Biology Reference
In-Depth Information
For example, LUVs made fromDPPC (16:0, 16:0 PC) have a sharp phase transition temper-
ature, T
m
, of 41.3
o
C. At temperatures well below T
m
, the LUVs are in the tightly packed gel
state and permeability is extremely low. At temperatures well above T
m
, the LUVs are in the
loosely packed liquid disordered state (l
d
, also called the liquid crystalline state) and perme-
ability is high. However, maximum permeability is not found in the l
d
state, but rather at the
T
m
[2]
. As the LUVs are heated from the gel state and approach the T
m
, domains of l
d
start to
form in the gel state. Solutes can then pass more readily through the newly formed l
d
domains than the gel domains, resulting in an increase in permeability. At T
m
there is
a maximum amount of coexisting gel and l
d
state domains that exhibit extremely porous
domain boundaries. It is through these boundaries that most permeability occurs. As the
temperature is further increased, the LUVs pass into the l
d
state and the boundaries disap-
pear, reducing permeability to that observed for the single component l
d
state. Thus
maximum permeability is observed at the T
m
.
Fick's First Law
The tendency for solutes to move from a region of higher concentration to one of lower
concentration was first defined in 1855 by the physiologist Adolf Fick (
Figure 14.2
). His
work is summarized in what is now the very well known Fick's Laws of Diffusion
[3]
. The
laws apply to both free solution and diffusion across membranes. Fick developed his laws
by measuring concentrations and fluxes of salt diffusing between two reservoirs through
tubes of water.
Fick's First Law describes diffusion as:
Diffusion rate
¼
DA dc
=
dx
Where:
D
¼
diffusion coefficient (bigger molecules have lower Ds)
cross sectional area over which diffusion occurs
dc/dx is the solute concentration gradient (diffusion occurs from a region of
higher concentration to one of lower concentration)
The relationship between a solute's molecular weight and its diffusion coefficient is shown
in
Table 14.1
. Large solutes have low diffusion coefficients and therefore diffuse more slowly
than small solutes. The diffusion rate for a particular solute under physiological conditions is
a constant and cannot be increased. This defines the theoretical limit for an enzymatic reaction
rate and also limits the size of a cell. If a solute starts at the center of a bacterial cell, it takes
~10
3
s to diffuse to the plasma membrane. For this reason, typical cells are microscopic (see
Chapter 1). At about 3.3 pounds and the size of a cantaloupe, the largest cell on Earth today is
the ostrich egg. However, a dinosaur egg in the AmericanMuseum of Natural History in New
York is about the size of basketball. Since an egg's only function is to store nutrition for a devel-
oping embryo, its size is many orders of magnitude larger than a normal cell.
A
¼
Osmosis
Osmosis is a special type of diffusion, namely the diffusion of water across a semi-
permeable membrane. Water readily crosses a membrane down its potential gradient
