The thylakoid membrane in a wide pH range

The structural organisation of thylakoid systems of pea chloroplasts subjected to osmotic shock after incubation in a wide range of pH values (from 1.0 to 12.0) was studied. It was found that the percentage of thylakoid profiles stacked into granas is 86 % at pH 4.0, 72 % at pH 7.4 and 25 % at pH 10.0. In the pH range from 6.0 to 10.0 under the conditions of osmotic shock, a complete disordering of thylakoid systems, unstacking, and fragmentation of thylakoids are observed. Decreasing medium pH to 5.5-3.5 leads to a shortening of agranal thylakoids, the appearance of numerous lipid drops, and a shrinkage of thylakoid systems, which cease to respond to osmotic shock. Thylakoid systems remain structured, and the stacking in granas is retained. At pH levels from 3.0 to 1.0, thylakoid systems appear as spherical clusters of tightly stacked vesicles, whose structural organisation also does not change under osmotic shock. Increasing the pH of the medium to 11-12 leads to a complete unstacking of thylakoids and a swelling of single thylakoid sheets, which also do not respond to osmotic shock. A correlation between the release of acyl lipids from the thylakoid membrane and the loss of the native response of the membrane to osmotic shock is discussed.