Correlation of ultrastructure with light-induced ion transport in chloroplasts

Small-Angle X-Ray and Neutron Scattering on Photosynthetic Membranes

Ultrastructural membrane arrangements in living cells and their dynamic remodeling in response to environmental changes remain an area of active research but are also subject to large uncertainty. The use of noninvasive methods such as X-ray and neutron scattering provides an attractive complimentary source of information to direct imaging because in vivo systems can be probed in near-natural conditions. However, without solid underlying structural modeling to properly interpret the indirect information extracted, scattering provides at best qualitative information and at worst direct misinterpretations. Here we review the current state of small-angle scattering applied to photosynthetic membrane systems with particular focus on data interpretation and modeling.

Protonation and chloroplast membrane structure

Light changes the structure of chloroplasts. This effect was investigated by high resolution electron microscopy, photometric methods, and chemical modification. (a) A reversible contraction of chloroplast membrane occurs upon illumination, dark titration with H⁺, or increasing osmolarity. These gross structural changes arise from a flattening of the thylakoids, with a corresponding decrease in the spacing between membranes. Microdensitometry showed that illumination or dark addition of H⁺ resulted in a 13–23% decrease in membrane thickness. Osmotically contracted chloroplasts do not show this effect. (b) Rapid glutaraldehyde fixation during actual experiments revealed that transmission changes are closely correlated with the spacing changes and therefore reflect an osmotic mechanism, whereas the light scattering changes have kinetics most similar to changes in membrane thickness or conformation. (c) Kinetic analysis of light scattering and transmission changes with the changes in fluorescence of anilinonaphthalene sulfonic acid bound to membranes revealed that fluorescence preceded light scattering or transmission changes. (d) It is concluded that the temporal sequence of events following illumination probably are protonation, changes in the environment within the membrane, change in membrane thickness, change in internal osmolarity accompanying ion movements with consequent collapse and flattening of thylakoid, change in the gross morphology of the inner chloroplast membrane system, and change in the gross morphology of whole chloroplasts.

Photophosphorylation as a function of illumination time. II. Effects of permeant buffers

(1) The amounts of orthophosphate, bicarbonate and tris (hydroxymethyl)-aminomethane found inside the thylakoid are almost exactly the amounts predicted by assuming that the buffers equilibrate across the membrane. Since imidazole and pyridine delay the development of post-illumination ATP formation while increasing the maximum amount of ATP formed, it follows that such relatively permeant buffers must also enter the inner aqueous space of the thylakoid. (2) Photophosphorylation begins abruptly at full steady-state efficiency and full steady-state rate as soon as the illumination time exceeds about 5 ms when permeant ions are absent or as soon as the time exceeds about 50 ms if valinomycin and KC1 are present. In either case, permeant buffers have little or no effect on the time of illumination required to initiate phosphorylation. A concentration of bicarbonate which would delay acidification of the bulk of the inner aqueous phase for at least 350 ms has no effect at all on the time of initiation of phosphorylation. In somewhat swollen chloroplasts, the combined buffering by the tris(hydroxymethyl) aminomethane and orthophosphate inside would delay acidification of the inside by 1500 ms but, even in the presence of valinomycin and KC1, the total delay in the initiation of phosphorylation is then only 65 ms. Similar discrepancies occur with all of the other buffers mentioned. (3) Since these discrepancies between internal acidification and phosphorylation are found in the presence of saturating amounts of valinomycin and KC1, it seems that photophosphorylation can occur when there are no proton concentration gradients and no electrical potential differences across the membranes which separate the medium from the greater part of the internal aqueous phase. (4) We suggest that the protons produced by electron transport may be used directly for phosphorylation without even entering the bulk of the inner aqueous phase of the lamellar system. If so, phosphorylation could proceed long before the internal pH reflected the proton activity gradients within the membrane.

The conformational basis of energy conservation in membrane systems. II. Correlation between conformational change and functional states

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