Water and Ion Transport in Plants

1. Water and Ion Transport in Plants

2. Differences Plants/Animals 1. The dominant ATPase is H+ ATPase not Na/K ATPase

3. 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.

4. 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.

6. Lateral Transport

7. Water can enter the cells from root hairs. Roots concentrate k and remove Na. Surface area is increased by the apoplastic route.

8. 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.

9. 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

10. 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.

21. Ion Movement in Excitable Plants Action potentials traveling through the Mimosa allows it to appear dead when touched.

22. 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.

23. 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 !

24. 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

25. In Chara , ions are transported to and from the cytoplasm across two membranes, the external plasma membrane and the internal vacuolar membrane.

26. 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)

27. At Rest -180 mv cytoplasm negative V V V -170 vacuole negative -10 mV cytoplasm negative

28. 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).

29. Ion Distributions Na 1:50:340 K 1:1,100;1,030 Cl 1:55:405 Ca 100:1:1200 Equilibrium Potentials Na -98.5 mV K -180 mV Cl +103 mV Ca +59 mV Nernst Equilibrium Potentials Ena = RT/zF ln [Nao/Nai] at 20oC Ena= 58(log) [1/50] or -98.5mV 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

30. 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.

31. 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).

32. 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.