108
Chapter 4
potential.
Table 4–2
provides a summary of the major char-
acteristics of the different pathways by which substances move
through cell membranes, while
Figure 4–15
illustrates the vari-
ety of commonly encountered channels and transporters associ-
ated with the movement of substances across a typical plasma
membrane.
Not included in Table 4–2 is the mechanism by which
water moves across membranes. The special case whereby this
polar molecule moves between body fl
uid compartments is
covered next.
Osmosis
Water is a polar molecule that diffuses across the plasma mem-
branes of most cells very rapidly. This process is facilitated by
a family of membrane proteins known as
aquaporins
that
form channels through which water can diffuse. The type and
concentration of these water channels differ in different mem-
branes. Consequently, some cells are more permeable to water
than others. In some cells the number of aquaporin channels,
and thus the permeability of the membrane to water, can be
altered in response to various signals.
The net diffusion of water across a membrane is called
osmosis.
As with any diffusion process, there must be a concen-
tration difference in order to produce a net fl ux. How can a differ-
ence in water concentration be established across a membrane?
The addition of a solute to water lowers the concentra-
tion of water in the solution compared to the concentration of
pure water. For example, if a solute such as glucose is dissolved
Table 4–2
Major Characteristics of Pathways by which Substances Cross Membranes
Diffusion
Mediated Transport
Through
Lipid Bilayer
Through Protein
Channel
Facilitated
Diffusion
Primary Active
Transport
Secondary Active
Transport
Direction of net fl ux
High to low
concentration
High to low
concentration
High to low
concentration
Low to high
concentration
Low to high
concentration
Equilibrium or
steady state
C
o
=
C
i
C
o
=
C
i
*C
o
=
C
i
C
o
C
i
C
o
C
i
Use of integral
membrane protein
No
Yes
Yes
Yes
Yes
Maximal fl
ux at
high concentration
(saturation)
No
No
Yes
Yes
Yes
Chemical specifi city
No
Yes
Yes
Yes
Yes
Use of energy and
source
No
No
No
Yes: ATP
Yes: ion gradient
(often Na
+
)
Typical molecules
using pathway
Nonpolar: O
2
,
CO
2
, fatty acids
Ions: Na
+
, K
+
, Ca
2+
Polar: glucose
Ions: Na
+
, K
+
,
Ca
2+
, H
+
Polar: amino acids,
glucose, some ions
*In the presence of a membrane potential, the intracellular and extracellular ion concentrations will not be equal at equilibrium.
Figure 4–15
Movement of solutes across a typical plasma membrane involving
membrane proteins. A specialized cell may contain additional
transporters and channels not shown in this fi
gure. Many of these
membrane proteins can be modulated by various signals, leading to
a controlled increase or decrease in specifi c solute fl
uxes across the
membrane. The stoichiometry of cotransporters is not shown.
Primary
active
transport
Secondary
active
transport
Ion
channels
Na
+
K
+
ADP
ATP
Ca
2+
ADP
ATP
ADP
ATP
Na
+
Na
+
Ca
2+
Cl
Ca
2+
Cl
HCO
3
Na
+
H
+
K
+
Na
+
Amino
acids
Facilitated
diffusion
Glucose
H
+
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