Biology Reference
In-Depth Information
FIGURE 10.4 (a) The activity of ion pumps creates a transepithelial potential difference of 30 e 40 mVacross the
intact cornea (and up to 100 mV across skin). (b) Wounding the tissue allows current to flow, shorting out the
potential difference across the epithelium in the immediate vicinity of the wound.
chemotactic sensor systems. These views may be reconciled by showing that receptor redis-
tribution is secondary to cell polarization, or it may be that all of these mechanisms are used
at different times and places.
ELECTRIC FIELDS IN LIVING SYSTEMS
Biological membranes are equipped with selective channels and pumps that are designed
to separate ions and create chemiosmotic gradients. The separation of charges involved in the
production of these gradients gives rise to strong electric fields; the 60 mV potential differ-
ence that presents across a typical 7 nm thick plasma membrane produces a field-strength
of about 8,600,000 V/m, more than a thousand times the peak field strengths that can be
found underneath overhead power lines. Most biological electric fields are local and operate
over tiny distances but, where ion fluxes are maintained across a complete tissue, as is the
case for many epithelia, small electric fields can be detected at the tissue scale. Mammalian
skin and cornea, for example, have a potential of difference of up to 100 mV between the
outside (
), 19 generating a field strength of up to 1000 V/m. )
The maintenance of transepithelial potential differences depends absolutely on the resis-
tance of intercellular tight junctions to leakage of ions in the opposite direction to which they
are being pumped. Wounding such an epithelium allows ions to equilibrate again as they
flow in a 'wound current' through the breach in the tissue ( Figure 10.4 ). This collapses the
) and the inside (
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) This can be demonstrated by placing a high-impedence (battery-operated) voltmeter between a saline
solution in which a finger is dipped and a fine hypodermic needle that is stuck through the skin of the hand,
preferably though an insulating layer such as candle wax. (The author and publisher stress that you, and not
they, are responsible for seeing that you meet all local ethical and safety policies if you do this.)
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