82
UNIT ONE
activity to link amino acids to form proteins (see
chapter 4, p. 132). Unlike many of the other
organelles, ribosomes are not composed of or
contained in membranes. They are scattered in the
cytoplasm and also bound to another organelle, the
endoplasmic reticulum.
2.
Endoplasmic reticulum.
The endoplasmic reticulum
(en-do-plaz
mik re-tik
u-lum) (ER) is a complex
organelle composed of membrane-bound fl
attened sacs,
contains abundant protein rods and tubules that form a sup-
portive framework called the
cytoskeleton
(si
to-skel-e
˘-ton).
The activities of a cell occur largely in its cytoplasm,
where nutrient molecules are received, processed, and used
in metabolic reactions. The following cytoplasmic organelles
have speciF
c functions:
1.
Ribosomes.
Ribosomes (ri
bo-so
¯mz) are tiny,
spherical structures composed of protein and RNA.
They provide a structural support and enzymatic
W
hat do abnormal pain intensity,
irregular heartbeats, and cystic f
bro-
sis have in common? All result From
abnormal ion channels in cell membranes.
Ion channels are protein-lined tunnels in the
phospholipid bilayer oF a biological membrane.
These passageways permit electrical signals to
pass in and out oF membranes as ions (charged
particles). Many ion channels open or close like
a gate in response to specif
c ions under specif
c
conditions. These situations include a change in
electrical Forces across the membrane, binding oF
a molecule, or receiving biochemical messages
From inside or outside the cell.
Ion channels are specif
c For calcium (Ca
+2
),
sodium (Na
+
), potassium (K
+
), or chloride (Cl
).
A cell membrane may have a Few thousand ion
channels specif
c For each oF these ions. Ten mil-
lion or more ions can pass through an ion chan-
nel in one second! Drugs may act by a±
ecting ion
channels (table 3A), and abnormal ion channels
cause certain disorders, including the Following:
Absent or Excess Pain
The ten-year-old boy amazed the people on the
streets oF the small northern Pakistani town.
He was completely unable to Feel pain and had
become a perFormer, stabbing knives through
his arms and walking on hot coals to entertain
crowds. Several other people in this community,
where relatives oFten married relatives, were also
unable to Feel pain. Researchers studied the con-
nected Families and discovered a mutation that
alters sodium channels on certain nerve cells. The
mutation blocks the channels so that the mes-
sage to Feel pain cannot be sent. The boy died at
age thirteen From jumping oFF a rooF. His genes
could protect him From pain, but pain protects
against injury by providing a warning.
A diFFerent mutation aFFecting the same
sodium channels causes drastically diFFerent
symptoms. In “burning man syndrome,” the chan-
nels become hypersensitive, opening and ²
ood-
ing the body with pain easily, in response to
exercise, an increase in room temperature, or just
putting on socks. In another condition, “paroxys-
mal extreme pain disorder,” the sodium channels
stay open too long, causing excruciating pain in
the rectum, jaw, and eyes. Researchers are using
the inFormation From these genetic studies to
develop new painkillers.
Long-QT Syndrome and Potassium
Channels
³our children in a Norwegian Family were born
deaF, and three oF them died at ages Four, Five,
and nine. All oF the children had inherited From
unaFFected “carrier” parents “long-QT syndrome
associated with deaFness.” They had abnormal
potassium channels in the heart muscle and in
the inner ear. In the heart, the malFunctioning
channels disrupted electrical activity, causing a
Fatal disturbance to the heart rhythm. In the inner
ear, the abnormal channels caused an increase in
the extracellular concentration oF potassium ions,
impairing hearing.
Cystic Fibrosis and Chloride
Channels
A seventeenth-century English saying, “A child
that is salty to taste will die shortly aFter birth,”
described the consequence oF abnormal chloride
channels in cystic Fibrosis (C³), inherited From
carrier parents. The major symptoms—di´
culty
breathing, Frequent severe respiratory inFections,
and a clogged pancreas that disrupts digestion—
result From buildup oF extremely thick mucous
secretions.
Abnormal chloride channels in cells lining
the lung passageways and ducts oF the pancreas
cause the symptoms oF C³. The primary deFect in
the chloride channels also causes sodium chan-
nels to malFunction. The result: very salty sweat
and abnormally thick mucus. Gene therapy is
directed at supplying patients’ lung-lining cells
with the instructions to produce normal chloride
channels.
3.1
CLINICAL APPLICATION
Faulty Ion Channels Cause Disease
TABLE
3A
|
Drugs That Af
ect Ion Channels
Target
Indication
Calcium channels
Antihypertensives
Antiangina (chest pain)
Sodium channels
Antiarrhythmias, diuretics
Local anesthetics
Chloride channels
Anticonvulsants
Muscle relaxants
Potassium channels
Antihypertensives, antidiabetics (non-insulin-dependent)
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