Importance of solute transport: Nutrition Guard cell control Pulvinus control (sleep movements) Growth

Transcription

1 Solute uptake (I)

2 Importance of solute transport: Nutrition Guard cell control Pulvinus control (sleep movements) Growth

3 The casparian strip enforces membrane control of solute uptake from soil

4 Importance of solute transport: Nutrition Guard cell control Pulvinus control (sleep movements) Growth Ψ s > Ψ s Ψ p < Ψ p

5 Importance of solute transport: Nutrition Guard cell control Pulvinus control (sleep movements) Growth 8 am 12 noon 5 pm

6 Importance of solute transport: Nutrition Guard cell control Pulvinus control (sleep movements) Growth Import of solutes produces Ψ s < 0, Ψ p > 0, and leads to inelastic extension Growth in Progress Please Do Not Touch

7 The movement of solutes across a membrane is influenced by electric potential, as well as concentration differences

8 Quantitative strength of an electric field Chemical potential of solute j:! j =! j * + RT ln a j + z j FE + V j P! j * = chemical potential under standard conditions F = Faraday's constant E = electric potential a j = activity (concentration) z j = charge V j = partial molal volume Force moving solute j along direction x: d! j /dx = RT d(ln a j )/dx + z j F de/dx + V j dp/dx

9 Quantitative strength of an electric field (2) Across membranes, simplify by using differences: Change of energy pushing molecule into a cell (final less initial energy):! j i -! j e = (! j i * -! j e *) + RT (ln a j i - ln a j e ) + z j F (E i - E e ) + V j (P i - P e ) (! j i * -! j e *) = 0 RT (ln a j i - ln a j e ) = RT ln (a j i /a j e ) = RT ln (c j i /c j e ) = effect of concentration gradient z j F (E i - E e ) = electrical potential effect = "membrane potential" V j (P i - P e ) = hydrostatic pressure effect (negligible)! j i -! j e =!! j = RT ln (c j i /c j e ) + z j F (E i - E e )

10 Quantitative strength of an electric field µ j i -µ j e = µ j = RT ln (c j i /c j e ) + z j F (E i - E e ) At equilibrium, µ j = 0, and RT ln (c j e /c j i ) = z j F (E i - E e ) Rearranging, E i - E e = RT/zF ln (c j e /c j i ), or c j e /c j i = exp [(zf/rt)*(e i - E e )] the Nernst equation E i - E e is the Nernst potential for ion j, the potential that supports the given concentration gradient c j e /c j i is the equilibrium concentration gradient for an ion across a membrane charged with a given potential E i - E e = 59 mv log 10 (c j e /c j i z = +1 For E i - E e = -59 mv and z = +1, log 10 (c j e /c j i ) = -1 and c j e /c j i = 1/10

11 K + is at equilibrium; the others are not!

12 A certain barley root cell maintains a potential difference of 88.5 mv across its plasma membrane. The following concentrations of ions (molar) inside and outside the cell were measured: Ion inside outside K Na Cl SO Ca (a) Which of the ions would be considered to be at or near electrochemical equilibrium by the formula given in lecture, assuming a temperature of 25 o C?

13 Use the Nernst equation: E i - E o = (2.3RT/zF)log 10 C o /C i, or E i - E o C i /C o = 10 ((Eo -E i )z/59) E o - E i = 88.5 mv For z = +1, C i /C o = 31.6 For z = +2, C i /C o = 1000 For z = -1, C i /C o = For z = -2, C i /C o = = (59/z mv)log 10 C o /C i A certain barley root cell maintains a potential difference of 88.5 mv across its plasma membrane. The following concentrations of ions (molar) inside and outside the cell were measured: Ion inside outside in/out (obs) in/out (predicted) K Na Cl SO Ca

14 (b) For each ion not in equilibrium, suggest a reason for its distribution (active uptake, active efflux, exclusion, magnetic effects, limited carrier capacity, no pressure for redistribution, etc.) (d) What will be the effect of adding the following chemicals: DNP (dinitrophenol, carries H + ions across membranes along their electrochemical gradient); valinomycin (carries K + ions across membranes along their electrochemical gradient); KCN (stops respiration and lowers the supply of ATP). A certain barley root cell maintains a potential difference of 88.5 mv across its plasma membrane. The following concentrations of ions (molar) inside and outside the cell were measured: Ion inside outside in/out (obs) in/out (predicted) K Na Cl SO Ca