60 SECTION ICellular & Molecular Basis of Medical Physiology
CHAPTER SUMMARY
■ The cell and the intracellular organelles are surrounded by a
semipermeable membrane. Biological membranes have a lipid
bilayer with a hydrophobic core and hydrophilic outer regions
that provide a barrier between inside and outside compartments
as well as a template for biochemical reactions. The membrane
is populated by structural and functional proteins that can be in-
tegrated into the membrane or be associated with one side of the
lipid bilayer. These proteins contribute greatly to the semiper-
meable properties of biological membrane.
■ Mitochondria are organelles that allow for oxidative phos-
phorylation in eukaryotic cells. They contain their own DNA,
however, proteins in the mitochondria are encoded by both mi-
tochondrial and cellular DNA. Mitochondria also are important
in specialized cellular signaling.
■ Lysosomes and peroxisomes are membrane-bound organelles
that contribute to protein and lipid processing. They do this in
part by creating acidic (lysosomes) or oxidative (peroxisomes)
contents relative to the cell cytosol.
■ The cytoskeleton is a network of three types of filaments that
provide structural integrity to the cell as well as a means for traf-
ficking of organelles and other structures. Actin is the funda-
mental building block for thin filaments and represents as much
as 15% of cellular protein. Actin filaments are important in cel-
lular contraction, migration, and signaling. Actin filaments also
provide the backbone for muscle contraction. Intermediate fila-
ments are primarily structural. Proteins that make up interme-
diate filaments are cell-type specific. Microtubules are made up
of tubulin subunits. Microtubules provide a dynamic structure
in cells that allows for movement of cellular components around
the cell.
■ There are three superfamilies of molecular motor proteins in the
cell that use the energy of ATP to generate force, movement, or
both. Myosin is the force generator for muscle cell contraction.
There are also cellular myosins that interact with the cytoskele-
ton (primarily thin filaments) to participate in contraction as
well as movement of cell contents. Kinesins and cellular dyneins
are motor proteins that primarily interact with microtubules to
move cargo around the cells.
■ Cellular adhesion molecules aid in tethering cells to each other
or to the extracellular matrix as well as providing for initiation
of cellular signaling. There are four main families of these pro-
teins: integrins, immunoglobulins, cadherins, and selectins.
■ Cells contain distinct protein complexes that serve as cellular
connections to other cells or the extracellular matrix. Tight
junctions provide intercellular connections that link cells into a
regulated tissue barrier. Tight junctions also provide a barrier to
movement of proteins in the cell membrane and thus, are im-
portant to cellular polarization. Gap junctions provide contacts
between cells that allow for direct passage of small molecules be-
tween two cells. Desmosomes and adherens junctions are spe-cialized structures that hold cells together. Hemidesmosomes
and focal adhesions attach cells to their basal lamina.
■ The nucleus is an organelle that contains the cellular DNA and
is the site of transcription. There are several organelles that em-
anate from the nucleus, including the endoplasmic reticulum
and the Golgi apparatus. These two organelles are important in
protein processing and the targeting of proteins to correct com-
partments within the cell.
■ Exocytosis and endocytosis are vesicular fusion events that al-
low for movement of proteins and lipids between the cell interi-
or, the plasma membrane, and the cell exterior. Exocytosis can
be constitutive or nonconstitutive; both are regulated processes
that require specialized proteins for vesicular fusion. Endocyto-
sis is the formation of vesicles at the plasma membrane to take
material from the extracellular space into the cell interior. Some
endocytoses are defined in part by the size of the vesicles formed
whereas others are defined by membrane structures that con-
tribute to the endocytosis. All are tightly regulated processes.
■ Membranes contain a variety of proteins and protein complexes
that allow for transport of small molecules. Aqueous ion chan-
nels are membrane-spanning proteins that can be gated open to
allow for selective diffusion of ions across membranes and down
their electrochemical gradient. Carrier proteins bind to small
molecules and undergo conformational changes to deliver small
molecules across the membrane. This facilitated transport can
be passive or active. Active transport requires energy for trans-
port and is typically provided by ATP hydrolysis.
■ Cells can communicate with one another via chemical messen-
gers. Individual messengers (or ligands) typically bind to a plas-
ma membrane receptor to initiate intracellular changes that lead
to physiologic changes. Plasma membrane receptor families in-
clude ion channels, G protein-coupled receptors, or a variety of
enzyme-linked receptors (eg, tyrosine kinase receptors). There
are additional cytosolic receptors (eg, steroid receptors) that can
bind membrane-permeant compounds. Activation of receptors
lead to cellular changes that include changes in membrane po-
tential, activation of heterotrimeric G proteins, increase in sec-
ond messenger molecules, or initiation of transcription.
■ Second messengers are molecules that undergo a rapid concen-
tration changes in the cell following primary messenger recog-
nition. Common second messenger molecules include Ca2+,
cyclic adenosine monophosphate (cAMP), cyclic guanine
monophosphate (cGMP), inositol trisphosphate (IP 3 ) and nitric
oxide (NO).