Ion Transport and Electrical Potentials in Plant Cells

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Abstract
The proper study of ion transport in plant cells is in its infancy. Any investigation which is concerned only with ion concentrations is practically certain to lead to incorrect deductions because electrical potential gradients are just as important as concentration gradients in determining passive ion movements. The 1st object of any investigation must be the determination of the electrochemical potential differences for each ion. The next questions to ask are: is the cell in flux equilibrium? what are the fluxes? Work is particularly required on higher plant ceils, on which one can do little more than speculate at the present time. It will be found that the important special feature developed by plant ceils is the inwardly directed anion pump located at the tonoplast. In the algae this is a chloride pump, but in higher plant cells it will have become adapted to pump organic acid anions. Anion pumps seem relatively rare in animal cells but in plant cells they are a logical evolutionary consequence of the development of a cell wall and the consequent ability to withstand high hydrostatic pressures. The anion pump is the chosen mechanism of producing the high internal osmotic pressure in the vacuole and hence the essential turgor pressure of plant cells. The cation pumps, are of secondary importance in plant cells and reflect the common origin of plant and animal cells from some primeval blob of protoplasm which had to develop a Na-extrusion pump to prevent it from being diluted out of existence. A Na-extrusion pump leads to a high internal K concentration which in its turn, in time, leads to enzyme adaptation to a K atmosphere. Hence these cation pumps must persist in plant cells when their osmotic cause has been removed.