TITLE:  Single site channel - ensemble

:This program models the behavior of an ensemble of channels.  The channels
:alternate between closed and open states, and when open permit interaction with
:a solute particle from either bath.

PARAMETER

{
	a	=	1e11 (fmol/cm3)  :concentration of solute in bath A
	b	=	0 (mol/cm3)      :concentration of solute in bath B
	k12	=	1 (/usec)
	k21	=	1 (/usec)
	k232	=	2.5e-12 (cm3/fmol/usec) :second order rate constant
	k32	=	1 (/usec)

:Definition of initial values of pseudo-first order rate constants and flows
	k23A0	=	2.5e-01 (/usec)
	k23B0	=	0 (/usec)
	j_AS0	=	0 (fmol/cm2/usec)
	j_SB0	=	0 (fmol/cm2/usec)
}

ASSIGNED
{
	k23A	(/usec)
	k23B	(/usec)
	j_AS	(fmol/cm2/usec)
	j_SB	(fmol/cm2/usec)
}

INDEPENDENT

{
	t FROM 0 TO 1 WITH 1000 (usec)
}

PLOT s1, s2, s3, j_AS, j_SB VS t

STATE
{
	s1 FROM 0 TO 1.2e-1 START 1e-1 (fmol/cm2) :closed channels
	s2 FROM 0 TO 1.2e-1             (fmol/cm2) :open channels (unoccupied)
	s3 FROM 0 TO 1.2e-1             (fmol/cm2) :open channels (occupied)
}

DERIVATIVE state
{
	k23A	=	k232 * a
	k23B	=	k232 * b

	s1'	=	-k12 * s1 + k21 * s2
	s2'	=	k12 * s1 - (k21 + k23A + k23B) * s2	+ k32 * s3
        s3'     =                        (k23A + k23B) * s2	- k32 * s3

	j_AS	= 	k23A * s2 - (k32/2) * s3
	j_SB	=	(k32/2) * s3 - k23B * s2
}

BREAKPOINT
{
  SOLVE state
}