Cells: The Living Units
(asbestos ﬁbers or glass, for example)
a particle binds to receptors on the cell’s surface, cytoplas-
mic extensions called pseudopods (soo
foot) form and ﬂow around the particle. Tis
forms an endocytotic vesicle called a
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
, 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
“changing shape”); that is, the ﬂowing of their cytoplasm into
temporary extensions allows them to creep along.
(“cell drinking”), also called
, 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.
shows the basic steps in endocytosis and trans-
An infolding portion of the plasma membrane,
, 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
“lattice clad”). Te clathrin coat (clathrin and some accessory
proteins) acts in both selecting the cargo and deforming the
membrane to produce the vesicle.
Te vesicle detaches, and
the coat proteins are recycled back to the plasma membrane.
Te uncoated vesicle then typically fuses with a sorting
vesicle called an
Some membrane components
and receptors of the fused vesicle may be recycled back to the
plasma membrane in a transport vesicle.
contents of the vesicle may (a) combine with a
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 (
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
sis; “cell eating”),
the cell engulfs some relatively large or solid material, such
as a clump of bacteria, cell debris, or inanimate particles
Primary active transport
The ATP-driven Na
stores energy by creating a steep
concentration gradient for Na
entry into the cell.
Secondary active transport
diffuses back across the membrane
through a membrane cotransporter protein, it
drives glucose against its concentration gradient
into the cell.
into the cytoplasm
Secondary active transport is driven by the concentration gradient created
by primary active transport.