The Physiology of Flowering Plants - KHAM PHA MOI

114 MINERAL NUTRITIONdetect the element only, with no information as to the form in which itis present. If cellular Cl – concentration is estimated by X-ray analysis,one can be confident that a count of the Cl wavelength X-rays gives ameasure of ion concentration. But X-rays at the P wavelength wouldcome from organic compounds as well as from Pi and would beunreliable as an estimate of the concentration of phosphate ions.Active accumulation: the electrochemical potential gradientIon uptake usually is a process of ion accumulation, plant cellsacquiring a total ion content greater than that in their environment,and this applies for many individual ions as well. For some ions, theinternal concentration can be very much higher than the external;several hundred-fold accumulation is common. Phosphate, which ispresent in very low concentrations in natural soils, can be accumulatedseveral thousand-fold. This immediately suggests movementagainst a free energy gradient, for higher concentration of a solutemeans higher free energy. However, there is another component tothe free energy gradient of ions, namely the electric field. For electricallycharged particles, an electric potential gradient is a freeenergy gradient. A cation, being positively charged, will movetowards a more electronegative region. An anion, being negativelycharged, will move towards an electropositive region. For ions thereforethe free energy gradient is the combined electrochemicalpotential gradient to which both the concentration of the ion(determining its chemical potential) and the electric potential contribute.This is highly relevant to ion uptake by plant cells, becausethere does exist an electric charge difference across the membranesof cells. As mentioned earlier, positively charged protons are pumpedto the outside of plant cells, into the walls; this results in the inside ofthe plasma membrane being left electronegative with respect to theoutside. The potential difference is in the range of 100–250 millivolts(mV). Similarly, pumping of protons from the cytoplasm across thetonoplast into the vacuole makes the cytosol side of the tonoplastelectronegative with respect to the vacuolar side.The cytoplasm is accordingly more electronegative than the apoplastoutside it. Therefore, when a cation, say K + , is found to be in ahigher concentration within a cell than outside it, the questionarises: has it been moving against the free energy gradient (as suggestedby the concentration gradient) or along the free energy gradient(as suggested by the electric potential gradient)? When the overallelectrochemical potential gradient is taken into account, it is foundthat the accumulation of cations, even allowing for the electronegativityof the cytoplasm, is very often, though not exclusively, againstthe electrochemical potential gradient. For anions, any accumulationinto the cytoplasm must be against the free energy gradient, foranions are negatively charged particles moving into a moreelectronegative area as well as against the concentration gradient.Movement against the free energy gradient is often termed active andrequires an input of energy by the cell. Where an ion is accumulated