1.30 1.35 1.40 1.45 1.50 1.55 0.5320 0.5340 0.5360 Lp ( x10 -6 m s -1 MP a -1) 0.5380 0.5400 0.5420 0.5440 P1 (MPa)
Fig. 4.16 Lpvs. P1 for fits to HPP Run 5, Cell 4. This is a positive pulse where the maximum pressure m = 0.5425MPa.
was or s MPa ), and corresponds to a fit to the later regions of the P
In §4.6.2b it was found that estimated Lp values from HPP experiments were higher
than that from OPP experiments for fits using the UL model with 2 ULs (Table 4.19 and Table 4.20). For the data set fit here, when (t0, P1) = (tm, Pm) the estimated Lp was
1.52 m s-1 MPa-1. However, when (t0, P1) = (tm +1.07, 0.5328), the estimated Lp
1.29 m s-1 MPa-1. This value is close to the average estimated Lp for the OPP data f
-1 -1 Cell 4 (of 1.31 m
observed HPP data (Fig. 4.17). One may hypothesise that Lp for both HPP and OPP
experiments are in fact the same for the same cell, but that the model has trouble fitting the initial steeper region. Lpas defined by the KK equations may govern the
ig. 4.17 Fit to an HPP data set using the UL model, where the fit begins at a point after (tm, Pm,) = (0,
.5425), namely (t0, P1,) = (1.07, 0.5328). Showing raw pressure data (---) and model fit (——).
r
han
• Limitations in the applicability of the KK equations. Evidence includes difficulties in fitting the shoulder of the HPP curve (§4.5.3, §4.6.1) and the superior fit given by a double exponential (§2.7.3a). A double exponential was also fit to an HPP curve for the data used in this chapter, and found to give a superior fit.
• Neglect of other dynamics in the experiment, such as possible pressure changes during the perturbation pulse, presence of some permeant solutes in the cell in HPP experiments (§4.8.6), influence of the tonoplast on the pressure
F 0
b) Differences in Lp between HPP and OPP experiments
The parameters Lp, ps and σ, being properties of the membrane, should be constant fo
each cell, assuming that there are no external factors affecting the parameters such as temperature changes or leaks in the apparatus. The results above in §4.8.3a lend weight to this claim for the case of Lp. Differences observed between HPP and OPP
experiments in the present study may be due to inadequacies in the model rather t a real physical difference in the parameters. Inadequacies may include:
dynamics, and influence of different values of D between APW and the cytoplasm (§4.8.4) e as that f used fo param Mem -9 2 -1 -9 2 -1
DULiby 1% resulted in a 0.7% change in Lp, a 1.2% change in ps, and a
D for the cell interior portant in parameter estimation. Using an incorrect value of D for the ULi may
ere e no current available data on the value of D for the interior of Chara, so the actual ex
As in §3.6.6, predictions
(Eq. (3.2)) were also calculated (Tab
significantly from m s given by the classical model, for which ps
= pT. This confirm again that Eq. (3.2) cannot be used to infer the membrane
permeability p (= p)based upon an estimated value of p obtained using the classical m
4.8.4 Impact of different D values on estimated parameters
It has been assumed that the diffusion coefficient D for the cell interior is the sam or APW. In fact it is likely to be less, and the extent to which values of D r the ULi may impact on the estimated parameters for fits to the data, using the UL model, are examined here. This analysis differs from that in §3.6.6 in that here
eters are estimated by fitting the model to observed data.
brane parameters were estimated for an OPP experiment (OPP Run 8 of Cell 2) for 3 different values of D in the ULi (DULi), where DULi = 0.9, 1.1, and 1.28 x 10
m s . D for the ULe (DULe) was kept constant at 1.28 x 10 m s . It was found that
the estimated membrane parameters varied significantly with DULi (Fig. 4.18).
Changing
1.2% change in σ. These results indicate that quantification of is im
also be a contributing factor to the finding that Lp values were higher for the HPP
experiments as compared to the OPP experiments (§4.6.2b, §4.8.3b), as ULs have a greater influence on the parameter estimation in OPP experiments than HPP
experiments (§4.6.1b). For OPP experiments, Fig. 4.18 reveals that a lower value of D for the ULi leads to a lower value of Lp, and results from the analysis in §3.6.6a
suggests a monotonic decrease of Lp with decreasing D for the ULi. However, th
appears to b
tent of the influence of D for the ULi cannot be verified.
of the total permeability pTusing the permeability equation
le 4.27). It was found that values of pT differed
that of 4.16 x 10-6 -1 -1 s
m s T
betwee strongl
1
Ta
n ps and pT in Table 4.27 also indicates that the permeability of the ULs are
y limiting transport across the membrane for this cell.
Fig. 4. 8 Plots of Lp (——), ps(——), and σ (——)vs. D in the ULi,for fits to OPP Run 8 of Cell 2.