Plant Mineral Nutrition: Solute TransportHORT 301 – Plant Physiology

September 22, 2010Taiz and Zeiger - Chapter 6, Appendix 1

paul.m.hasegawa.1@purdue.edu

1.5 (A) The plasma membrane, endoplasmic reticulum, and other endosomes

Integral membrane transport proteins – responsible for movement of ions across membranes

Molecular diffusion - net movement of mineral nutrients and other molecules down the chemical potential gradient, passive transport

6.2 Development of a diffusion potential and a charge separation between two compartments

K+ > in B right and Cl- > in left, electrical gradient

K+ and Cl- move across membrane, concentration gradient

No net ion movement across membrane

Chemical potential gradient () – forces that drive diffusionMineral ions and charged molecules – concentration and electrical potential gradientsNeutral molecules – concentration gradient, unaffected by charge

APOPLASTpH 5.5

CYTOSOLpH 7.2

ΔE=-100 to -200 mV

PLASMAMEMBRANE

Membrane potential gradient (E) – electrical potential gradientDifferential ion accumulation on sides of the membrane

Inside negative membrane potential across the plasma membrane

Antiporter

6.14 Overview of the various transport processes on the plasma membrane and tonoplast (Part 2)

APOPLASTpH 5.5

CYTOSOLpH 7.2

ΔE=-100 to -200 mVPLASMAMEMBRANE

pH gradient - primarily responsible for the plasma membrane potential gradientpH gradient requires energyChemical energy (ATP hydrolysis) is coupled to H+-transport against the electrochemical gradient

ADP + Pi

ATP

ATP

ADP + Pi

H+

H+

pumps

H+

6.14 Overview of the various transport processes on the plasma membrane and tonoplast (Part 2)

APOPLASTpH 5.5

6.14 Overview of the various transport processes on the plasma membrane and tonoplast (Part 2)

Ion and solute transport across the plasma membrane coupled to ∆pH 6.14 Overview of the various transport processes on the plasma membrane and tonoplast (Part 2)

Translation of a membrane potential gradient into a force for diffusion∆E can drive diffusion of ions

At equilibrium – ion concentration gradient is balanced by the voltage difference

∆E (electrical potential/membrane potential) = 2.3RT/zF log Co/Ci

At equilibrium 2.3RT/zF = 59 mV, monovalent ion

∆E = 59 mV log Co/Ci

if Co/Ci = 10, log 10 = 1then ∆E = 59 mV x 1

An inside negative, membrane potential of -59 mV (~60 mV) can translate into a 10-fold concentration difference (monovalent cation)

Plasma membrane potential effects on a monovalent anion(e.g. Cl-), a membrane potential of -120 mV (inside negative) requires that [Cl-]apoplast must be >100X relative to [Cl-]cytosol for passive transport

Each ion has its own electrochemical potential

Specificity is due to unique concentration activity

Divalent (Ca2+ or SO42-) ions have 2X the electrical potential

6.4 Ion concentrations in the cytosol and the vacuole

pH 5.5

pH 7.2

- 100/ -200 mV

pH 5.5

+30 mV

Passive and active ion transport across the plasma membrane and tonoplast Dependent on concentration and membrane potential gradientIntracellular distribution of essential elements due to passive (dashed, -----) or active (solid line, →) transport

Transport protein categories – channels, carriers and pumps

Channels – diffusion inwards or outwards across the membrane

K+ channel

Primary active transport – energy production is coupled to ion transport H+ electrochemical gradients across the plasma membrane and tonoplast

Smith et al. (2010) Plant Biology

(pH 5.2) -100 to -200 mV

Apoplast Cytosol

Vacuolar H+-ATPase

ADP + Pi

ATP

H+

PM H+-ATPase

Secondary active transporters (carriers) – couple H+ transport to ion transportDown the H+ electrochemical gradientSymporter – same direction, antiporter – opposite directions

Model of H+-sucrose symporter function

Raven et al. (2005) Biology of Plants

H+-ATPase and H+-sucrose symporter coordination

Transport proteins at the plasma membrane

Smith et al. (2010) Plant Biology

pH 7.4-100 to -200 mV

pH 5.5

Tonoplast transport proteins

Smith et al. (2010) Plant Biology

pH 7.2

6.14 Overview of the various transport processes on the plasma membrane and tonoplast6.14 Overview of the various transport processes on the plasma membrane and tonoplast

+30 mV

Radial ion transport from soil solution to the xylem