46
Chapter 3
a membrane offers to the passage of substances can be altered
to allow increased or decreased fl ow of molecules or ions across
the membrane in response to various signals.
In addition to acting as a selective barrier, the plasma
membrane plays an important role in detecting chemical sig-
nals from other cells and in anchoring cells to adjacent cells
and to the extracellular matrix of connective-tissue proteins.
Membrane Structure
All membranes consist of a double layer of lipid molecules
containing embedded proteins (
Figure 3–6
). The major
membrane lipids are
phospholipids,
which are amphipathic
molecules. One end has a charged or polar region, and the
remainder of the molecule, which consists of two long fatty
acid chains, is nonpolar. The phospholipids in cell membranes
are organized into a bimolecular layer with the nonpolar fatty
acid chains in the middle. The polar regions of the phospho-
lipids are oriented toward the surfaces of the membrane as a
result of their attraction to the polar water molecules in the
extracellular fl
uid and cytosol.
No chemical bonds link the phospholipids to each other
or to the membrane proteins. Therefore, each molecule is free
to move independently of the others. This results in consider-
able random lateral movement of both membrane lipids and
proteins parallel to the surfaces of the bilayer. In addition, the
long fatty acid chains can bend and wiggle back and forth.
Thus, the lipid bilayer has the characteristics of a fl
uid, much
like a thin layer of oil on a water surface, and this makes the
membrane quite fl exible. This fl exibility, along with the fact
that cells are fi lled with fl
uid, allows cells to undergo moder-
ate changes in shape without disrupting their structural integ-
rity. Like a piece of cloth, a membrane can be bent and folded
but cannot be stretched without being torn.
The plasma membrane also contains about one molecule
of cholesterol for each molecule of phospholipid, whereas intra-
Nucleus
Plasma membranes
Organelles
(a)
Cytoplasm
(b)
Cytosol
Figure 3–5
Comparison of cytoplasm and cytosol. (a) Cytoplasm (colored area)
is the region of the cell outside the nucleus. (b) Cytosol (colored
area) is the fl uid portion of the cytoplasm outside the cell organelles.
Table 3–1
Functions of Cell Membranes
1. Regulate the passage of substances into and out of cells and
between cell organelles and cytosol
2. Detect chemical messengers arriving at the cell surface
3. Link adjacent cells together by membrane junctions
4. Anchor cells to the extracellular matrix
Figure 3–6
(a) Electron micrograph of a human red blood cell plasma membrane. Cell membranes are 6 to 10 nm thick, too thin to be seen without the
aid of an electron microscope. In an electron micrograph, a membrane appears as two dark lines separated by a light interspace. The dark lines
correspond to the polar regions of the proteins and lipids, whereas the light interspace corresponds to the nonpolar regions of these molecules.
(b) Arrangement of the proteins and lipids in a membrane.
From J. D. Robertson in Michael Locke (ed.),
Cell Membranes in Development,
Academic Press, Inc., New York, 1964.
(a)
Plasma
membrane
Red blood
cell cytosol
(b)
Proteins
Phospholipid
bilayer
Extracellular fluid
Intracellular fluid
Polar regions
Fatty acids
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