Ion and Solute Transport across Plant Cell MembranesHORT 301 – Plant Physiology

October 10, 2007Taiz and Zeiger, Chapter 6, Web Chapter 2 (p 1-10), Web Topic 6.3

paul.m.hasegawa.1@purdue.edu

Plant Mineral Nutrition Lectures

Lecture 1 – mineral nutrients/essential elements, plant nutrient status, acquisition by plant roots

Lecture 2 – transport across cell membranes, transport proteins

Lecture 3 – mineral nutrient absorption, assimilation and translocation

Mineral Nutrient (Ion) and Solute Transport across Cellular Membranes

Transport into or out of plant cells is across the plasma membrane lipid bilayer, composed of phospholipids

Phospholipids – fatty acids are linked to two carbons in glycerol (ester linkages) and a head group (phosphate, phosphate-linked molecule) is attached to the third carbon

Fatty acids - hydrophobic membrane region (low or no affinity for water) and head groups - hydrophilic membrane region (affinity for water)

1.5 (C) Chemical structures and space-filling models of typical phospholipids

Plasma membrane - lipid molecules are joined end-to-end in the hydrophobic regions, with the hydrophilic phospho head groups on the outside of the membrane, i.e. cytoplasm and apoplast (outside)

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

Hydrophobic region - restricts diffusion of ions and other water soluble molecules directly across the membrane bilayer solution

Transport proteins - embedded across the membrane and have specific affinities for ions and other solutes, i.e. Na+, sucrose, etc.

Chemical potential (j mol-1) is the driving force (energy) for ion and solute transport across membranes – chemical potential components are:Solute potential Hydrostatic/pressure And electrical potentialGravity effect is negligible

Passive transport is down (with/downhill) the chemical (electrochemical) potential gradient and active transport is up (against/uphill) the chemical potential gradient

6.1 Relationship between chemical potential and the transport of molecules

Water (H2O) - chemical potential = water potential (w)

w = solute/osmotic potential (s) + hydrostatic pressure/pressure potential (p)

H2O - no net charge (localized positive and negative charges) = no electrical potential

Ions and solutes - negligible hydrostatic pressure/pressure potential

Uncharged solutes have no electrical potential like water, e.g. sucrose, starch

Charged solutes – electrical gradient contributes to the chemical potential to drive transport of ions (charged atoms or molecules)

Mineral nutrients are absorbed by plants as ions, electrical potential

Membrane potential – electrical gradients that buildup across a membrane, differential accumulation of ions on sides of the membrane

Chemical potential (electrochemical potential) of ions:

Δµ (electrochemical potential) = RT ln Ci/Co (concentration activity) +

zF∆E (electrical potential)

Ci and Co – concentration inside and outside, respectively, R – gas

constant, T – temperature (°K), z = electrostatic charge of the ion (+ or -), F = Faraday’s constant, ∆E = membrane potential

Electrical potential across the plasma membrane of plant cells (steady-state) is inside negative, about -120 mV

Therefore, cations (positively charged ions) move passively into the cell, even against a concentration gradient, and anions (negatively charged) must be actively transported into the cell

Transformation of an electrical gradient (membrane potential) into a concentration gradient – at equilibrium defined by the Nernst equation, ∆E = -2.3RT/zF log Co/Ci

Univalent cation (+, e.g. Na+) - membrane potential of about -59 mV (inside negative) = the energy to drive a 10-fold concentration gradient

∆E = 59 mV log Co/Ci, membrane potential of 59 mV (Co/Ci = 10, log 10 =

1)

Membrane potential across the plasma membrane – usually about -120 mV, Na+ accumulates 102 (100-fold) greater concentration in the symplast relative to the apoplast based on the electrical potential

For a univalent anion (-, e.g. Cl-) a membrane potential of -120 mV (inside negative) requires that a Cl-apoplast must be >100X relative to Cl-

symplast for passive transport

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

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

Each ion has its own electrochemical potential

Differential membrane permeability for an ion causes a membrane potential

Movement of Ions with the electrochemical potential gradient

Major cellular ion (solute) compartments of a cell - apoplast, cytosol and vacuole

Apoplast - variable in size relative to the symplast

Symplast - cytosol = 5-10% and vacuole = 90-95%, fully expanded cell 6.4 Ion concentrations in the cytosol and the vacuole

Active proton (H+) transport generates membrane potential (electrical) and pH (H+) gradients that facilitate passive and active transport of ions and solutes - H+ electrochemical potential gradient

Transport across these membranes is mediated by electrongenic H+-ATPases and pyrophophatases, pumps

Electrogenic transport - transfer of charged atoms/molecules unequally across a membrane, causing a membrane potential and a concentration gradient for that ion

H+-ATPases and pyrophosphatases - hydrolyze high energy phosphoester bonds, ATP or pyrophosphate (PPi)

