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When a stoma’s guard cells try to swell and open the stomata, they actively pump out protons outside of the cell. This causes a membrane potential, where the inside of the cell is negative with respect to the outside. This then causes potassium ions, which are positive, to rush into the cell, mitigating that potential. However, my texts say that anions such as chloride and malate ions follow potassium into the cell as well; their rationale is that positive ions must be balanced by the presence of negative ions. I do not understand why this is the case. If you extruded positive ions (protons) out of the cell and allowed positive ions (potassium) back in, there would be no ionic imbalance to draw the anions from where they’re idling. Could someone clear this up for me?

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The ion concentrations are different inside and outside of the cell, allowing a potential. The cell becomes even more polarized by the pumping out of H+ ions, making the cell even more negative (potential difference). Then this negative potential happens to cause voltage-gated channels to open and the uptake of potassium occurs. This will depolarize the cell (return it to the base potential difference). It is usually very important for the cell to maintain the negative potential after it recovers. To combat the uptake of potassium, negative charges can be used to balance the increased positive ion influx so the resting potential is stabilized.

These mechanisms are usually mediated by special proteins that are regulated by actual electrical changes in the membrane.

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You may find it an interesting sidelight that in many plants with potassium deficiency, we see necrotic leaf spots or leaf tip or marginal burn. The plant can't adequately close the stomates, so you get localized dessication in the leaf and therefore necrotic leaf spots. In this way, drought stress and potassium deficiency are somewhat related.

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    $\begingroup$ do you have any references that one can read for more information on your answer? $\endgroup$ May 9, 2016 at 17:07

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