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