Handbook of Plant and Crop Physiology

Electrochemical potential difference:The driving force for the transport of solutes across plant cell

membranes. It is the electrochemical potential difference across the membrane (
j

ijJo,

where i is inside and o is outside). Contributions of the pressure term (PV
) to the electro-

chemical potential difference of ions across biological membranes are usually negligible com-

pared with the electrical contribution and hence can generally be ignored [2]. The electrochem-

ical potential difference across a membrane then is


j(io)
2.3RTlog

a

a

j

o

ij

zjF(io) (2)

Flux of solute Jj(mol sec^1 m^2 ): This is the unidirectional rate of solute movement across a unit

membrane area. The net flux Jnet, or uptake, is the difference between the influx (Jin) and efflux

(Jout):Jnet
JinJout

Accumulation of a solute:Specifies a higher concentration (not necessarily higher electrochemical

potential) of the solute inside.

Active transport:Transport of a solute against its electrochemical potential gradient [3]. Such trans-

port always needs energy input.

Passive transport:Transport of a solute along its electrochemical potential gradient.

Metabolic transport:Any transport that depends on metabolic energy supply; it is inhibited by in-

hibitors of energy metabolism [4].

Electrogenic transport:Transport of an ion unaccompanied by equal opposite charge, thus creating

an electrical potential difference or changing it.

Electrophoretic transport:Transport of an ion in response to a preexisting electrical potential dif-

ference. Such transport is also electrogenic and results in a change of the electrical potential

difference.

III. FREE SPACE AND OSMOTIC VOLUME

In experiments on the time course of salt uptake by plant tissues, two phases are revealed: (1) a rapid ini-
tial phase that is completed within a few minutes and (2) a slower phase that may proceed for several
hours at a constant rate. The initial rapid uptake is into the free space [5], namely the extramembranal
space of the plant tissue. The free space consists of the cell walls and the intercellular spaces. Uptake into
the free space is reversible and nonmetabolic. All the anions and part of the cations that are absorbed in
the first uptake phase can be washed out with water, and the remaining cations can be exchanged with an-
other cation. Uptake in the second phase is into the osmotic volume [5], namely the space that is sur-
rounded by plasma membranes. The latter is usually a metabolic process.
Cation exchange in the free space results from the presence of immobile negative charges in the cell
walls. Dissociated carboxylic groups, in particular those of polygalacturonic acid, are responsible for
these charges [5]. The presence of immobile negative charges in the cell wall, adjacent to the external
aqueous phase, results in an electrical potential difference, the Donnan potential [6].

338 JACOBY AND MORAN

TABLE 1 Composition of Pond Water and of the Sap of the Alga Nitella clavata

Growing in the Pond

Pond Sap

Ion (mol m^3 ) (mol m^3 ) Ratio: sap /pond

Mg^2  1.5 5.5 3.6

Ca^2  0.7 7.0 10

Na 1.2 49 41

K 0.5 49 97

H 2 PO 4  0.008 1.7 212

Cl 1.0 101 101

SO^24  0.34 6.5 20