Post on 21-Dec-2015
Water Transport into Plant Cells & Cell ExpansionHORT 301 – Plant Physiology
September 3, 2008Taiz and Zeiger - Chapter 3 (p. 41-52) & Chapter 15 (p. 364-372), Web
Topics 3.3 through 3.7, Passioura (2001) Encyclopedia of Life Sciencespaul.m.hasegawa.1@purdue.edu
Diffusion, Bulk Flow and Osmosis – water transport processes in plants
Water Potential Drives Water Transport Into and Out of Cells – chemical and pressure potentials that drive water movement
Water Potential and Turgor Pressure: Cell Volume Regulation/Cell Expansion – plant fresh weight growth
Diffusion, Bulk Flow and Osmosis – major water transport processes in plants
Diffusion – random motion of molecules that results in net movement from high to low concentration
3.7 Thermal motion of molecules leads to diffusion
Diagram depicts red and blue molecules each concentrated to one side (initial), at an intermediate period and after equilibrium
Note: random motion continues even after equilibrium
Diffusion rate – concentration dependent
Fick’s law , Js = -Ds Δcs/ΔxJs – transport rate/flux density, amount of s crossing a unit area x per
timeΔcs – concentration gradient of substance sΔx – distance between the separated concentrationsDs – diffusion coefficient - capacity of a substance to move through a
specific medium- (minus sign) - indicates movement is down a concentration gradient
(high to low concentration), mol m-2 s-1
Diffusion facilitates 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
Bulk flow (mass flow) – movement of a large number of molecules en masse usually driven by pressure, but other forces (e.g. gravity) may drive bulk flow
Examples of bulk/mass flow of water – river flow, rainfall, and pressure- driven movement of water through a garden hose
Pressure-driven bulk flow is the primary process for long distance water transport, movement from roots to shoots through the xylem and water transport in soil
Osmosis – movement of water (any liquid) through a selectively permeable membrane, diffusion or pressure-driven bulk flow
Osmosis is the process that moves water into and out of living plant cells and, in most instances, is mediated by diffusion
Water diffuses across a plant membrane from high to low water concentration
Solutes decrease the water concentration of a solution
Water move across the plasma membrane (into or out of the cell) from higher → lower water concentration (lower → higher solute concentration)
Water channels (aquaporins) facilitate water (transport) across the plasma membrane – diffusion of water into or out of the cell from high to low water concentration
3.13 Water can cross plant membranes by diffusion
Aquaporin pore opening and closing (gating) - regulated by different stimuli, e.g. pH, Ca2+, etc, movement (transport)
Water channels (aquaporins) – transmembrane protein pores across membrane lipid bilayers
Water does not readily diffuse across the hydrophobic lipid bilayer
Water Potential (Ψw ) Is the Principal Force for Water Transport in Plants – nomenclature used by plant physiologists for the force that drives water transport
Water potential (Ψw) gradient defines the free energy gradient for
passive movement of water, higher to lower free energy
Water potential (Ψw) is the free energy of water per volume, expressed as pressure units, bar or pascal (Pa), one bar = 0.1 megapascals (MPa)
Reference state for water potential (Ψw) - pure water at ambient temperature and pressure, Ψw = 0,
Water containing solutes has a negative water potential (-Ψw), lower free energy than pure water
Pure water diffuses into aqueous solutions containing solutes (negative water potential, Ψw < 0 (-MPa)
Water transport Summary: higher → lower water concentration (lower → higher solute conc)higher → lower Ψw (more negative)
Plant cell water potential (Ψw) 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, 8.32 J mol-1 K-1
T – Kelvin (K), 0°C = 273 Kcs – osmolality (mol of solute per liter, ideal solute = 1, includes
dissociation constant correction)
Ψp (hydrostatic pressure/pressure potential/turgor pressure)
Pure water at ambient pressure - Ψp = 0
Positive hydrostatic pressure/pressure potential (push/turgor in cells)
Negative hydrostatic pressure/pressure potential (tension)
Ψg (gravity, gravitational pull) - causes water to move downwards but has negligible impact on water transport in plants
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, the following equation defines water potential: Ψw = Ψs + Ψp
