79
CHAPTER THREE
Cells
respond to incoming messages, in a process called
signal
transduction.
Membrane Structure
The cell membrane is mainly composed of lipids and pro-
teins, with some carbohydrate. Its basic framework is a dou-
ble layer (bilayer) of phospholipid molecules (see F g. 2.15)
that self-assemble so that their water-soluble (hydrophilic)
“heads,” containing phosphate groups, form the surfaces
of the membrane, and their water-insoluble (hydrophobic)
In addition to maintaining the integrity of the cell, the
cell membrane controls the entrance and exit of substances,
allowing some in while excluding others. A membrane that
functions in this manner is
selectively permeable
(per
me-
ah-bl). The cell membrane is crucial because it is a conduit
between the cell and the extracellular fl
uids in the body’s
internal environment. It allows the cell to receive and
FIGURE 3.4
A transmission electron microscope.
The maximum ef
ective magniF
cation possible using a light microscope
is about 5,000×. A con±ocal microscope is a type o± light microscope that
passes white or laser light through a pinhole and lens to impinge on
the object, which greatly enhances resolution (ability to distinguish F
ne
detail). A transmission electron microscope (TEM) provides an ef
ec-
tive magniF
cation o± nearly 1,000,000×, whereas a scanning electron
microscope (SEM) can provide about 50,000×. Photographs o± micro-
scopic objects (micrographs) produced using the light microscope and
the transmission electron microscope are typically two-dimensional,
but those obtained with the scanning electron microscope have a
three-dimensional quality
(F
g. 3.5)
. Scanning probe microscopes work
dif
erently ±rom light or electron microscopes. They move a probe over
a sur±ace and translate the distances into an image.
Red
blood cells
(a)
(b)
Red
blood cells
Blood
vessel wall
Red
blood cells
(c)
FIGURE 3.5
Human red blood cells as viewed using (
a
) a light
microscope (1,200×), (
b
) a transmission electron microscope (2,500×),
and (
c
) a scanning electron microscope (1,900×).
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