EFB530 Plant Physiology

Solute transport

We have discussed the movement of water, which occurs by diffusion, by osmosis, or by bulk flow from the soil to the leaves; none of which utilize the direct input of energy to move water

Plants also take up the mineral nutrients they need from the soil

• such as nitrogen (as NO₃ ^-), potassium (as K⁺), calcium (as Ca²⁺), magnesium (as Mg²⁺), phosphorus (as H₂PO₄^-), sulfur (as SO₄^2-), or chlorine (as Cl^-)

Its very unlikely that the growing medium will provide the optimal concentrations of all of the minerals that a plant will need to carry out its metabolism

• some will be too high, more likely they will be too low in concentration

Plants need a mechanism to selectively concentrate or exclude minerals

• they do that at the level of the plasma membrane (root epidermis or endodermis)

When discussing the transport of dissolved solutes, we need to recall the properties of membranes

• selectively permeable

Membranes allow the slow diffusion of water and small, uncharged molecules, but are highly impermeable to the movement of charged molecules

Chemical potential

m = m^* + RTlnC + zFE + VP + mgh

• pressure and gravity terms are negligible

when discussing potential across a membrane, compare m^inside to m^outside

Dm = m^outside - m^inside = RTln (C^outside / C^inside) + zF (E^outside - E^inside)

therefore, chemical potential, which drives ion movement, is determined by both a concentration gradient and by an electrical gradient = electrochemical potential

this means that ions can move against a concentration gradient if a voltage is applied

we can solve for the electrical gradient term to get the Nernst potential (DE_n)

DE_n = (2.3RT)/zF log (C^outside / C^inside)

considering K⁺ (z=+1), DE_n = 59 log (C^outside / C^inside) [in mV]

therefore, by applying 59 mV, a 10-fold concentration difference can be maintained
[the log of 10 = 1]

all living cells maintain an asymmetric ion distribution across the plasma membrane, which results in an electrical potential across the membrane or membrane potential

• relatively easy to measure the membrane potential, sticking a microelectrode into a cell, with a reference electrode in the bathing solution, then measuring voltage
• can also measure the internal concentration of an ion, know the conc. in bath
• if the concentration gradient is appropriate for the membrane potential (according to the Nernst equation), then that ion is moving by passive transport

This potential can develop due to differential diffusion rates of differently charged ions

• for example: when there is a concentration difference of KCl across a membrane, the K⁺ ion may diffuse across the membrane faster than the Cl^- ion, generating a difference in the charge balance, which is called the diffusion potential

If an ion is maintained in a distribution inappropriate for its Nernst potential, then it is probably transported by active transport, requiring energy input to maintain a concentration against an electrochemical gradient

for example: measure the membrane potential of a pea root = -110 mV

• submerged in a known mineral solution
• predict (Nernst equation) what the internal concentration of a mineral should be
• measure the internal concentration of solutes

see Table 6.1

note the specificity for individual ions and ability to concentrate ions

ions flow across biological membranes faster than across synthetic lipid membranes

• movement of ions across a biological membrane occurs through membrane-bound proteins (three types)

• channels are selective for particular ions based on size and charge
• do not directly bind to the ion

• binding site confers specificity
• can act to transport a single molecule each time through passive diffusion
• or can transport two different molecules each time = cotransport
• can carry those in the same direction, as symports
• or in opposite directions, as antiports
• cotransport often utilizes the H⁺ electrochemical gradient established by the H⁺-ATPase (higher H⁺ concentration outside the cell) to drive the transport of another molecule against its electrochemical gradient (this would represent secondary active transport, since the carrier is not using the energy (usually ATP) directly

• generate membrane potential (electrogenic)
• sort of the reverse of ATP synthesis in chloroplasts/mitos, but very different proteins
• in plants, usually move H⁺, can also move Ca²⁺
• pumps perform primary active transport, since they are using the energy directly to move ions against their electrochemical potential
• pumps are a type of carrier, so they bind and release the ions, this binding site confers selectivity

Vacuole also has transporters- pumps, channels, and carriers

• inside of vacuole is more acidic
• accumulates solutes to maintain turgor pressure

The kinetics of different transporter mediated movement show different types of kinetics

• channels show a linear relationship between rate and ion concentration (within biological range)
• carriers show saturable kinetics, as one sees with enzymes
• similar to enzyme kinetics, with K_m and V_max
• K_m can change in different solute concentrations (high affinity-works at low concentration vs. low affinity-works at high concentrations with faster kinetics)

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