solutes, regardless of the
concentration of membrane-penetrating
solutes present
Hypertonic
A solution that causes cells to shrink; one
that contains greater than 300 mOsmol/L
of nonpenetrating solutes, regardless of the
concentration of membrane-penetrating
solutes present
Hypotonic
A solution that causes cells to swell; one
that contains less than 300 mOsmol/L of
nonpenetrating solutes, regardless of the
concentration of membrane-penetrating
solutes present
Isoosmotic
A solution containing 300 mOsmol/L
of solute, regardless of its composition of
membrane-penetrating and nonpenetrating
solutes
Hyperosmotic
A solution containing greater than 300
mOsmol/L of solutes, regardless of its
composition of membrane-penetrating and
nonpenetrating solutes
Hypoosmotic
A solution containing less than 300
mOsmol/L of solutes, regardless of its
composition of membrane-penetrating and
nonpenetrating solutes
*These terms are defi ned using an intracellular osmolarity of 300 mOsm, which is within the range
for human cells but not an absolute fi xed number.
Plasma
membrane
Endocytosis
Exocytosis
Extracellular fluid
Intracellular
fluid
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