Many biologically important molecules, including a number of pharmaceutical agents, are organic weak acids that can cross the membrane in their protonated, uncharged form but not in their charged form (i.e., as the conjugate base).
mˈɛni bˌaɪəlˈɑːdʒɪkli ɪmpˈoːɹtənt mˈɑːlɪkjˌuːlz, ɪŋklˈuːdɪŋ ɐ nˈʌmbɚɹ ʌv fˌɑːɹmɐsˈuːɾɪkəl ˈeɪdʒənts, ɑːɹ ɔːɹɡˈænɪk wˈiːk ˈæsɪdz ðæt kæn kɹˈɔs ðə mˈɛmbɹeɪn ɪn ðɛɹ pɹˈoʊtənˌeɪɾᵻd, ʌntʃˈɑːɹdʒd fˈɔːɹm bˌʌt nˌɑːt ɪn ðɛɹ tʃˈɑːɹdʒd fˈɔːɹm ˈaɪ.ˈiː., æz ðə kˈɑːndʒuːɡˌeɪt bˈeɪs.