Energy from hydrolysis is used for active transport of H+s (against the H+ electrochemical potential gradient, uphill) to establish membrane potential and pH gradients

Membrane potential and pH gradients drive transport of nearly all ions and solutes in plants, via transport proteins

Exceptions:Ca2+-ATPases and ATP–binding cassette (ABC) transporters that mediate active transport of Ca2+ and macromolecules, respectively

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

Distribution of some essential mineral nutrients within the cell, e.g. transport across the plasma membrane for cations (Na+, K+, Ca2+) is passive (dashed) and for anions (Cl-, NO3

-, H2PO4-) is active (solid)

6.4 Ion concentrations in the cytosol and the vacuole

Transport Proteins – individual proteins or multi-subunit structures (quaternary structure) that are embedded in the membrane

Facilitate passive and active transport across membranes

Transport proteins are usually highly specific – transport a particular ion or solute with high specificity, tightly control active or passive transport of ions and solutes

~450 Arabidopsis genes encode transport proteins

Transport protein categories – channels, carriers and pumps

6.7 Three classes of membrane transport proteins: channels, carriers, and pumps

Channel – selective pore that transports an ion or solute by diffusion (passive), usually restricted to ions or small molecules

Transport is due to gating (opening and closing of the channel pore)

The “gate” is a component of protein structure – gating is regulated by stimuli, voltage (membrane potential changes), osmotic, hormones, Ca2+, light

Specific channels may transport ions or H2O inwards (inward

rectifying) or outwards (outward rectifying)

Carrier – specific substrate binding site on one side of the membrane, protein undergoes a conformational change that exposes the substrate to the opposite side

Substrate binding site confers high specificity (affinity) for transport

Transport rate of carriers is between 100 to 1000 ions or molecules per second, about 106 times slower than transport through channels

Carrier mediated transport is passive diffusion (uniport) or

6.11 Two examples of secondary active transport coupled to a primary proton gradient

Secondary active (discussed after pumps) e.g. symport or antiport

Pump – transport protein that couples energy production to the movement of a solute against the chemical (electrochemical) potential, primary active transport

Proton (H+)-ATPases (plasma membrane and tonoplast) - most common pumps in plants, plasma membrane and tonoplast membrane

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

Transport protein categories – channels, carriers and pumps

6.7 Three classes of membrane transport proteins: channels, carriers, and pumps

H+-ATPases transducers of ATP hydrolysis → ADP + Pi - unidirectional electrogenic H+ transport, pH gradients and membrane potential across the plasma membrane (apopolast) and tonoplast membrane (vacuole)

6.14 Overview of the various transport processes on the plasma membrane and tonoplast (Part 1)

Tonoplast pyrophosphatase - H+ pump hydrolyzes PPi to 2Pi, energy is used electrogenic H+ transport to the inside of the vacuole

H+ pumps generate proton (H+) electrochemical gradients (ΔpH and membrane potential) across membranes that facilitate secondary active ion and solute transport in plants

ATP binding cassette (ABC) transport proteins – active transport of large molecules (secondary products, flavonoids, anthocyanins, xenobiotics) by the transduction of energy from ATP hydrolysis

Ca2+-ATPases - localized in the plasma membrane, tonoplast membrane and endomembranes, couple ATP hydrolysis to active transport of Ca2+ from the cytosol

6.14 Overview of the various transport processes on the plasma membrane and tonoplast (Part 1)

Primary and Secondary Active Transport of Ions and Solutes – active transport mechanisms in plants

H+ pumps - primary active transport to generate H+ electrochemical gradients (ΔpH and membrane potential) across the plasma membrane and tonoplast

Plasma membrane – ΔpH ~2 units (apoplast - pH 5.5 and cytosol - pH 7.2, membrane potential ~-120 mV (cytosol negative relative to apoplast)

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

Tonoplast - ΔpH ~2 units (vacuole - pH 5.5 and cytosol - pH 7.2 – cytosol), membrane potential ~+30 mV (vacuole positive relative to cytosol)

Electrophoretic flux – passive transport of an ion that at equilibrium is defined by the Nernst equation,

-120 mV (inside negative) - K+ can accumulate to 100-fold in the cytosol relative to the apoplast

Secondary active transport (carrier mediated) – active transport of an ion or solute (against the electrochemical gradient) by coupling to passive transport of H+

s (down the H+ electrochemical gradient)

Antiporter - H+ and ion/solute transport is in the opposite direction, H+ electrochemical gradient is greater than the electrochemical gradient of the substrate

6.10 Hypothetical model for secondary active transport (Part 1)

Red arrow denotes the direction of the electrochemical gradient

Symporter – H+ and ion/solute transport is in the same direction

6.10 Hypothetical model for secondary active transport (Part 2) 6.11 Two examples of secondary active transport coupled to a primary proton gradient

Transport proteins in planta, expression in heterologous systems or loss-or gain-of-function genetics

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