How are essential mineral nutrients taken up and distributed in the plant?
    The environment does not supply all the nutrients at the right concentrations, so there is a selective and discriminatory process.
Importance of ion transport:
    a.  Acquire suitable amounts of essential nutrients
    b.  Use ions to serve as  important signals for communication. e.g. electrical potential and [Ca]
    c.  Use ions to regulate turgor and movement, e.g.  guard cell movement control stomatal aperture.
For growth and cell expansion, e.g. positive pressure due to ion uptake followed by water uptake.

CONCEPTS

1. LIPID BILAYERS ARE IMPERMEABLE TO IONS AND CHARGED MOLECULES.

2. PROTEINS CATALYSE THE TRANSPORT OF SOLUTES ACROSS MEMBRANES, JUST AS ENZYMES CATALYZE CHEMICAL REACTIONS.
    High specificity: High affinity, low Km;
    Fast Rate : Vmax

There are 3 main classes of transport proteins.
Channels,
Pumps
Carriers

3. PROTEIN-MEDIATED TRANSPORT CAN BE ACTIVE OR PASSIVE.
Passive transport is movement down an electrical and a concentration gradient..
Active transport is movement against an electrical and concentration gradient. .

    Q?  How do you know whether an ion is moving downhill or uphill?
        What is the energy source for uphill movement?

What is an electrochemical gradient?
How is a gradient formed?
Electrochemical gradient exists when there is a conc. and an electrical gradient.

Conc. Gradient: exists when there is unequal conc. across a membrane.  E

Electrical Gradient results when there is unequal distribution of charges across a membrane.  When an ion (C+ or A-)  moves in one direction, and there is no accompanying  flux of another ion that neutralizes the charge.

(A) Diffusion potential : is formed when an ion moves down its electrochemical gradient (passive).
(B) Active pump potential : is formed when an ion is actively pumped against its electrochemical gradient.

NERNST EQUATION PREDICTS THE DISTRIBUTION OF IONS AT EQUILIBRIUM at a given electric potential.
    A very important equation. It is used to determine: a) which ion is entering a cell passively or actively; and b)  which ion leaves the cell passively or actively, when the ion conc. and electrical potential difference are known.
 

Major Transport Proteins in Plants
1. H⁺ PUMPS ARE THE MAJOR ION PUMPS IN PLANTS.

They generate an electrical (-inside) and pH gradient (acid outside).
    a) PLASMA MEMBRANE H+-ATPase EXTRUDE H+ out of the cell.
    b) VACUOLAR H+-ATPase ACIDIFIES THE VACUOLE
    c) VACUOLAR H+-PPase ALSO ACIDIFies THE VACUOLE
The stored energy generated by H+ pumps us used for active transport of other ions and metabolites and for passive transport.

2. ACTIVE TRANSPORT OF MANY NUTRIENTS AND METABOLITES IS DEPENDENT ON H⁺-COUPLED COTRANSPORT.
    Symport:
    Antiport

3. CHANNELS ALLOW RAPID, PASSIVE TRANSPORT OF IONS, METABOLITES AND WATER.
        e.g. Light stimulated opening of stomatal pore..

4.  Water is transported via WATER CHANNELS or AQUAPORINS in membranes that conduct large volumes of water rapidly.
        e.g. endodermis

5. Guard cell movement controls stomatal aperture.
Example of how pumps, carriers, channels and water transport work together to regulate stomatal aperture.
a. Opening
b. Closing
                                    ----------------
Reading: Taiz & Zeiger 2006
   ch. 6. Solute transport
    ch. 2 for review of enzymes and energy (online at www.plantphys.net)
            esp. electrochemical potential, and
            enzyme.

                ************                    ***************
Lecture Review
Nernst Equation states that at equilibrium the difference in conc.of an ion between two compartments is balanced by the voltage difference between the two compartments.

DE = -2.3 RT/ zF log [C]i /[C]o         when z = valence of ion;  e.g. K+ = +1
                                                                C = conc. of a charged species (mM, mM or M).
                                                               T = 25 C or 298 K;
                                                              F = 23 cal/mV. mole;
                                                             R = 1.987 cal/mole.deg

Free energy change of 1 charged species (C) moving from o to i.
DG = RT ln [Ci /Co] + zFDE             DE =    Ei -Eo   in mV  (millivolts, )
                                                            DG  = cal, if you use the units shown above.

Free energy change of 1 uncharged species S moving from out to in.
 DG = RT ln [Si /So]

1. Distinguish between simple diffusion, facilitated diffusion, and active transport.  Which of these three mechanisms would  most probably account for:
a.  entry of a small lipid-soluble solute (e.g. ethanol)
b.  extrusion of Ca ions from the cell
c.  rapid entry of glucose (neutral sugar) down a concentration gradient.
d.  influx of 1 mM K⁺ into a plant cell containing 100 mM KCl.

2.   See Table 6-1. 
a.  Use the Nernst equation to calculate the predicted cell ion conc.  if that ion was at equilibrium with the external conc.  Note the membrane potential of most plant cells is -120 mV.
Is 1 mM external K⁺   taken up passively or actively?  Why?
1 mM Na⁺
1 mM Ca⁺⁺
1 mM NO₃^-

Can you predict whether each ion is moving in passively or actively? Explain.

3. [ Taken from Taiz 1998, Fig. 6-11].   A plant cell has a membrane potential of -250 mV.  When 50 mM glucose is added, the membrane potential changes to -145 mV, and the external pH of 5.7 becomes 6.3.  Can you explain what  happened?  (try first without peeking at the legend)   Glucose is taken up into the cell by a pump, a cotransport or a channel?  Why?   Explain with logical reasoning.

4.    K+ can be taken up into cells down the electrical chemical gradient.  The Nernst equation predicts the distribution of an ion at equilibrium.
a)  When ext K conc is 1.0 mM, what is the predicted cytosolic K conc. if the cell has a electrical potential of -120 mV.
How does K get into the cell under these conditions?  pump, cotransport or channel?  why?

b)  Now let’s say the soil K+ has been depleted, so the soil K+ conc. is 0.1 mM.  Plants will survive and grow in this environment. Assume [K+] inside is maintained at 75-100 mM, and the electrical potential is -120 mV.  How would plants take up this ion?  why?

c).  In cell under (a), when morning sunlight hits the plant, it activates the PM proton pump.  The elec. potential orig. at -120 mV becomes -150 mV.
Why is the pot. more negative?

How would the change in elec. potential affect the cell K+, assuming passive distribution of this ion.?

5.  Definitions:  Know the difference between
a)  active and passive transport of an ion.
b)  pump, cotransport and channel
c)  antiport and symport
d)  Plasma membrane  H+-pumping ATPase and vacuolar H+-pumping ATPase

Why are proton pumps important for plant cells?

BSCI 442