1. Water Movement through Plant Cells HORT 301 – Plant Physiology
September 24, 2007
Taiz and Zeiger, Chapter 3 (p ), Web Topics 3.3 through 3.7

2. Diffusion, pressure-driven bulk flow and osmosis
Water potential drives water movement (transport) into and out of cells
Cell volume homeostasis
Water movement (transport) in plants
Plant water status affects critical physiological processes

3. Diffusion, pressure-driven bulk flow and osmosis – water movement (transport) processes
Diffusion – net movement of molecules from high concentration to low concentration, down a concentration gradient

4. Diffusion rate - movement (transport) of a substance, concentration dependent, Fick’s law , Js = -Ds Δcs/Δx
Δcs – concentration gradient of substance s
Δx – distance moved
Ds – diffusion coefficient - capacity of a substance to move through a medium
Minus (-) sign - indicates movement is down a concentration gradient (high to low concentration)
Diffusion facilitates substance/solute movement over short distances, i.e. into and out of cells and within cells, but is too slow for long distance transport (root to the shoot)
Glucose diffusion across the cell (~50 µm) takes 2.5 seconds but over one meter takes 32 years

5. Pressure-driven bulk flow (mass flow) – movement of a large number of molecules en masse that is driven by pressure
Example - pressure driven movement of water through a garden hose
Bulk flow is the primary process for long distance water transport in plants (movement from roots to shoots through the xylem) and water transport in soil

6. Osmosis – movement of water (any liquid) through a selectively permeable membrane, diffusion or pressure-driven bulk flow
Plasma membrane (other cell membranes) - semi-permeable to water and solutes
Osmosis is the process by which water moves into and out of a plant cell
Water diffuses across a plant membrane from high to low water concentration
Solutes decrease the water concentration of a solution
i.e. water moves to the side of a membrane with higher solute conc

7. Water potential, driving force of water movement (transport) into and out of plant cells – water potential (Ψw), nomenclature used by plant physiologists, principal force responsible for osmosis into and out of plant cells
Thermodynamically, water potential (Ψw) gradient defines the free energy/chemical potential gradient for passive movement of water, higher to lower free energy

8. Water potential (Ψw) is the free energy of water per volume, expressed as pressure units, bar or pascal (Pa), one bar = 0.1 megapascal (MPa)

9. Reference state for water potential (Ψw) - pure water at ambient temperature and pressure, Ψw = 0,
i.e. water containing solutes has a negative water potential (Ψw)
Pure water diffuses into aqueous solutions containing solutes (negative water potential (lower free energy), Ψw < 0 (-MPa)

10. Water potential (Ψw) in plants is affected primarily by solute concentration, pressure and gravity
Ψw = Ψs + Ψp +Ψg
Ψw (water potential) - free energy of water, Ψw of pure water = 0, more negative water potential (Ψw) is lower free energy
Ψs (solute (osmotic) potential) – solute concentration effect on Ψw, dissolved solutes lower free energy of water by reducing the concentration of water
van’t Hoff equation - Ψs = RTcs
R – gas constant, T – Kelvin, cs – osmolality

11. Ψp (hydrostatic pressure/pressure potential/turgor pressure)
Ambient pressure = 0
Positive hydrostatic pressure/pressure potential (turgor in cells) - push
Negative hydrostatic pressure/pressure potential (tension) - pull

12. Ψg (gravity, gravitational pull) - causes water to move downwards but has negligible impact on water transport in plants
i.e., the effect of gravity on water at the top of 10 m column is 0.1 MPa, seawater has a solute potential of ~-2.8 MPa
So for functional simplification - Ψw = Ψs + Ψp
See Web Topic 3.5 for discussion of matric potential, also disregarded for our consideration of water potential in plants

13. Water transport into and out of plant cells – passive, along (down) the water potential (Ψw) gradient, higher (less negative) to lower (more negative) Ψw and free energy
Water potential equilibrium (outside and inside of the cell) - Ψw(outside/apoplast) = Ψw(inside/symplast), no water movement

14. Water potential (Ψw) of a solution becomes more negative by the addition of sucrose
Pure water - water potential (Ψw) = 0, Ψw = Ψs + Ψp = 0
Ψs = 0 and Ψp = 0
Solution of 0.1 M sucrose - Ψw = MPa
Ψs = RTcs = MPa, Ψp = 0 MPa

15. Water movement into a cell along the water potential (Ψw) gradient - flaccid cell immersed into a 0.1 M sucrose solution
Water moves into the cell because the Ψw inside the cell (symplast) is more negative than outside of the cell (apoplast)
Flaccid cell - Ψw = MPa, Ψs = MPa and Ψp = 0 (no turgor)
Cell immersed into 0.1 M sucrose solution (Ψw = MPa)
Water moves into the cell until the symplastic and apoplastic Ψw reach equilibrium, MPa

16. Initially, the protoplasm volume increases until constrained by the cell wall
Water potential (Ψw) equilibrium is established due to hydrostatic pressure/pressure potential/turgor (Ψp) build-up, Ψp = MPa

17. Water movement from the cell - turgid cell in a 0
Water movement from the cell - turgid cell in a 0.1 M sucrose solution, then sucrose concentration (apoplast) is increased to 0.3 M
Turgid cell in 0.1 M sucrose solution, Ψw(apolast) = Ψw(symplast) = MPa
Cell after increasing solution to 0.3 M sucrose,
Ψw(apoplast) = MPa,Ψs = MPa and Ψp = 0 MPa)
Water potential equilibrium, - Ψw(apoplast) = Ψw(symplast) = MPa, turgor decreases to zero, water is lost until Ψw equilibrium ( MPa) is reached

18. Water movement into and out of cells summary:
Water moves from less negative to more negative Ψw
Ψw(apolast) = Ψw(symplast)
No water movement
Ψw(apoplast) higher (less negative) than Ψw(symplast)
Water moves into the cell and hydrostatic pressure increases
Ψw(apoplast) lower (more negative) than Ψw (sympast)
Hydrostatic pressure decreases and water moves out of the cell

19. Cell volume homeostasis - rigid cell wall regulates cell volume
Positive hydrostatic pressure/pressure potential/turgor (Ψp) – facilitates cell expansion
Increases the capacity of a cell to retain water (minimize volume reduction) as the apoplastic water potential Ψw fluctuates, e.g. light period of the day/night cycle, drought

20. Water loss (cell volume decrease) increases as Ψp approaches 0
Plant wilts (becomes flaccid) when cell turgor approaches 0

21. Water Movement through Plant Cells HORT 301 – Plant Physiology
September 24, 2007
Taiz and Zeiger, Chapter 3 (p ), Web Topics 3.3 through 3.7

22. Diffusion, pressure-driven bulk flow and osmosis
Water potential drives water movement (transport) into and out of cells
Cell volume homeostasis
Water movement (transport) in plants
Plant water status affects critical physiological processes

23. Water movement (transport) in plants – transport rate is dependent on the driving force (water potential) and hydraulic conductance
Jv = Lp(ΔΨw), Lp – hydraulic conductivity (resistances to water movement), ΔΨw – water potential gradient
Membranes are principal resistances to hydraulic conductance

24. Water channels facilitate water movement (transport) across the plasma membrane – into and out of the cell
Water channels (aquaporins) – membrane protein pores that facilitate water movement across lipid bilayer of membranes
Aquaporin pore opening and closing (gating) - regulated by different stimuli, e.g. pH, Ca2+, etc, movement (transport)

25. Plant water status affects critical physiological functions – water potential (Ψw) is a “signal” for numerous physiological processes