Fig. 5From: First report on Vitamin B9 production including quantitative analysis of its vitamers in the yeast Scheffersomyces stipitisProposed putative pathway for ammonium and urea uptake in S. stipitis. Ammonium (left) is internalized in the cell by facilitated diffusion via ammonium permease; to maintain the potential difference across the plasma membrane the H+ -ATPase Pma1 expels a proton, causing acidification of the growth medium. Urea (right) is internalized through a symport with an H+ ion (causing an increase in the pH of the growth medium), and it is converted into ammonium via urea amidolyaseBack to article page