Effects of pH and light on the membrane conductance measured in the acid and basic zones ofChara |
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| Authors: | J R Smith N A Walker |
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| Institution: | (1) Biophysics Laboratory, School of Biological Sciences A12, University of Sydney, 2006, New South Wales, Australia;(2) Present address: Biophysics Laboratory, School of Physics, University of New South Wales, 2033 Sydney, N.S.W., Australia |
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| Abstract: | Summary The area-specific coductance of the membrane in the acid and basic zones (denoted byG
A
andG
B
, respectively) ofChara cells was measured in flowing solutions, containing 5mm zwitterionic buffer, as a function of the external pH(denoted by pH0). During illuminationG
A
was  1 S/m2 for pH0 in the range 5 to 8.5, and increased markedly to 3 to 6 S/m2 at higher pH0.G
B
, however, was always larger thanG
A
during illumination with a typical magnitude of 5 to 15 S/m2 for pH0 6 to 12. Thus under many experimental conditions it is possible that there is no single correct value for the membrane area-specific conductance. A flow of current in the external medium between the acid and basic regions was found to be associated with pH  banding , and also withG
B
exceedingG
A
. This current could be present in flowing solutions without added HCO
3
–
over a wide range of pH0 and at high (25mm) buffer concentration. Combining measurements ofG
A
andG
B
with measurements of the currents in the acid and basic zones (denoted byJ
A
andJ
B
, respectively), it was estimated that the resting (i.e. in the absence of net current flow) potential difference (PD) across the membranes within the individual zones (denoted byU
A
andU
B
) was –265±20 and –183±5 mV, respectively, during illumination. Upon the removal of illumination at pH0-7.5,G
A
,G
B
andJ
B
were found to decrease rapidly during the initial few hundred seconds. During this period (U
B
–V
m
) remained relatively constant. A transient hyperpolarization ofV
m
often occurred, the magnitude of which was correlated with the magnitude ofJ
B
prior to the removal of illumination. After some 0.5 to 1 ksec of darkness,G
A
andG
B
had both decreased considerably and nowG
A
G
B
andU
A
U
B
V
m
. Eventually, after 2 to 8 ksec of darkness, the membrane conductance was effectively homogeneous with a much smaller magnitude (typically<0.2S/m2) andV
m
was depolarized by typically 5 to 15 mV. |
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| Keywords: | conductance Chara banding membrane pH light current |
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