Biomedical Engineering Reference
In-Depth Information
Passive Channels
Na +
Cl
K +
Outside
Na +
Inside
Cl
K +
A
FIGURE 12.5 Idealized cross section of a selectively permeable membrane with channels for ions to cross the
membrane. The thickness of the membrane and the size of the channels are not drawn to scale. When the diagram
is drawn to scale, the cell membrane thickness is 20 times the size of the ions and 10 times the size of the channels,
and the spacing between the channels is approximately 10 times the cell membrane thickness. Note that a potential
difference exists between the inside and outside of the membrane, as illustrated with the
and - signs. The mem-
brane is selectively permeable to ions through ion-specific channels—that is, each channel shown here only allows
one particular ion to pass through it.
þ
Active Channel
Closed Active Channel
K +
Cl
K +
Outside
Cl
Inside
K +
K +
FIGURE 12.6 Passive and active channels provide a means for ions to pass through the membrane. Each channel is
ion-specific. As shown, the active channel on the left allows
K þ to pass through the membrane, but the active channel
on the right is not open, preventing any ion from passing through the membrane. Also shown is a passive
Cl channel.
12.4 BASIC BIOPHYSICS TOOLS AND RELATIONSH IPS
12.4.1 Basic Laws
Two basic biophysics tools and a relationship are used to characterize the resting poten-
tial across a cell membrane by quantitatively describing the impact of the ionic gradient and
electric field.
Fick's Law of Diffusion
The flow of particles due to diffusion is along the concentration gradient, with particles
moving from high concentration areas to low ones. Specifically, for a cell membrane, the
flow of ions across a membrane is given by
J ð diffusion Þ¼ D d ½
dx
ð
12
:
1
Þ
Search WWH ::




Custom Search