A model of the photosynthetic membrane from higher plants is presented. The different photosystems, PSIa, PSIB, PSIIa and PSII/7, are located in separate domains. The photosystems with the largest antenna systems, the alpha systems, are in the grana and the other in the stroma lamellae. In each grana disc PSIa is located in a flat annulus surrounding a circular PSIIa domain. In this the PSIIa units with the largest antennae are found in the center. The model is consistent with results from recent membrane fractionation experiments.Photosynthesis; Thylakoid structure; Membrane domain; Photosystem I; Photosystem II; CytochromefThe structure of the photosynthetic membrane (the thylakoid) from plants is very complex and it has to be since it carries out diverse functions under varying conditions of light and temperature. Its main function is to capture light quanta and to convert light energy into useful chemical energy. To this end the light energy is used to drive a series of redox reactions whereby water is oxidized to protons and oxygen while ferredoxin is reduced concomitantly with the production of ATP.The thylakoid membrane consists of essentially two compartments: the grana and the stroma lamellae interconnecting the grana. One can also distinguish between appressed and stroma-exposed membranes or between different membrane domains such as the partitions (appressed), the margins of both grana and stroma lamellae, the grana end membranes (stroma exposed) and the zone at the border between the grana and the stroma lamellae (Fig. 1).It was shown long ago that, following mechanical press treatment of thylakoid membranes, centrifugation could separate a small vesicle fraction containing PSI from a heavier grana-rich fraction containing both PSI and PSII. It was concluded that the small vesicle fraction originated from the stroma lamellae, that these contain PSI but very little PSII, and that the grana of the native thylakoid contains both PSI and PSI1 [l].Separation of press-treated thylakoids by aqueous two-phase partition resulted in the isolation of insideout vesicles of the size of about 0.5 pm, which were highly enriched in PSI1 [2-61. The vesicles were assum- The thylakoid membrane, however, is much more complex in its structure.The photosystems are heterogenous and there are different types of both PSI and PSII. We here present a model for the structure and function of the thylakoid membrane which is based on recent fractionation experiments and which takes into account the heterogeneity of the photosystems. We assume the presence of two main types of PSI (CY and /3) and two main types of PSI1 (a and /3). We also discuss the model with respect to both the Z-scheme of electron transport where the two photosystems cooperate in series and the alternative scheme of Arnon in which they function independently in parallel.Photosystem II is heterogenous, both with regard to its antenna size and its redox properties. For a review see Black et al. [13]. One can distinguish between at least two types. One type, PSIIa...
The relative proportion of stroma lamellae and grana end membranes was determined from electron micrographs of 58 chloroplasts from 21 different plant species. The percentage of grana end membranes varied between 1 and 21% of the total thylakoid membrane indicating a large variation in the size of grana stacks. By contrast the stroma lamellae account for 20.3 +/- 2.5 (sd)% of the total thylakoid membrane. A plot of percentage stroma lamellae against percentage of grana end membranes fits a straight line with a slope of zero showing that the proportion of stroma lamellae is independent of the size of the grana stacks. That stroma lamellae account for about 20% of the thylakoid membrane is in agreement with fragmentation and separation analysis (Gadjieva et al. Biochim. Biophys. Acta 144: 92-100, 1999). Chloroplasts from spinach, grown under high or low light, were fragmented by sonication and separated by countercurrent distribution into two vesicle populations originating from grana and stroma lamellae plus end membranes, respectively. The separation diagrams were very similar lending independent support for the notion that the proportion of stroma lamellae is constant. The results are discussed in relation to the composition and function of the chloroplast in plants grown under different environmental conditions, and in relation to a recent quantitative model for the thylakoid (Albertsson, Trends Plant Sci. 6: 349-354, 2001).
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Thylakoid membranes from spinach were fragmented mechanically and separated into vesicles originating from grana and stroma-exposed lamellae (Andreasson et al. (1988) Biochim Biophys Acta 936: 339-350). The grana vesicles were further fragmented and separated into smaller vesicles originating from different parts of the grana (Svensson and Albertsson (1989) Photosynth Res 20: 249-259). All vesicles so obtained were analyzed with respect to chlorophyll and carotenoid composition by reverse phase HPLC. For all fractions the following relations (mole/mole) were found: 1 carotenoid per 4 chlorophyll (a+b), 2 lutein per 5 chlorophyll b and 5 violaxanthin per 100 chlorophyll (a + b). The contents of lutein and neoxanthin were each linearly related to chlorophyll b and β-carotene was linearly related to chlorophyll a.
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