76
UNIT ONE
ever, consider a hypothetical composite cell that includes
many known cell structures
(f
g. 3.3)
.
The three major parts of a cell—the
nucleus
(nu
kle-us),
the
cytoplasm
(si
to-plazm), and the
cell membrane
—if
appropriately stained are easily seen under the light micro-
scope. In many cell types the nucleus is innermost and is
enclosed by a thin membrane called the nuclear envelope.
The nucleus contains the genetic material (DNA), which
directs the cell’s functions. The cytoplasm is composed of
specialized structures called cytoplasmic organelles (or-
gan-elz) suspended in a liquid called
cytosol
(si
to-sol).
Organelles divide the labor in a cell by partitioning off
certain areas or providing speciF
c functions, such as dis-
mantling debris or packaging secretions. The cytoplasm
surrounds the nucleus and is contained by the cell mem-
brane (also called a plasma membrane).
PRACTICE
1
What is a dif
erentiated cell?
2
Name the major parts oF a cell.
3
State the general Functions oF the cytoplasm and nucleus.
3.1
INTRODUCTION
An adult human body consists of about 50 to 100 trillion
cells, the basic units of an organism. All cells have much in
common, yet they come in at least 260 different varieties.
Different cell types interact to build tissues, which interact to
form organs.
Cells with specialized characteristics are termed
differ-
entiated.
Such specialized cells form from less specialized
cells that divide. A cell is like the Internet, harboring a vast
store of information in its genome. However, like a person
accessing only a small part of the Internet, a cell uses only
some of the information in its genome as instructions for
building its characteristic structures.
Cells vary considerably in size, which we measure in units
called
micrometers
(mi
kro-me
terz). A micrometer equals one
thousandth of a millimeter and is symbolized µm. A human
egg cell is about 140 µm in diameter and is just barely visible
to an unaided eye. This is large when compared to a red blood
cell, about 7.5 µm in diameter, or the most common types of
white blood cells, which are 10 to 12 µm in diameter. Smooth
muscle cells are 20 to 500 µm long
(f g. 3.1)
.
Differentiated cells have distinctive shapes that make pos-
sible their functions
(f g. 3.2)
. ±or instance, nerve cells that
have threadlike extensions many centimeters long transmit
nerve impulses from one part of the body to another. Epithelial
cells that line the inside of the mouth are thin, fl attened, and
tightly packed, somewhat like fl oor tiles. They form a barrier
that shields underlying tissue. Muscle cells, slender and rod-
like, contract and pull structures closer together.
3.2
A COMPOSITE CELL
It is not possible to describe a typical cell, because cells vary
greatly in size, shape, content, and function. We can, how-
reproducing itselF at the cell’s expense. Later during inFection, the virus begins
to use yet another type oF receptor to enter cells.
The cells oF about 1% oF people oF European ancestry lack CCR5 and are
immune to HIV inFection; the cells oF another 13% make halF the normal number
oF CCR5 receptors. IF they become inFected, it takes two years longer than the
average For AIDS to develop. The mutation is rare in other population groups.
As soon as the mutation was discovered that could naturally protect
against HIV inFection, pharmaceutical companies began a race to mimic the
ef
ect in a drug. It took more than a decade, with the ±
rst drug approved in 2007
(Selzentry, or generic maraviroc). The drug is used For people, about 65,000 indi-
viduals in the U.S., who have developed resistance to the other three classes oF
anti-HIV drugs. It is a twice-daily pill, and patients must undergo testing to be
certain that their strains oF HIV use the CCR5 doorway. Studying the roles oF cells
in health and disease is a common route to developing drugs.
U
nderstanding how HIV (Human ImmunodeFiciency Virus),
the virus that causes AIDS (Acquired Immune DeFiciency
Syndrome), enters human cells has led to development oF
a new type oF drug to treat the disease. In 1996, investiga-
tions oF people in high-risk groups repeatedly exposed to
the virus but never inFected Found that they were protected because HIV was
unable to enter their cells. The reason: an inherited mutation blocks produc-
tion oF certain proteins that Function as receptors, or doorways oF sorts, on
speci±
c human cells. The virus was essentially kept out.
When inFection begins, HIV typically enters CD4 helper T cells, which
control many Facets oF the immune response. The viruses ±
rst bind to CD4
receptors on these cells, then also attach themselves to nearby receptors oF
another type, called CCR5. Only then can the virus enter the cell and begin
FROM NATURAL PROTECTION AGAINST HIV TO A NEW DRUG
Cells with nuclei, such as those oF the human body, are termed
eukary-
otic,
meaning “true nucleus.” In contrast are the
prokaryotic
(“beFore
nucleus”) cells oF bacteria. Although bacterial cells lack nuclei and
other membrane-bound organelles and are thus simpler than eukary-
otic cells, the bacteria are widespread and have existed much longer
than eukaryotic cells. Viruses are simpler than cells. They consist oF
genetic material in a protein coat and cannot reproduce outside oF a
host cell.
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