fundamentals of engineering supplied-reference handbook - Ventech!

Transfer Across Membrane Barriers
Mechanisms
Passive diffusion – affected by lipid solubility (high solubility
increases transport), molecular size (decreased with
molecular size), and ionization (decreased with ionization).
Passive diffusion is influenced by:
1. Partition coefficient (indicates lipid solubility; high lipid
solubility characterizes materials that easily penetrate
skin and other membranes).
2. Molecular size is important in that small molecules tend
to transport much easier than do large molecules.
3. Degree of ionization is important because, in most cases,
only unionized forms of materials transport easily
through membranes. Ionization is described by the
following relationships:
Acids
Base
⎡ ⎤
⎣ ⎦
nonioinized form HA
pKa − pH = log10 = log
⎢ 10
ionized form ⎥ A
+
⎡ ionized form ⎤ HB
pKa − pH = log10 = log
⎢ 10
nonionized form ⎥ B
⎣ ⎦
Facilitated diffusion – requires participation of a protein
carrier molecule. This mode of transport is highly compound
dependent.
Active diffusion – requires protein carrier and energy and is
similarly affected by ionization and is highly compound
dependent.
Other – includes the specialized mechanisms occurring in
lungs, liver, and spleen.
77
BIOLOGY (continued)