PAs accumulate in the lumen and buffer the lumen pH during photophosphorylation.

Photosynthetic reactions produce protons that are vectorially released in the lumen. Lumen acidification shifts equilibrium 1 to the right (production of charged Put in lumen). Depletion of free putrescine urges new free Put to diffuse from stroma into lumen (Le Chatelier principle equilibrium 2). Finally, free Put in stroma is replaced by charged Put in stroma which is ionized (equilibrium 3). The net result of this process (i.e., new poise of the 3 equilibria) is ion trapping. That is the accumulation of Put in lumen up to 100 times. The final concentration of Put depends mainly on ΔpH and counterion concentration (such as Cl-).