12
UNIT 1
Basic Principles of Cell Structure and Function
Introduction and Key Concepts for
Cell Components
The
cell
is the basic structural and functional unit of the body,
and variations in
cell structure
account for the remarkable diver-
sity in the morphology and function of the body’s basic
tissues
and organs. The cell’s repertoire of constituents is limited, but
the possible combinations of quantities and arrangements of
these constituents are numerous.
Nucleus (Fig. 2-1)
Nuclear envelope
: This is a double-membrane structure consist-
ing of a perinuclear cistern sandwiched between the inner and
the outer nuclear membranes. The nuclear envelope protects the
genetic material and separates it from the diverse molecules and
structures of the cytoplasm. The nuclear envelope is continuous
with the rough endoplasmic reticulum.
Nuclear pore
: These gaps in the nuclear envelope are bridged by
a pore complex of numerous proteins. This structure controls
the trafF c of molecules and particles between the nucleus and
the cytoplasm.
Heterochromatin
: This tightly coiled chromatin is inaccessible
for transcription, and it appears as basophilic clumps in speci-
mens prepared for light microscopy.
Euchromatin
: This is chromatin in a more extended form,
accessible to transcription. In the light microscope, it appears
as less densely stained, basophilic regions of the nucleus.
Nucleolus
: One or more of these spherical, basophilic structures
develop in the nucleus if the cell is generating ribosomes.
Cytoplasm (Fig. 2-1)
Ribosome
: This is an egg-shaped particle (~20
3
30 nm) that
attaches to a messenger RNA (mRNA) molecule and generates
a polypeptide according to the code provided by the nucleotide
sequence of the mRNA strand.
Polyribosome
: This is formed by the alignment of several ribo-
somes along the length of an mRNA molecule. When an mRNA
for a cytosolic or mitochondrial protein is being translated, the
polyribosome is not associated with endoplasmic reticulum, but
rather it is suspended in the cytosol as a free polyribosome.
Rough
endoplasmic
reticulum
(
RER
): This typically consists of
attened membrane-delimited cisternae that are studded with
ribosomes. As polypeptides are synthesized by the ribosomes,
they are sequestered by insertion into the cisternae. Small shut-
tle vesicles (transport vesicles) move the newly synthesized poly-
peptides from the RER to the Golgi complex.
Golgi
complex
: This consists of a stack of fl attened membra-
nous saccules (cisternae). The Golgi complex receives poly-
peptides from the RER, modiF es them (e.g., by sulfation,
glycosylation), and sorts and packages them according to their
destination as constituents of lysosomes, secretory granules
(e.g., pancreatic acinar cell), or cytoplasmic granules (e.g.,
neutrophilic
leukocyte).
Secretory vesicle
: Also called
secretory granules
, these are
membrane-delimited packages of secretory products that can
be either stored in the cytoplasm or immediately secreted by
exocytosis of the vesicle at the cell surface.
Lysosome
: This is a membrane-delimited vesicle characterized
by its low pH and the content of a variety of hydrolases that can
digest proteins, lipids, and polysaccharides. Lysosomes function
both in the turnover of intrinsic cellular components and in the
breakdown of the material ingested by endocytosis.
Other
types
of
membrane
-
delimited
vesicles
: Substances other
than the secretory products of secretory granules and the
hydrolytic enzymes of lysosomes may be sequestered within
vesicles. Examples are proteases within proteasomes, unF
nished
polypeptides in transfer vesicles, and oxidases and catalase in
peroxisomes.
Smooth
endoplasmic
reticulum
(
SER
): This is a membrane-
delimited labyrinth, usually in the form of tubules rather than
fl attened cisternae. Its multiple functions include synthesis of
membrane phospholipids, degradation of some hormones and
toxic substances (liver), synthesis of steroid hormones, and
sequestration of calcium stores (striated muscle).
Mitochondrion
: This is a double-membrane organelle (like the
nucleus) that generates adenosine triphosphate (ATP) to fuel
energy-requiring activities of the cell. It also functions in some spe-
cialized synthetic pathways such as that for steroid
hormones.
Cell Surface (Fig. 2-1)
Cilia
: These are cylindrical extensions about 200 nm thick
and 5 to 10
μ
m long. They are usually motile, with a whipping
motion that consists of a power stroke and a recovery stroke.
An axoneme of microtubules in a nine-doublet, two-singlet
arrangement provides the stiffness and movement of cilia.
Microvilli
: These are cylindrical, nonmotile extensions, typi-
cally about 80 nm in diameter and 1
μ
m long. In some cells,
they are several micrometers long, in which case, they are called
stereocilia
. Actin F
laments form the cores of microvilli.
Zonula occludens
: Also called
tight junctions
, these are sites
of fusion between plasmalemmas of adjacent epithelial cells
that separate the lumen from the underlying connective tissue.
Tight junctions prevent materials from passing from one com-
partment to another by diffusing through the space between
adjacent epithelial cells.
Zonula
adherens
: These are sites of mechanical adhesion
between adjacent epithelial cells. They are generally associated
with and parallel to tight junctions. Actin F
laments of the ter-
minal web are anchored by proteins at the cytoplasmic faces of
the membranes of the two cells.
Macula
adherens
: Also called
desmosomes
, these, like zonula
adherens, are sites of mechanical adhesion between cells, but
they are conF
gured as spots rather than bands. Intermediate
F laments are anchored to proteins at the cytoplasmic surfaces
of the membranes of the two cells.
Gap
junctions
: Also called
nexus junctions
, these are sites of
close (~2 nm) apposition between plasmalemmas of adjacent
cells. The small space between the membranes is bridged by
connexons, each of which is a cylindrical complex of proteins
forming a pore. Gap junctions allow free fl
ow of small ions
between cells, so that the cells are electrically coupled.
Basolateral folds
: Infoldings of the plasmalemma at the basal
surface of an epithelial cell provide greatly increased surface area
in support of extensive trafF c of substances between the cytosol
and the underlying interstitial compartment. Such folds are par-
ticularly prominent in epithelial cells that function in removing
water and ions from a lumen. Lateral folds can have an additional
function of contributing mechanical strength to a layer of cells by
interlacing with complementary folds of an adjacent cell.
Basal lamina
: This is an extracellular layer at the interface
of a cell and the adjacent connective tissue. Basal laminae are
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