Abstract
The complexity of the structure of the inner membranes of the chloroplast, the thylakoids, is described elsewhere in these proceedings by Barber. The thylakoids are the site of the early reactions of photosynthesis and contain a number of pigment-protein complexes which function in the conversion of light energy into chemical energy (1). The resultant transfer of electrons from H2O to NADP+ requires the cooperation of two reaction centres acting in series which are known respectively as photosystem two (PS2) and photosystem one (PS1). Each photosystem has its own light harvesting array of chlorophyll molecules serving as an antenna to the reaction centre. The most recent advance in the understanding of thylakoid structure is the realisation that PSI and PS2 are inhomogenously distributed within the plane of the thylakoid membrane (2,3). PSI and its light harvesting complex (LHC1) are probably restricted to the non-appressed membranes and PS2 and the third major pigment-protein complex, the light harvesting chlorophyll a/b protein (LHC2), are found preferentially in the appressed regions of the grana. At neutral pH the thylakoids carry a net negative charge and the maintenance of the intricate membrane structure requires the presence of a sufficient level of screening cations. The segregation of the photosystems within the plane of the membrane is due to the difference in surface charge density of the various chlorophyll-protein complexes. The appressed regions of the membranes presumably have a lower net negative charge density (σ) than the non-appressed regions. This is discussed in detail by Barber elsewhere in these proceedings.
Published Version
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