78
UNIT ONE
Microtubules
Flagellum
Nuclear envelope
Basal body
Chromatin
Ribosomes
Cell membrane
Mitochondrion
Cilia
Microtubules
Microtubule
Golgi
apparatus
Secretory
vesicles
Centrioles
Microvilli
Lysosomes
Smooth
endoplasmic
reticulum
Rough
endoplasmic
reticulum
Nucleolus
Nucleus
Phospholipid bilayer
FIGURE 3.3
A composite cell illustrates the organelles and other structures found in cells. Specialized cells diF
er in the numbers and types of
organelles, re±
ecting their functions. Organelles are not drawn to scale.
Cell Membrane
The cell membrane is the outermost limit of a cell. Not just a
simple boundary, the cell membrane is an actively function-
ing part of the living material. Many important metabolic reac-
tions take place on its surfaces, and it harbors molecules that
enable cells to communicate and interact. The chapter open-
ing vignette on page 76 offers one example of the importance
of understanding the cell membrane in treating HIV infection.
General Characteristics
The cell membrane is extremely thin—visible only with the
aid of an electron microscope
(f
g. 3.4)
—but it is fl
exible
and somewhat elastic. It typically has complex surface fea-
tures with many outpouchings and infoldings that increase
surface area. The cell membrane quickly seals tiny breaks,
but if it is extensively damaged, cell contents exit and the
cell dies.
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