Acidity of the thylakoid lumen in plastids makes sense from an evolutionary perspective

An acid pH in the lumen of chloroplast thylakoids is necessary in order to derive the required amount of CO2 to account for the observed rates of carb on fixation. We point out that the endosymbiotic derivation of the chloropl ast fr-om a cyanobacterium would have resulted in the lumen of the thylakoi d having an acid pH. The thylakoids of cyanobacteria are continuous with th e plasma membrane, resulting in the lumen of the thylakoid being open to th e outside of the cell. Endosymbiosis resulted in the cyanobacterium being t aken up into a food vacuole of a protozoan. The vacuole would have had an a cid pH, probably around pH 5, so the endosymbiotic bacterium would have bee n surrounded by an environment with an acidic pH. The lumen of the thylakoi ds would have been at an acid pH since they were open to the exterior of th e cell, and to the contents of the vacuole.