Chapter 3
Cells: The Living Units
75
3
(asbestos fibers or glass, for example)
(Figure 3.13a)
. When
a particle binds to receptors on the cell’s surface, cytoplas-
mic extensions called pseudopods (soo
9
do-pahdz;
pseudo
5
false,
pod
5
foot) form and flow around the particle. Tis
forms an endocytotic vesicle called a
phagosome
(fag
9
o-
sōm; “eaten body”). In most cases, the phagosome then fuses
with a lysosome and its contents are digested. Any indigest-
ible contents are ejected from the cell by exocytosis.
In the human body, only macrophages and certain white
blood cells are “experts” at phagocytosis. Commonly referred
to as
phagocytes
, these cells help protect the body by ingest-
ing and disposing of bacteria, other foreign substances, and
dead tissue cells. Te disposal of dying cells is crucial, because
dead cell remnants trigger inflammation in the surrounding
area or may stimulate an undesirable immune response. Most
phagocytes move about by
amoeboid motion
(ah-me
9
boyd;
“changing shape”); that is, the flowing of their cytoplasm into
temporary extensions allows them to creep along.
Pinocytosis.
In
pinocytosis
(“cell drinking”), also called
fluid-
phase endocytosis
, a bit of infolding plasma membrane (which
begins as a protein-coated pit) surrounds a very small volume of
extracellular fluid containing dissolved molecules (Figure 3.13b).
Tis droplet enters the cell and fuses with an endosome. Unlike
phagocytosis, pinocytosis is a routine activity of most cells, af-
fording them a nonselective way of sampling the extracellular
fluid. It is particularly important in cells that absorb nutrients,
such as cells that line the intestines.
As mentioned, bits of the plasma membrane are removed
when the membranous sacs are internalized. However, these
membranes are recycled back to the plasma membrane by ex-
ocytosis as described shortly, so the surface area of the plasma
membrane remains remarkably constant.
Protein-coated vesicles provide the main route for endocy-
tosis and transcytosis of bulk solids, most macromolecules, and
fluids. On occasion, these vesicles are also hijacked by patho-
gens seeking entry into a cell.
Figure 3.12
shows the basic steps in endocytosis and trans-
cytosis.
1
An infolding portion of the plasma membrane,
called a
coated pit
, progressively encloses the substance to be
taken into the cell. Te coating found on the cytoplasmic face of
the pit is most o±en the bristlelike protein
clathrin
(klă
9
thrin;
“lattice clad”). Te clathrin coat (clathrin and some accessory
proteins) acts in both selecting the cargo and deforming the
membrane to produce the vesicle.
2
Te vesicle detaches, and
3
the coat proteins are recycled back to the plasma membrane.
4
Te uncoated vesicle then typically fuses with a sorting
vesicle called an
endosome
.
5
Some membrane components
and receptors of the fused vesicle may be recycled back to the
plasma membrane in a transport vesicle.
6
Te remaining
contents of the vesicle may (a) combine with a
lysosome
(li
9
so-
sōm), a specialized cell structure containing digestive enzymes,
where the ingested substance is degraded or released (if iron or
cholesterol), or (b) be transported completely across the cell and
released by exocytosis on the opposite side (
transcytosis
). ²rans-
cytosis is common in the endothelial cells lining blood vessels
because it provides a quick means to get substances from the
blood to the interstitial fluid.
Based on the nature and quantity of material taken up and
the means of uptake, three types of endocytosis that use clathrin-
coated vesicles are recognized: phagocytosis, pinocytosis, and
receptor-mediated endocytosis.
Phagocytosis.
In
phagocytosis
(fag
0
o-si-to
9
sis; “cell eating”),
the cell engulfs some relatively large or solid material, such
as a clump of bacteria, cell debris, or inanimate particles
1
2
Primary active transport
The ATP-driven Na
+
-K
+
pump
stores energy by creating a steep
concentration gradient for Na
+
entry into the cell.
Secondary active transport
As Na
+
diffuses back across the membrane
through a membrane cotransporter protein, it
drives glucose against its concentration gradient
into the cell.
K
+
Na
+
Na
+
Na
+
Na
+
Na
+
Na
+
Na
+
Na
+
Na
+
Na
+
Na
+
Na
+
-glucose
symport
transporter
loads glucose
from extracellular
fluid
Na
+
-glucose
symport transporter
releases glucose
into the cytoplasm
Glucose
Na
+
-K
+
pump
Cytoplasm
Extracellular fluid
ATP
Figure 3.11
Secondary active transport is driven by the concentration gradient created
by primary active transport.
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