We have investigated the structure of the photosynthetic membrane in a mutant of barley known to lack a chlorophyll-binding protein. This protein is thought to channel excitation energy to photosystem II, and is known as the "light-harvesting chlorophyll-protein complex." Extensive stacking of thylakoids into grana occurs in both mutant and wild-type chloroplasts. Examination of membrane internal structure by freeze-fracturing indicates that only slight differences exist between the fracture faces of mutant and wild-type membranes. These differences are slight reductions in the size of particles visible on the EFs fracture face, and in the number of particles seen on the PFs fracture face. No differences can be detected between mutant and wild-type on the etched outer surface of the membrane. In contrast, tetrameric particles visible on the etched inner surface of wild-type thylakoids are extremely difficult to recognize on similar surfaces of the mutant. These particles can be recognized on inner surfaces of the mutant membranes when they are organized into regular lattices, but these lattices show a much closer particle-to-particle spacing than similar lattices in wild-type membranes.Although several interpretations of these data are possible, these observations are consistent with the proposal that the light-harvesting chlorophyll-protein complex of photosystem II is bound to the tetramer (which is visible on the EFs face as a single particle) near the inner surface of the membrane. The large tetramer, which other studies have shown to span the thylakoid membrane, may represent an assembly of protein, lipid, and pigment comprising all the elements of the photosystem II reaction. A scheme is presented which illustrates one possibility for the integration of the light reaction across the photosynthetic membrane.The light reaction of photosynthesis is localized within the thylakoid membranes found in higher plants and green algae. The structure of these membranes is exceedingly complex, but now seems to be understood in general terms. At least two types of particles exist within the membrane, and can be visualized by the freeze-fracture technique (10,14,22). The larger of these particles is found principally in stacked (grana) regions of the chloroplast (14, 33, footnote 1), spans the thylaStaehelin, L. A. 1976. Reversible particle movements associated with unstacking and restacking of chloroplast membranes in vitro.

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The localization of the chlorophyll-protein complexes inside the thylakoid membrane of Acetabularia rnediterranea was determined by fractionating the chloroplast membrane with EDTA and Triton X-100, by using pronase treatment, and by labeling the surface-exposed proteins with 1251.

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