Biology Reference
In-Depth Information
FIGURE 14.4
Basic types of membrane transport: simple passive diffusion; facilitated diffusion (by channels
and carriers); and active transport
[8]
.
Even for simple passive diffusion it requires energy to cross a bilayer membrane. In order
to cross a membrane the solute must first lose its waters of hydration, diffuse across the
membrane, and then regain its waters on the opposite side. The limiting step involves
the energy required to lose the waters of hydration.
Table 14.2
shows the relationship
between the waters of hydration (proportional to the number of
OH groups on a homol-
ogous series of solutes) and the activation energy for trans-membrane diffusion. As the
number of waters of hydration increases from glycol
e
erythritol, the activation
energy for diffusion also increases. The activation energy compares very well with the
energy of hydration.
However, water diffusion does not fit this model. Water permeability is just too high.
Several possibilities have been suggested to account for the abnormally high membrane
permeability of water:
<
glycerol
<
1. Water is very small and so just dissolves in bilayers better than larger solutes.
2. Due to its size, water can enter very small statistical pores (~4.2
˚
in diameter) more
readily. Statistical pores result from the simultaneous lateral movement of adjacent
TABLE 14.2
Relationship Between the Waters of Hydration (Related to the Number
of eOH Groups on a Homologous Series of Solutes) and the Activation
Energy for Trans-Membrane Diffusion.
Solute
e
OH groups
Activation energy (KJ/mol)
Glycol
(HO-CH
2
-CH
2
-OH)
2
60
Glycerol
(HO-CH
2
-CH(OH)-CH
2
-OH)
3
77
Erythritol
(HO-CH
2
-CH(OH)-CH(OH)-CH
2
-OH)
4
87
