96
UNIT ONE
take in water and the particles dissolved in it, such as pro-
teins, that otherwise might be too large to enter.
Phagocytosis
(fag
o-si-to
sis) is similar to pinocytosis,
but the cell takes in solids rather than liquid. Certain types of
cells, including some white blood cells, are called
phagocytes
because they can take in solid particles such as bacteria and
cellular debris. When a phagocyte F rst encounters such a par-
ticle, the particle attaches to the cell membrane. This stimu-
lates a portion of the membrane to project outward, surround
Active transport is similar to facilitated diffusion in that it
uses carrier molecules within cell membranes. As
f gure 3.28
shows, these carrier molecules are proteins that have bind-
ing sites that combine with the specific particles being
transported. Such a union triggers release of cellular energy,
and this energy alters the shape of the carrier protein. As a
result, the “passenger” molecules move through the mem-
brane. Once on the other side, the transported particles are
released, and the carrier molecules can accept other pas-
senger molecules at their binding sites. They transport sub-
stances from regions of lower concentration to regions of
higher concentration, so these carrier proteins are some-
times called “pumps.” A sodium/potassium pump, for
example, transports sodium ions out of cells and potassium
ions into cells.
Particles moved across cell membranes by active trans-
port include sugars, amino acids, and sodium, potassium,
calcium, and hydrogen ions. Some of these substances are
actively transported into cells, and others are actively trans-
ported out. Movements of this type are important to cell
survival, particularly maintenance of homeostasis. Some of
these movements are described in subsequent chapters as
they apply to speciF
c organ systems.
Endocytosis
Cellular energy is used to move substances into or out of a cell
without actually crossing the cell membrane. In
endocytosis
(en
do-si-to
sis), molecules or other particles that are too large
to enter a cell by diffusion or active transport are conveyed in
a vesicle that forms from a section of the cell membrane.
The three forms of endocytosis are pinocytosis, phago-
cytosis, and receptor-mediated endocytosis. In
pinocytosis
(pi
-no-si-to
sis), cells take in tiny droplets of liquid from
their surroundings
(f
g. 3.29)
. When this happens, a small
portion of cell membrane indents (invaginates). The open
end of the tubelike part thus formed seals off and produces
a small vesicle about 0.1 µm in diameter. This tiny sac
detaches from the surface and moves into the cytoplasm. ±or
a time, the vesicular membrane, part of the cell membrane,
separates its contents from the rest of the cell; however, the
membrane eventually breaks down, and the liquid inside
becomes part of the cytoplasm. In this way, a cell is able to
Carrier protein
with altered shape
Carrier protein
Binding site
Region of higher
concentration
Region of lower
concentration
Phospholipid
molecules
(a)
(b)
ell membran
e
Transported
particle
Cellular
energy
FIGURE 3.28
Active transport moves molecules against their
concentration gradient.
(
a
) During active transport, a molecule or an ion
combines with a carrier protein, whose shape changes as a result. (
b
) This
process, which requires cellular energy, transports the particle across the
cell membrane.
Cell
membrane
Fluid
Fluid-filled
vesicle
Cytoplasm
Nucleolus
Nucleus
FIGURE 3.29
A cell may take
in a tiny droplet of F
uid from its
surroundings by pinocytosis.
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