See Web Topic 3.5 for discussion of matric potential, also disregarded for our consideration of water potential in plants
Water potential (Ψw) of a solution becomes more negative by the addition of sucrose
3.9 Five examples illustrating the concept of water potential and its components (Part 1)
Solution of 0.1 M sucrose: - Ψw = -0.244 MPaΨs = RTcs = -0.244 MPa, Ψp = 0 MPa-0.244 MPa (Ψw) = -0.244 (Ψs) + 0 (Ψp)
Pure water: water potential (Ψw) = 0Ψw = Ψs + Ψp = 0Ψs = 0 and Ψp = 0
Ψw = Ψs whenΨp = 0
Water transport into and out of plants cells is driven by the water potential (Ψw) gradient – passive, water moves from higher to lower (more negative) Ψw
Water movement into a cell - flaccid cell (Ψw = -0.732 MPa) immersed into a 0.1 M sucrose solution (Ψw = -0.244 MPa)
Water moves into the cell because the Ψw inside the cell (symplast) is more negative than outside of the cell (apoplast)
Flaccid cell: - Ψw = -0.732 MPa, Ψs = -0.732 MPa and Ψp = 0 (no turgor pressure)
Cell immersed into 0.1 M sucrose solution (Ψw = -0.244 MPa): water moves into the cell until the symplastic and apoplastic Ψw reach equilibrium, -0.244 MPa
Initially, the protoplasm volume increases until constrained by the cell wall
Water potential (Ψw) equilibrium is established due to hydrostatic pressure/turgor pressure (Ψp) build-up, Ψp = 0.488 MPa
3.9 Five examples illustrating the concept of water potential and its components (Part 2)
Cell walls facilitate Ψp increase
Ψp – driving force for cell expansion (volume increase), which is due to uptake of water, Ψw gradient between the apoplast and symplast
(0.1 M sucrose solution)
Water movement from the cell - turgid cell in a 0.1 M sucrose solution, then immersed into 0.3 M sucrose solution causing water loss and volume reduction
Turgid cell in 0.1 M sucrose solution: Ψw(apolast) = Ψw(symplast) = -0.244 MPa, Ψs = -0.732 MPa and Ψp = 0.488 MPa
Cell after immersion into 0.3 M sucrose solution:,Ψw(apoplast) = -0.732 MPa,Ψs = -0.732 MPa and Ψp = 0 MPa
Water potential equilibrium, - Ψw(apoplast) = Ψw(symplast) = -0.732 MPa, turgor pressure decreases to zero, water is lost until Ψw equilibrium (-0.732 MPa) is reached
3.9 Five examples illustrating the concept of water potential and its components (Part 3)
Water movement into and out of cells summary:
Water moves from higher (less negative) to lower (more negative) Ψw
Ψw(apolast) = Ψw(symplast)
No water movement
Ψw(apoplast) higher (less negative) than Ψw(symplast)
Water moves into the cell and turgor pressure increases
Ψw(apoplast) lower (more negative) than Ψw (sympast)
Turgor pressure decreases and water moves out of the cell
Water Transport into Plant Cells & Cell ExpansionHORT 301 – Plant Physiology
September 3, 2008Taiz and Zeiger - Chapter 3 (p. 41-52) & Chapter 15 (p. 364-372), Web
Topics 3.3 through 3.7, Passioura (2001) Encyclopedia of Life Sciencespaul.m.hasegawa.1@purdue.edu
Diffusion, Bulk Flow and Osmosis – water transport processes in plants
Water Potential Drives Water Transport Into and Out of Cells – chemical and pressure potentials that drive water movement
Water Potential and Turgor Pressure: Cell Volume Regulation/Cell Expansion – plant fresh weight growth
Water Potential and Turgor – Cell Volume Regulation and Cell Expansion – semi-rigid cell wall mechanically restricts water loss (cell volume reduction) as the plant is subjected to more negative Ψw, e.g. during daylight, drought
Turgor pressure (Ψp) – “buffers” the cell from water loss as apoplastic Ψw decreases (more negative)
Ψp decreases and Ψw equilibrium is established with minimal symplastic water loss
Water loss from the cell and reduction in cell volume increases as Ψp (turgor pressure) approaches 0
3.10 Hoffler diagrams
Plant wilts (becomes flaccid) when cell turgor pressure approaches 0
~15% volume reduction in this example
Ψw decreasing
Ψpdecreasing
Turgor pressure (p) pushes on walls forcing cell wall expansion
w gradient facilitates water uptake for cell volume increase/expansion (fresh weight gain)
Turgor and growth rate
TTurgor Pressure (MPa)
Growth rate is defined by the formula:GR = m(p –Y)
GR – growth ratem – wall extensibilityp – turgor pressureY – yield threshold
m is due to turgorpressure and biological processes (hydrolysis and biosynthesis
Plant water status affects critical physiological functions – water potential (Ψw) is a “signal” for numerous physiological processes
3.14 Water potential of plants under various growing conditions