When the extracellular environment is more acidic than the neutral pH within the cell, certain acids will exist in their neutral form and will be membrane soluble, allowing them to cross the phospholipid bilayer.
wˌɛn ðɪ ˌɛkstɹəsˈɛljʊlɚɹ ɛnvˈaɪɹənmənt ɪz mˈoːɹ ɐsˈɪdɪk ðɐn ðə nˈuːtɹəl pˈiː ˈeɪtʃ wɪðˌɪn ðə sˈɛl, sˈɜːʔn̩ ˈæsɪdz wɪl ɛɡzˈɪst ɪn ðɛɹ nˈuːtɹəl fˈɔːɹm ænd wɪl biː mˈɛmbɹeɪn sˈɑːljuːbəl, ɐlˈaʊɪŋ ðˌɛm tə kɹˈɔs ðə fˈɑːsfəlˌɪpɪd bˈɪleɪɚ.