The internal sacs of the lemon fruit hold a very acidic liquid, considerably more acidic than the cells in other plants. In order to create and maintain the high concentration of hydrogen ions inside these internal sacs, lemons employ enzymes (V-ATPases) associated with the outer membrane of the cells, that use the plants energy to pump hydrogen ions through the otherwise semi-impermeable membrane and into the cell.
"During maturation, the vacuolar pH of the juice sac cells of lemon fruits declines gradually from pH 6.0 to pH 2.2, a process that may take over a year. This prolonged acidification is accompanied by citrate accumulation in the form of the free acid to a final concentration of about 300 mM. Since citrate typically enters vacuoles as citrate3-(28), it would tend to bind protons and raise the vacuolar pH rather than lower it. Thus, the dramatic decline in pH is brought about by active proton pumping, rather than by citrate uptake." (Müller et al. 1996:1921-1922)
"The vacuolar H+-ATPases (V-ATPases) of eukaryotic cells are large, multimeric proton pumps composed of 10 to 13 different subunits organized into a hydrophilic catalytic complex, V1, and a hydrophobic transmembrane H+ channel, Vo...V-ATPases normally operate far from thermodynamic equilibrium and are therefore considered to be under kinetic regulation. Kinetic regulation of the V-ATPase may involve inhibitors, V1 dissociation from the membrane, slip induced by ΔμH+, and variable H+/ATP coupling...Although the juice sac V-ATPase became sensitive to inhibitors after being solubilized, purified, and reconstituted into liposomes, its pH-dependent slip rate was still about one-half that of the epicotyl V-ATPase. We therefore proposed that the low slip rate of the juice sac V-ATPase might be one of the factors which allows the lemon fruit V-ATPase to build up its steep equilibrium pH gradient...Thus, a lower rate of slip in the presence of a pH gradient remains the most likely factor allowing the generation of a steeper ΔpH by the fruit V-ATPase. Although the coupling ratios of the fruit and epicotyl V-ATPases measured in the absence of a pH gradient were equal, the fact that the V-ATPase of juice sacs retained some coupling in the presence of nitrate, whereas the epicotyl V-ATPase did not, may be significant biologically. Nitrate may be mimicking the effect of another stress, such as low cytosolic pH, which would tend to inactivate a normal V-ATPase. The ability of the fruit V-ATPase to adjust to adverse conditions may be a key factor in the overall regulation of vacuolar pH in lemon." (Müller et al. 1999:10706, 10707)