Ernest G. Uribe

Membrane permeability and internal volume as factors in ATP synthesis by spinach chloroplasts

Abstract Osmotic shock of frozen and thawed spinach chloroplasts produces separable “heavy” and “light” fractions. The “heavy” fraction is completely uncoupled: although electron transport is possible in the light, neither illumination nor an acidbase transition can cause either ATP formation, or activation of ATPase. The “light” fraction is competent in both reactions, activated in either fashion. The loss of energy coupling in “heavy” particles is correlated with a loss of osmotic response (swelling or shrinking in response to graded sucrose concentrations) and with a loss in the ability to store succinate in a space inaccessible to inulin. When “light” particles are shrunken by exposure to sucrose, their ability to store succinate, and the amount of ATP formed after an acid-base transition, are both decreased more markedly than are other functions. Chloroplasts uncoupled by EDTA retain their osmotic properties and their ability to store succinate, so the effect of this treatment is not perceptibly due to an alteration of membrane properties. These data are interpreted as illustrating the need for an intact, osmotically sound graua membrane as a necessary but not a sufficient prerequisite for energy coupling. A sufficient free internal space is needed for massive storage of organic acids used as buffers at pH 4, and these in turn are needed for ATP formation upon return to alkali.