AUDEMC04_0131005065.QXP
12/8/03
5:32 PM
Page 67
How Do Substances Move Across Membranes?
67
(c) Phagocytosis
(a) Pinocytosis
(extracellular fluid)
(extracellular fluid)
food particle
pseudopods
1
3
2
1
vesicle containing
extracellular
fluid
(cytoplasm)
(cytoplasm)
1 A dimple forms in the plasma membrane, which 2 deepens
and surrounds the extracellular fluid. 3 The membrane
encloses the extracellular fluid, forming a vesicle.
2
food
vacuole
3
1 The plasma membrane extends pseudopods toward an
extracellular particle (for example, food). 2 The ends of
the pseudopods fuse, encircling the particle. 3 A vesicle called
a food vacuole is formed containing the engulfed particle.
(b) Receptor-mediated endocytosis
(extracellular fluid)
nutrients
receptors
1
coated pit
2
3
(cytoplasm)
4
coated vesicle
1 Receptor proteins for specific molecules or complexes of
molecules are localized at coated pit sites. 2 The receptors bind
the molecules and the membrane dimples inward. 3 The coated
pit region of the membrane encloses the receptor-bound
molecules. 4 A vesicle ("coated vesicle") containing the bound
molecules is released into the cytoplasm.
Phagocytosis Moves Large Particles into the Cell
Cells use phagocytosis (which means “cell eating”) to pick
up large particles, including whole microorganisms (see
Fig. 4-7c). When the freshwater protist Amoeba, for example, senses a tasty Paramecium, Amoeba extends parts of
its surface membrane. These membrane extensions are
(a)
extracellular particles
bound to receptors
(b)
FIGURE 4-7
Three types of endocytosis
QUESTION Compare and contrast receptor-mediated endocytosis with
active transport.
called pseudopods (Latin for “false foot”).The pseudopod
ends fuse around the luckless Paramecium, and the prey is
carried into the interior of the Amoeba inside a vesicle—
called a food vacuole—for digestion. Like Amoeba, white
blood cells also use phagocytosis and intracellular digestion to engulf and destroy invading bacteria.
(c)
(d)
coated vesicle
(extracellular fluid )
(cytoplasm)
protein
coating
FIGURE 4-8
coated pit
plasma membrane
Receptor-mediated endocytosis
These electron micrographs illustrate the sequence of events in receptor-mediated endocytosis. (a) The shallow depression in the plasma membrane is coated on the inside with a protein (dark, fuzzy substance in the micrographs)
and bears receptor proteins on the outside (not visible). (b, c) The pit deepens and (d) eventually pinches off as a
coated vesicle. The protein coating is recycled back to the plasma membrane.
0.1 micrometer