32
SECTION I
Cellular & Molecular Basis of Medical Physiology
other techniques. When cells are homogenized and the result-
ing suspension is centrifuged, the nuclei sediment first, fol-
lowed by the mitochondria. High-speed centrifugation that
generates forces of 100,000 times gravity or more causes a
fraction made up of granules called the
microsomes
to sedi-
ment. This fraction includes organelles such as the
ribosomes
and
peroxisomes.
CELL MEMBRANES
The membrane that surrounds the cell is a remarkable struc-
ture. It is made up of lipids and proteins and is semipermeable,
allowing some substances to pass through it and excluding
others. However, its permeability can also be varied because it
contains numerous regulated ion channels and other trans-
port proteins that can change the amounts of substances mov-
ing across it. It is generally referred to as the
plasma
membrane.
The nucleus and other organelles in the cell are
bound by similar membranous structures.
Although the chemical structures of membranes and their
properties vary considerably from one location to another,
they have certain common features. They are generally about
7.5 nm (75 Å) thick. The major lipids are phospholipids such
as phosphatidylcholine and phosphatidylethanolamine. The
shape of the phospholipid molecule reflects its solubility
properties: the head end of the molecule contains the phos-
phate portion and is relatively soluble in water (polar,
hydro-
philic
) and the tails are relatively insoluble (nonpolar,
hydrophobic
). The possession of both hydrophilic and
hydrophobic properties make the lipid an
amphipathic
mole-
cule. In the membrane, the hydrophilic ends of the molecules
are exposed to the aqueous environment that bathes the exte-
rior of the cells and the aqueous cytoplasm; the hydrophobic
ends meet in the water-poor interior of the membrane (Figure
2–2). In
prokaryotes
(ie, bacteria in which there is no
nucleus), the membranes are relatively simple, but in
eukary-
otes
(cells containing nuclei), cell membranes contain various
glycosphingolipids, sphingomyelin, and cholesterol in addi-
tion to phospholipids and phosphatidylcholine.
Many different proteins are embedded in the membrane.
They exist as separate globular units and many pass through
the membrane
(integral proteins),
whereas others
(peripheral
proteins)
stud the inside and outside of the membrane (Figure
2–2). The amount of protein varies significantly with the func-
tion of the membrane but makes up on average 50% of the mass
of the membrane; that is, there is about one protein molecule
per 50 of the much smaller phospholipid molecules. The pro-
teins in the membranes carry out many functions. Some are
cell adhesion molecules
that anchor cells to their neighbors or
to basal laminas. Some proteins function as
pumps,
activelyFIGURE 2–1
Diagram showing a hypothetical cell in the center as seen with the light microscope.
Individual organelles are ex-
panded for closer examination.
(Adapted from Bloom and Fawcett. Reproduced with permission from Junqueira LC, Carneiro J, Kelley RO:
Basic Histology,
9th ed.
McGraw-Hill, 1998.)
Secretory granulesCentriolesSmooth
endoplasmic
reticulumGolgi
apparatusLipid
dropletsRough
endoplasmic
reticulumLysosomesMitochondrionGlobular headsNuclear envelopeNucleolus