From Cellular Function to Industrial Products
17
- c j
(
2
.
1
)
Dmem
is the diffusion coefficient for the compound under consideration in the lipid membrane, and
ca
and
ch
are the concentrations of the compound in, respectively, the abiotic phase (extracellular
medium) and the biotic phase (the cytoplasm). The ratio
DmanKvsJdmeni
is called the permeability
coefficient
P,
and it is frequently used for calculation of the mass transport (Stein, 1990). A
collection of permeability coefficients for a few compounds in the cytoplasmic membrane of the
plant cell
Char a ceratophylla
is given in Table 2.3. In the absence of experimental data for a
particular permeability coefficient, one may use the following to get a rough estimate:
P ^ M l
= 0 .0 2 8 /:;'
(
2
.
2
)
Table 23 Permeability coefficients for compounds in membranes of the plant cell
Chara ceratophylla
and
the olive oil-water partitioning coefficient. To evaluate the permeability of other compounds one may use
certain rough measures of how chemical groupings on a permeant can be expected to affect the membrane
permeability (Stein, 1990): An extra hydroxyl group on the molecule decreases the permeability 100- or
1000-fold. A carboxyl group has an even larger effect. An extra amide group is more or less equivalent to
two extra hydroxyl groups. Conversely, an extra methyl group in the compound is likely to increase the
permeability five-fold, while a doubling of molecular volume decreases the permeability 30-fold.
Compound
Permeability coefficient
(cm s'll
Partitioning coefficient
Carbon dioxide
4.5 10*'
Bicarbonate
5.0 10
'7
Water
6.6
ltf*
Urea
2.8
1 0 7
1.5 10
4
Methanol
2.5 10
'4
Ethanol
1.4 10
'4
Ethanediol
1.7 10
'5
4.9 10
'4
1,2 Propanediol
1.7 10
"1
1,3 Propanediol
2.1
10 '3
Formic acid
1.5 10-
Acetic acid
3.0
10
':
Propionic acid
1.5 IO''
Butyric acid
4.4 10
1
Acetamide
1.4 10
'5
8.3 10
“1
Formamide
2.0
10'5
7.6
10
^
Lactamide
1.5 10'&
Butyramide
5.0 10 s
Glucose
5.0 10
'8
Glvcerol
2.2
IO
'7
7.0 10°
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