Biomedical Engineering Reference
In-Depth Information
transferred from one solution with a concentration c 1 to a solution with concentration
c 2 is easily calculated as
c 1
c 2
W
=
nRT ln
(1.4)
where c 1 is the salt concentration in the cell and c 2 is the salt concentration in the
extracellular space.
Osmotic pressure is also used by the cells of plants and, in particular, trees. Tree
roots have a high osmotic pressure inside them which leads to absorption of water
from the soil. A key role is also played, it is believed, by osmotic pressure in the
growth of plants. The openings on the surfaces of cell leaves, called stomata, are
bordered by guard cells that can regulate their internal pressure by controlling the
potassium concentration. Water absorption causes these cells to swell under osmotic
pressure and the stomata are closed.
Contained within the cytoplasm are the components of the cytoskeleton and certain
smaller compartments known as organelles which are specialized to perform their
respective functions. We refer the reader to cell biology and cell biophysics text
books for information on these important subcellular structures.
A typical cell membrane maintains a transmembrane potential which is of the
order of 100 mV. The value of this potential varies between different cells. The
transmembrane potential across cancer cell membranes may vary dramatically from
the normal cell membranes due to different electrical and metabolic conditions. Sig-
nificant depolarization of the membrane potential has been found in cancerous breast
biopsy tissues and in transformed breast epithelial cells when compared to normal
cells [ 10 ]. In the case of mitochondria, a proton gradient exists across the mito-
chondrial inner membrane which determines the membrane potential there. An early
stage study [ 8 ] suggests that mitochondria-specific interactions of cationic fluores-
cent probes (molecules) are dependent on the high transmembrane potential (negative
inside the membrane) maintained by functional mitochondria. Marked elevations in
mitochondria-associated probe fluorescence have been observed in cells engaged in
active movement. These results obtained through various investigations suggest that
membrane potentials vary considerably between various types of membranes, e.g.,
normal cell membranes, cancerous cell membranes, mitochondrial membranes, etc.
Like membrane potentials, the thicknesses of various membranes in normal cells,
cancerous cells, mitochondria, etc., also vary on a nanometer (nm) scale. The mem-
brane thickness is of the order of 3-6 nm. Taken together, the membrane potential
being of the order of 100 mV and the membrane thickness of the order of several nm,
results in the electric field across the cell membrane being in the range of
10 7 V/m.
It is worth relating the above number to our everyday experience where the elec-
tric potential we use is 120 V in the Americas and 220
240 V in the rest of the
world for lighting homes and offices. A comparison between the above potentials
suggests that a cell membrane appears to act like a cellular power plant. Nature
has given us this energy-generating nanoscale component which is present in each
of our body's cells. The electric energy created in this power plant is enough to
regulate the functions of many biological processes such as ion movements across
 
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