when the only solute present is a weak acid , ha , which penetrates as molecules only into a living cell according to a curve of the first order and eventually reaches a true equilibrium we may regard the rate of increase of molecules inside as see pdf for equation where p ( m ) is the permeability of the protoplasm to molecules , m ( o ) , denotes the external and m ( i ) the internal concentration of molecules , a ( i ) denotes the internal concentration of the anion a ( - ) and see pdf for equation ( it is assumed that the activity coefficients equal 1 . )
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