Download Class Notes 2

Water and Ion Transport in Plants
Differences Plants/Animals
1. The dominant ATPase is
H+ ATPase not Na/K ATPase
2. Both Cell walls and cell membranes control movment, especially
of water
3. Plant physiologists view water potentials not osmotic potentials.
Water will move from the area of as higher potential (less solute) to
an area of lower potential (more solute). If there is a hydraulic
pressure on the water -- a push-- its potential is greater. If there is a
negative pressure -- a draw--its potential is reduced.
A megapascal is
aproximately 10
atmospheres.
4. Plants cells are a
three compartment
system -- they have a
large central vacuole
separated from the
cell by the
membrane of the
tonoblast.
Lateral Transport
Water can enter the
cells from root hairs.
Roots concentrate k
and remove Na.
Surface area is
increased by the
apoplastic route.
Lateral movement of water and ions can be1) through the a
cell and across all membranes
2) Via the symplast, a continuum of cytoplasm within plant
tissue connected by plasmodesmata (cytoplasmic channels)
3) Via the apoplast, an extracellular route along the cell walls.
Ions are processes as they cross membranes, so routes 1 and 2
mean more control. Access to the central stele of xylem
vessels is only via 1 and 2 since the Casparian strip blocks
flow from the apoplast.
Root Pressure
The center of the root with its xylem is called the stele. The
area behaves like an osmometer. At night when
transpiration is low, root cells are still pumping fluid and
minerals in. The endodermis prevents a back leak of ions so
the water potential factors movement into the xylem.
Guttation is the exudation of droplets from this root
pressure
Xylem movement of water can
be 15 meters/hour and plants
can loose 200 liter/hour in the
summer.
Xylem movement is due to
root pressure and to
transpiration.
Ion Movement in Excitable Plants
Action potentials traveling through
the Mimosa allows it to appear
dead when touched.
Venus flytrap’s lobes are
covered with trigger
hairs. If an insect steps on
two hairs or the same
hair twice, an action
potential is generated.
Water movements follow
ion movements and the
grasshopper is captured.
The alga Chara (duckweed) responds to
environmental stimuli, and mehcanical
stimulation. An action potential across the cell
membrane causes the internal fluid to jel and
prevents it from leaking out small holes or
tears. So the plant avoids loss of fluids – it
eaten by a duck !
In a cell at rest, the protoplasm streams around the
cell at 100 u/sec. When the cell is damaged, an action
potential is generated and the streaming stops.
Protoplasmic streaming is produced by actinomyosin
as found in animal muscle. Streaming is inhibited
when Ca++ moves into the cytoplasm activating a
protein kinase that phosphorylates myosin so it can’t
bind with actin.
Cytoplasm
Vacuole
In Chara , ions are transported to and from
the cytoplasm across two membranes, the
external plasma membrane and the
internal vacuolar membrane.
There are three kinds of potential :
The Resting Potential
The Receptor Potential
The Action Potential
Receptor potentials are caused by changes in
temperature, UV, mechanical stimulation but they
decay with time and distance.
Action potentials are triggered by the receptor
potentials and travel over long distances unchanged
in magnitude.
The Resting potential across the plasma
membrane is -180 mV (cytoplasm
negative)The Resting potential across the
vacuolar membrane is -10 mV
(cytoplasm negative)
At Rest
V
-180 mv
cytoplasm
negative
V
V
-170 vacuole
negative
-10 mV
cytoplasm
negative
In an action potential, the difference in voltage across the
plasma membrane disappears. The -180mV becomes 0 mV.
The vacuolar membrane becomes more negative on the inside
from -10 mV to -50 mV. (Actually the vacuole is becoming
more positive with respect to the rest of the system.
The AP has two components -- ion movement across the
plamsa membrane (fast)and ion movment across the vacuolar
membrane (slow).
Ion Distributions
Na
1:50:340
K
1:1,100;1,030
Cl
1:55:405
Ca
100:1:1200
Nernst Equilibrium Potentials
Ena = RT/zF ln [Nao/Nai]
at 20oC Ena= 58(log) [1/50] or -98.5mV
Equilibrium Potentials
Na
-98.5 mV
K
-180 mV
Cl
+103 mV
Ca
+59 mV
If suddenly Na gates were open in the
plasma membrane, it would move out of the
cell until the cell sap became -98mV with
respect to the outside. Then the negativity of
the inside would balance the concentration
gradient and Na movement would stop
K would move out until the inside was -180
mV. Since this is the resting potential, K
moves little
In the AP the PD across the plasma membrane goes
from -180 to 0. NA would only take it to -100, K would
not move it from -180. Ca++ moving in would take the
PD to +59 and Cl moving out would take it to +103. So
movement of either ion is possible in an AP.
Using an isotope to sort out the differences, it was found
that Cl leaves the cell in an AP, but Ca++ is required to
keep the channel open.
In an action potential, the difference in voltage across the
plasma membrane disappears. The -180mV becomes 0 mV.
The vacuolar membrane becomes more negative on the inside
from -10 mV to -50 mV. (Actually the vacuole is becoming
more positive with respect to the rest of the system.
The AP has two components -- ion movement across the
plamsa membrane (fast)and ion movment across the vacuolar
membrane (slow).
A trigger allows Ca++ to enter
from the external fluid
Cl leaves the cytoplasm in Ca++
dependent channels.
Cl also moves into the cell
from the vacuole, K moves out
across both membranes ,
Ca++ ions are pumped out,
protoplasmic streaming starts
again.