Cells: The Living Units
Check Your Understanding
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:
Relate plasma membrane structure to active and passive
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.
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
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.
of adjacent cells
(a) Tight junctions:
prevent molecules from passing through
the intercellular space.
Anchoring junctions bind
adjacent cells together like a molecular
“Velcro” and help form an internal tension-
reducing network of fibers.
(c) Gap junctions:
allow ions and small molecules to pass
for intercellular communication.
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