Introduction to Health Physics: Fourth Edition - Ruang Baca FMIPA UB

Na +
K +
BIOLOGICAL BASIS FOR R ADIATION SAFETY 287
pure-water side. Water will therefore flow through the semipermeable barrier from
the pure-water side into the solution side by osmosis, and the water in the solution
arm will rise. Water will continue to flow through the barrier, and the solution column
will continue to rise, until the hydrostatic pressure due to the increased height of
the column of water is great enough to prevent further inflow of water through
the semipermeable barrier. The hydrostatic pressure needed to prevent further flow
through the barrier is called the osmotic pressure of the solution. Osmotic pressure
is determined by the concentration of the solution and increases with increasing
concentration.
Biological membranes may be selectively permeable to various different
molecules. This selective permeability, which allows only certain ions to pass, is
responsible for the electrical potential differences that form the basis for the operation
of the nervous system and muscles, including the cardiac muscle. Consider
the arms of a U tube that are separated by a membrane that allows only K + ions to
pass through, as in Figure 7-3. Let us pour a solution containing a relatively high
concentration of KCl and a low concentration of NaCl into one arm of the U tube,
and a KCl–NaCl solution that is relatively more concentrated in Na + than in K +
into the other arm. The solutions on both sides of the membrane are electrically
neutral. However, since the membrane is permeable to K + ions, the K + ions will
diffuse through the membrane from the more to the less concentrated K + solution.
This transfer of K + ions results in a net loss of positive ions on one side of the membrane
and a net gain of positive ions on the other side, thus upsetting the electrical
neutrality. Since one side will now have a net positive charge and the other a net
negative charge, these opposite charges will attract each other and will migrate to
the membrane separating the two solutions. This accumulation of positive ions on
one side and negative ions on the other side of the membrane leads to an electrical
potential difference across the membrane. This potential difference continues to
increase as more K + ions move through the membrane until the built-up positive
potential is great enough to prevent more positively charged K ions from passing
through the membrane. Passage of ions from a region of high to low concentration
is accomplished without the expenditure of energy. However, transfer of ions against
a concentration gradient, which is called active transport, requires the expenditure
of energy.
Na +
K +
Cl - Cl -
–
–
–
–
–
–
–
–
+
+
+
+
+
+
+
+
Polarized
Membrane
Figure 7-3. Schematic illustration of ionic transport, in which
the membrane is permeable only to the K + ion. The accumulation
of positive charges on one side of the membrane and negative
ions on the other side leads to a potential difference across the
membrane.