Chapter 3
Cells: The Living Units
67
3
Check Your Understanding
6.
Which two types of cell junctions would you expect to find
between muscle cells of the heart?
For answer, see Appendix H.
The Plasma Membrane:
Membrane Transport
Relate plasma membrane structure to active and passive
transport processes.
Compare and contrast simple diffusion, facilitated
diffusion, and osmosis relative to substances transported,
direction, and mechanism.
pulling forces. Desmosomes are abundant in tissues subjected
to great mechanical stress, such as skin and heart muscle.
Gap Junctions
A
gap junction
, or
nexus
(nek
9
sus; “bond”), is a communicat-
ing junction between adjacent cells. At gap junctions the adjacent
plasma membranes are very close, and the cells are connected
by hollow cylinders called
connexons
(kŏ-nek
9
sonz), composed
of transmembrane proteins. Te many different types of con-
nexon proteins vary the selectivity of the gap junction channels.
Ions, simple sugars, and other small molecules pass through these
water-filled channels from one cell to the next (Figure 3.5c).
Gap junctions are present in electrically excitable tissues, such
as the heart and smooth muscle, where ion passage from cell to
cell helps synchronize their electrical activity and contraction.
Interlocking
junctional
proteins
Intercellular
space
Intercellular
space
Plasma membranes
of adjacent cells
Microvilli
Intercellular
space
Basement membrane
Plaque
Linker
proteins
(cadherins)
Intermediate
filament
(keratin)
Intercellular
space
Channel
between cells
(formed by
connexons)
(a) Tight junctions:
Impermeable junctions
prevent molecules from passing through
the intercellular space.
(b) Desmosomes:
Anchoring junctions bind
adjacent cells together like a molecular
“Velcro” and help form an internal tension-
reducing network of fibers.
(c) Gap junctions:
Communicating junctions
allow ions and small molecules to pass
for intercellular communication.
Figure 3.5
Cell junctions.
An epithelial cell is shown joined to adjacent cells by three common
types of cell junctions. (Note: Except for epithelia, it is unlikely that a single cell will have all three
junction types.)
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