At least two mechanisms may contribute. Only the non-polar undissociated acid can diffuse through the cytoplasmic membrane. In the relatively high pH in the cytoplasm the acid dissociates and reduces the pH. The cell tries to control the intracellular pH by pumping out protons, which costs energy in form of ATP. Too high diffusion rate of undissociated acid into the cell therefore uncouples energy from growth. It can not be ruled out that inhibition of enzymatic reactions in the declining cytoplasmic pH also plays a role. A second mechanism, relevant for respirating organisms, may be that the declining intracellular pH also diminishes the proton gradient over the cell membrane which is needed for the ATP generation in respiration.
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