Biological Membranes - A. N. Ogurtsov 2012

Electrogenesis of Biomembranes
Secondary Transport by Cotransporters
Transporters in Plant Vacuoles

The acidity of the vacuolar lumen (pH = 3 : 6) is higher than that of the plant Cytosol (pH = 7.5). Vacuolar acidity is maintained by V-type ATP Pumps and plant-specific PP_i-pumps that utilize the energy of Pyrophosphate Hydrolysis. Both of these pumps are located on the vacuolar membrane and import protons into the vacuole against their concentration gradient.

The vacuolar membrane also contains channels for Cl^- and NO^-₃, which transport these anions from the cytosol into the vacuolar lumen. The driving force for the movement of these anions is the positive electrical potential generated by protons pumped into the vacuole by proton pumps (Figure 105).

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Figure 105 - Transport systems of The plant Cell vacuole

The combined action of proton pumps and anion channels maintains a vacuolar Membrane Potential of 20 mV and the increased acidity of the lumen. The proton gradient across the vacuolar membrane is used (similarly to the sodium ion gradient across The Plasma Membrane) as an energy source to drive cotransporters.

Figure 105 shows three such proton antiporters, which use the energy of proton flow from the vacuole into the cytosol (down their concentration gradient and driven by the excess positive charge of the vacuolar lumen) to pump sodium and calcium cations, as well as sucrose molecules, into the vacuole. In this way, the excess sucrose produced in the leaves during the day by Photosynthesis is stored in The Cell vacuoles. At night, sucrose is transported into the Cytoplasm and metabolized to CO₂ and H₂O, during which ATP is synthesized from ADP and P_i.